Modern and fossil critters, trace fossils, shell morphology
Many, but not all the terms included here are associated with articles posted on this website.
Adapical orientation (Cephalopod) Describes the facing direction of sutures and external ribs towards the apex or protoconch (the first chamber). It applies to straight and coiled forms.
Adductor muscle scars (Bivalve) : Scars where the animal’s adductor muscles are attached to the shell. Two muscle scars are present in most equivalve species near the anterior and posterior margins of both valves. Valves are opened and closed by the adductor muscles, in combination with tension in the ligament.
Aboral surface (Echinoderm) Defined where there is an opening for the anus, the term is used to orient specimens – there is no dorsal-ventral or anterior-posterior position because of the radial symmetry of most echinoderms.
Abyssal zone: The deepest parts of the oceans at 4000-8000 m, covering 83% of Earth’s total ocean area. It is perpetually dark – well below the photic zone. Temperatures generally range from 2-3o C. These depths are also close to or greater than the calcite and aragonite compensation depths.
Ambulacral grooves (Echinoderm) Grooves in the test surface that allow the tube feet to pass food to the mouth.
Ambulacral plates (Echinoderm) Polygonal-shaped plates, each a single calcite crystal, arranged petal-like as part of the five-fold symmetry. Ambulacra contain the pores that allow tube feet to extend; they do not contain spine tubercles. cf. interambulacra.
Ammonoid (Cephalopod) An extinct group of cephalopods that first appeared in the Devonian and disappeared at the end of the Cretaceous. They display a diverse array of coiling geometries. They are distinguished from Nautiloids by the position of the siphuncle that tracks the outer margins of chambers, and their complex septa.
Anal canal (Gastropod) The opening in the aperture margin, opposite the siphonal canal, that allows the egress of waste.
Angiosperm: Plants that produce flowers and seed-bearing fruits. They first appeared in the Late Cretaceous, about 125-100 Ma. The evolution of pollinating insects closely parallels this event. They are the largest group of all extant green plants.
Annelid: A segmented worm including earthworms, leeches, and marine species that secrete a calcareous tube.
Anoxia : Anoxic conditions usually applied to aqueous environments (water masses as well as connate water) where there is none, or insufficient dissolved oxygen for respiration; usually measured at less than 0.5 ml/L. Under these conditions, the sources of oxygen via bacterial reduction are from nitrates and sulphates. Once these sources are depleted carbon dioxide becomes an important source during reduction to methane. Deep waters in lakes where there is no turnover of the water mass, can become anoxic. Anoxia events are also implicated in some of Earth’s major extinctions, such as the Late Permian – Triassic event. Early Precambrian oceans and lakes were probably anoxic. e.g., März and Brumsack, 2015.
Anterior (Cephalopod) The anterior position is shown with the shell upright and aperture downward and facing the observer, The opposite margin is posterior.
Anterior-posterior (Brachiopod) Posterior orientation is applied to the beak-hinged part of both valves. The opposite margin is anterior.
Anterior-posterior (Trilobite) Looking down on the carapace (dorsal view) the margin of the head – cephalon is anterior, and the tail – pygidium is posterior.
Aperture (Cephalopod)The opening of the last grown chamber, the body chamber, in which the animal resides, feeds, and swims. cf. aperture in gastropods.
Aperture: (Gastropod) The open end of a shell through which the animal feeds and excretes. The base of the aperture margin (when holding the shell upright) is commonly interrupted by a siphonal canal that channels water for respiration; the anal canal is located at the top of the aperture. It is usually protected by an operculum.
Aperture: (Graptolite) The open end of the theca that is secreted by graptolite zooids, through which the zooid feeding filters extend. Graptolite aperture margins are commonly adorned with nodes and spines.
Apical orientation (Cephalopod) Describes the facing direction of sutures and external ribs towards the aperture. It applies to straight and coiled forms.
Arthropods: A Phylum of animals characterized by a segmented body and an exoskeleton composed of articulated plates that is moulted at different stages of life and growth. The five main groups include trilobites (extinct), chelicerates including spiders, scorpions and ticks (arachnids) and horseshoe crabs, hexapods (mainly insects), crustaceans (crabs, lobsters, shrimp etc.), and myriapods (e.g. centipedes).
Articulates (Brachiopod) Bivalved brachiopods where the valves possess a well defined hinge, teeth and sockets. This is the most abundant group of brachiopods. cf. Inarticulates.
Assemblage zone: A biozone that contains at least three groups of fossil taxa that distinguish the interval from adjacent strata.
Asteroids The common star fish, containing 5 arms, or arms in multiples of five. In some species the arms can be replaced if broken off or predated. Ambulacral and interambulacral plates are much smaller than in the echinoid cousins. Like the echinoids they use tube feet to move, pass food to the mouth, and for respiration. The oral surface is usually down.
Autotrophs: Organisms that derive energy from light or chemical reactions. Predominantly in the plant domain where the principal mechanism is photosynthesis. In the absence of light, chemotrophic organisms will obtain their energy and carbon for growth from chemical reactions with compounds such as sulphur and ammonia, or carbon dioxide. cf. heterotroph.
Axial lobe (Trilobite) The central, longitudinal, segmented part of the thorax, bound on each side by left and right pleural lobes. Left and right pleura are determined about an imaginary line of bilateral symmetry with the dorsal surface facing and pygidium down (or towards you).
Beak (Bivalve): The pointy end of the shell (always dorsal) that represents the initial stage of animal growth. It points towards the anterior margin.
Beak (Brachiopod) The pointy end of the pedicle and brachial valves, where shell growth was initiated (similar to pelecypods).
Benthic: (adjective) An ecological term applied to organisms that live on a sediment-water interface, or within sediment. It includes invertebrates, vertebrates, and plants (particularly algae and cyanobacteria). The most prolific benthic zones are located within the photic zone that constrains the limits of photosynthesis.
Bioimurration: The process where the skeletal or encrusting material (commonly calcium carbonate) overgrows another organism. The process has the potential to preserve fine details of the substrate structure – this is important where the substrate is easily biodegraded (e.g., plants).
Biomineralization: The process where an organism secrets minerals to form an internal skeleton, exoskeleton, shell, teeth, or tusks – most commonly precipitated as calcium carbonate and calcium phosphate (including hydroxyapatite). These structural components have relatively high preservation potential.
Bioturbation: The reworking of sediment and soil by plants and the burrowing and grazing activities of animals, primarily invertebrates. Bioturbation destroys primary depositional structures and textures in sediment, creating new structures in the process (e.g., lebenspurren). It can also affect sediment-soil redox conditions and redistribute nutrients. Bioturbation has been important driver of invertebrate evolution.
Biserial: (Graptolite) Graptolites that grow two sets of theca on both side of their stipes.
Bivalvia: A major Class within the phylum Mollusca. The majority of bivalves (pelecypods) have two valves equal in size and shape and hinged along their dorsal margin. An (imaginary plane of symmetry separates the valves. Exceptions to this arrangement are commonly manifested as inequivalve taxa where the largest valves is cemented or attached to a substrate (e.g., oysters). Classification is based mainly on gill structure in living taxa and dentition. They are primarily filter feeders. Pelecypod shells consist of layered calcite or aragonite crystals.
Blue-Green algae: Another name for cyanobacteria.
Body chamber (Cephalopod) The last grown chamber that contains the aperture, in which the animal resides.
Body whorl: (Gastropod) Usually the largest whorl that contains the aperture and operculum.
Boring: (Trace fossils) Burrows drilled into hard substrates such as wood, bedrock, shells, carbonate hardgrounds, or amber. Common examples are the cylindrical or bulb-shaped tubes formed by common ship worms (Teredo), Pholad bivalves, and mollusc shell borings made by carnivorous gastropods. Microborings (mm to sub mm scale), for example in shells, can be produced by endolithic algae and certain species of sponge.
Brachial folds (Brachiopod) A distinctive fold (positive, raised, anticline-like structure) in the brachial valve; there is a corresponding sulcus (depression) in the pedicle valve. The axes of both structures are approximately parallel to the bilateral plane of symmetry. Both structures are common in the Rhynchonellids and Spiriferids. Cf. ribs which are superficial ornaments on the shell surface.
Brachial valve (Brachiopod) (dorsal valve) The smaller of the two valves. Sockets occur on this valve if dentition is present.
Brachidium (Brachiopod) An internal, coiled skeletal structure in many species, composed of calcium carbonate, that supports the softer lophophore used for feeding and respiration. The coil looks a bit like an expanded slinky. It will only be observed in broken or sectioned shells.
Burrow: (Trace fossils) The result of sediment excavation by an animal, mostly invertebrates, during any of several life activities (e.g., feeding). Burrows assume a variety of tube-shapes; straight, curved, or sinuous, single or branched, lined or unlined, actively back-filled or passively filled. The multiplicity of burrow types reflects animal behaviour.
Burrow lining: (Trace fossils) Sediment or fecal pellets added to a burrow wall as the animal moves or burrows. A good example is the lumpy, pellet-lined burrows of Ophiomorpha. Linings commonly have a different colour and/or texture to other burrow filling.
Calice, calyx: (Corals) The uppermost cup- or chalice-like depression of a corallum or corallite that provides space for the living polyp. The base of a calice is commonly bound by a tabula and dissepiments.
Callus: (Gastropod) A calcareous growth extending along the inner margin of the aperture that partly or completely covers the umbilicus.
Cambrian: The first period in the Paleozoic Eon, dating from 541 to 485.4 Ma, preceded by the latest Precambrian Ediacaran. The Cambrian witnessed an explosion of invertebrate animals including abundant shelly faunas, and primitive vertebrates. The Burgess Shale is an iconic example of soft-body preservation on some of these species. Prominent among invertebrate species were the Trilobites, molluscs, and brachiopods. It is hypothesized that the dramatic change occurred during a rise in atmospheric oxygen, abruptly, or as more recent investigations suggest, more gradually (Stockey et al., 2024; OA) The Cambrian ended with major extinctions caused by an oxygen crisis. The period was named by Adam Sedgwick in 1835.
Carboniferous: A subdivision (Period) in the Paleozoic, named from the ubiquitous coal deposits in fluvial-related strata. It spans 359.3 Ma to 289.9Ma. It is underlain by the Devonian and overlain by the Permian. The two main subperiods the older Mississippian and Pennsylvanian represent two different plate configurations where the assembly of Pangea was complete only in the later part of the Pennsylvanian. Glaciation was more prominent in the younger subperiod.
Cephalon (Trilobite) The trilobite head that was composed of a central lobe, or glabella that divided the head laterally, fixed and free cheeks, and a pair of compound eyes. All these segments were fused together (no flexibility). The cephalon margin is anterior.
Cnidaria: The phylum of animals includes hard and soft corals, sea anemones, siphonophores (e.g., Blue Bottles), and true jellyfish. Most Cnidarian taxa use stinging cells (nematocysts) for defense or to capture prey. The phylum includes the abundant Scleractinia, Rugose, and Tabulate corals.
Coccoliths: Marine phytoplankton that secrete calcium carbonate skeletons; they are one of the main constituents in natural chalk. Coccospheres are algal cells surrounded by coccoliths arranged into spheres tubes and cup-shaped bodies, up to 100 microns in diameter. They are one of the culprits responsible for marine algal blooms.
Codiacean algae: A group of green algae that precipitate aragonite needles 2-3µm long. Two common species are Halimeda and Penicillus that, across carbonate platforms and reefs, produce large volumes of aragonite mud. Cf. coralline algae.
Coenosteum: (plural Coenostea) The porous calcareous structure that connects corallites in all three groups of colonial corals. It is part of the corallum in these colonies. In coenosteum-dominant species the corallites are spread out, connected by broad coenostea – the opposite applies to corallite-dominant species. The coenosteum structure is sponge-like and constructed of calcareous tubes, spines, the extensions of costae, and networks of small plates.
Coiling (Cephalopod) There is a range of coil geometries in nautiloids and ammonoids.
- Orthoconic – straight shells
- Evolute – Coils are in contact with their neighbours but do not overlap.
- Convolute – Each new chamber partly overlaps earlier coils.
- Involute – The latest part of the coil, including the body chamber, overlaps all previous coils.
- Lituiticone – Coiling is initially tight and involute with later chambers more loosely coiled and hook-like.
Coiling:(Gastropod) Hold the shell upright with the aperture facing you. Apertures that open on the right have dextral coiling (the most common type); those that open on the left are sinistral. Coiling is centred around a central column – the columella. Most common is helical coiling. Shells where the body whorl completely envelops the earlier whorls have involute coiling. Planispirally coiled shells have bilateral symmetry (similar to ammonites). Non-coiled forms include the common limpet.
Columella: (Corals) Present in Rugose and Scleractinian corals but not Tabulate corals. In Rugose corals it is a central pillar-like structure from which the septa radiate towards the wall. In the Scleractinia, the columella, if present, is a mesh of tooth-like structures extending from the edge of the septa.
Columella (Gastropod) The central column that connects all whorls. It is only visible in broken shells, or along the inner margin of the aperture. It may be ornamented with raised ridges, or columellar folds.
Commissure (Brachiopod) The line of contact between the two valves. It may be smooth or disrupted by plications (folds in both valves).
Compound eyes (Trilobite) Eyes of great complexity that appear quite suddenly as fossils in the Early Cambrian. They are located either side of the glabella. The eyes were constructed from multiple prismatic calcite crystals, each crystal acting as a lens, to give an almost 360° field of view.
Coquina: A limestone made up of shells, shell fragments and other bioclasts, with a degree of sorting that indicates relatively high depositional energy. Where the fragments are mostly sand-sized, the Dunham limestone classification equivalent is grainstone.
Corallite: An individual skeletal tube that houses a single polyp, within a coral colony. Corallites are separated from each other by the coenosteum.
Coralline algae:Calcite and high magnesium calcite precipitating red algae, that build upon substrates such as bioclasts and rock surfaces and other algae. All begin life as encrusters, but grow to different forms such as articulated, flexible, bush-like branches, or nodular clusters around shells or pebbles (e.g. Lithothamnion). They are an important contributor to cool-water bioclastic limestones. Both types contribute to temperate and tropical carbonate sediment. They are important components of coral and bryozoan reefs.
Corallum: Corallum (plural Coralla) refers to the entire skeletal structure of a solitary coral (that is constructed by a single polyp). It is discoid, tube-, vase-, horn- or cone-shaped. The term also applies to the entire structure of corallites in a colonial coral.
Costae: (corals) Rib-like structures aligned longitudinally along the epitheca of solitary corals; they are the extension of septa through the coral wall.
Crawling traces: (Trace fossils) A behavioural trait exhibited by invertebrates that produce trails, grooves and burrows when moving from A to B – basically just getting somewhere. Traces are fairly simple, lacking systematic patterns.
Cretaceous: The geological period that witnessed the end of some major fossil groups (e.g. dinosaurs, ammonites) and ushered in modern animal and plant groups – the first flowering plants, many insect groups, and mammals. It extends from 145 – 66 Ma, the last period of the Mesozoic Era. The climactic finale was an asteroid impact – the Chicxulub impact. Pangea was in its final breakup throes. There were no polar icecaps. Arctic regions were home to many boreal plant species and animals including those more attuned to warmer climes like snakes and crocodilians.
Crinoids Probably the most spectacular group of echinoderms. There are two groups – the sea lilies that are attached to a substrate, and the feather stars that can move about. Both groups have a calyx, a cup like structure that contains the viscera and from which the arms extend. The calyx is made up of ambulacral and interambulacral plates. The calyx is attache to a stem made up of calcite discs – columnals. The flexible arms also consist of interconnected columnals.
Crossed lamellar structure (Bivalve) : Layers in molluscan shells where aragonite crystallites are organized in sheets. The crystallites in one sheet are oriented at relatively high angles to adjacent sheets. The overall effect can sometimes resemble a zig-zag pattern.
Cryptalgal laminates: A general term for laminated mats composed primarily of cyanobacteria, but like includes other microbes. The laminates may be flat and uniform, or tufted, pustulose, or polygonal, resulting from desiccation or, in arid environments, evaporite precipitation. In the rock record they are commonly found with stromatolites. The term microbialite is generally used in modern examples because there are several groups of microbes including bacteria, cycanobacteria, and red and green algae.
Cyanobacteria: Microscopic, single cell or colonial, prokaryotic organisms that today are aquatic and photosynthetic. They are likely the first known photosynthetic organisms on Earth, and were the primary builders of stromatolites and cryptalgal laminates (or microbialites) the oldest being about 3.4 Ga; as such they were responsible for producing free (molecular) oxygen in Earth’s ancient atmosphere. Precambrian fossil microbes, best preserved in cherts, are an assortment of filaments and coccoid colonies.
Declined: (Graptolites) Graptoloids where thecae have been added obliquely downward from the sicula, on stipes that have inverted V- or U-shapes.
Delthyrium (Brachiopod) A triangular shaped region just below the beak on the pedicle valve (posterior), through which the fleshy pedicle exits. It is analogous to the pedicle foramen.
Dendroid: One of the main Graptolite orders (the other is the graptoloids). They were the first graptolites, appearing in the Middle Cambrian, dying out late in the Carboniferous (outlasting the graptoloids). They were mostly sessile and grew bush or fan-like structures consisting of many stipes (branches), each stipe connected laterally by dissepiments (horizontal rods); a few species became planktic. Stipes were commonly bifurcate (unlike the graptoloids).
Dentition (Bivalve): An array of teeth and sockets along the dorsal hinge line.The teeth and sockets on one valve have corresponding sockets and teeth on the other valve. The two most common types are heterodont and taxodont dentition. Heterodont dentition (the most common) consists of two or three largish cardinal teeth and corresponding sockets that grow immediately below the beak. In many species lateral teeth also extend along the hinge line towards the anterior and posterior margins. Taxodont dentition consists of many small teeth and sockets arranged in a row, usually on either side of the beak.
Dentition (Brachiopod) Small knob-like structures on one valve, with corresponding sockets on the other valve.
Diatom: One of the largest groups of phytoplankton; single cell but commonly occur in chains or colonies, moving freely within the water column or attached to substrates (Benoiston et al., 2017, OA). They are composed of opaline silica, an amorphous, hydrated form of silica (SiO2). Diatoms occur in nearly all freshwater, fully marine and paralic waters (Armbrust, 2009), but in nearly all cases within the photic zone (they are photosynthetic autotrophs). They are important for the production of oxygen to both water and atmosphere.
Dissepiments: (Corals) Cup- or dome-shaped plates that grow between septa and separate the calice of a growing polyp from earlier growth stages. It has a similar function to the flatter tabulae. Both dissepiments and tabulae can occur together, with the former developed around the corallite periphery.
Dissepiments: (Graptolites) Horizontal rods of collagen or chitin that connect stipes, primarily in the dendroid graptolites.
Dorsal-ventral (Bivalve): The part of the shell containing the umbo, beak and hinge-ligament structures is dorsal; the opposite, outer shell margin is ventral.
Dorsal-ventral (Brachiopod) The alternative names for brachial and pedicle valves respectively.
Dorsal-ventral (Cephalopod) With the shell upright and aperture downward and facing the observer, the dorsal margin is at the top, the ventral at the bottom.
Dorsal-ventral (Trilobite) The carapace surface is dorsal, and the soft under-belly is ventral.
Dwelling traces: (Trace fossils) Mostly expressed as burrows and borings produced by invertebrates for somewhere to live. Commonly built by suspension feeders.
Echinodermata: Modern echinoderms include star fish (Asteroids), sea urchins and sand dollars (Echinoids, brittle stars or Ophiuroids, feather stars and sea lilies or Crinoids, and the oddball group, the Holothuroids or sea cucumber. Their ancestors first appeared in the Early Cambrian, and possibly the Ediacaran.
Echinoids A major group of echinoderms that includes extant sea urchins and sand dollars. They have no arms; instead they have ambulacral and interambulacral areas arranged in petal-like five fold symmetry. Most have spines. Most are grazers and hence the oral surface is usually down.
Ecosystem: The totality of organisms, micro- and macro-, plant and animal, their biological interactions (e.g., symbiotic relationships, food web), and their interactions and influence with the abiotic, physical and chemical environment. Common marine ecosystems include coral reefs, mangrove forests, and seagrass meadows. Common terrestrial systems include boreal and tropical forests, and Arctic tundra.
Ediacaran fauna: Late Precambrian metazoan fossils that appear petal-, feather-, or sea-pen-like, soft-bodied animals, commonly with a central stem and holdfast that was attached to the sediment substrate. They are found in a variety of shapes – fan-, bush-, and comb-shaped, or simple domes or discs. Some attained lengths of 2 m and more. They are found in rocks 575-542 million years old, i.e., immediately prior to the Cambrian. The name Ediacara is probably Australian Aboriginal. They are always found as impressions on bedding. Their preservation hints at a lack or paucity of scavengers.
Eelgrass: The common name for thin bladed seagrasses like Zostera and Posidonia.
Endemic (species): A species that is found naturally and is indigenous to only one region – cf. native species that are also indigenous but may occur in more than one region (more cosmopolitan).
Endolithic algae: Eukaryotic algae that live in micropores of skeletons and shelly material, and in pore throats of granular sediment. They are capable of dissolving calcium carbonate and promoting early diagenesis or weakening organic structures that leads to their fragmentation. They also play a role in micritisation of bioclasts. The term also applies to endolithic fungi and bacteria.
Epifauna: Marine and non-marine benthic organisms that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. They may be permanently attached (e.g., barnacles, forams, calcareous algae), or mobile (e.g., gastropods, many bivalves, forams, ostracods).
Epiflora: Marine and non-marine benthic plants that live on a substrate – the sediment-water interface, shells, aquatic plants, other organisms. Common examples include macroalgae, calcareous red and green algae.
Escape structures: Fugichnia (Trace fossils) Burrows constructed by invertebrates to keep pace with sudden increases in sedimentation or erosion. Burrows may show packing structures, or spreiten as the animal moves from side-to-side, oriented depending on whether the animal is moving up of down through sediment.
Escutcheon (Bivalve): A narrow, oval-shaped depression along the dorsal margin of both valves. It is posterior to the beak and approximately parallels the hinge line and the ligament.
Ethology (Trace fossil): Applied to trace fossils, it is the study of animal behaviour as recorded by the traces and burrows it creates – it asks the question “what was the critter doing when it made a particular trace?”. Ichnology considers the following behaviours: grazing, feeding, crawling (moving from A to B), resting, dwelling, and escaping. Depending on its behaviour, the same animal can create different structures.
Euhaline Aquatic systems with salinity of 30.0-40 parts per thousand derived primarily from marine salts.
Eukaryote: Organisms containing cells that have organelles separated by membranes, including a nucleus that contains strands of DNA. Other organelles include mitochondria. Additional organelles in plants are chloroplasts that aid photosynthesis. Cf. Prokaryotes. The eukaryotes includes most life forms – animals, plants, protists (single celled), fungi, and most algae.
Euxinic conditions: Ocean waters that are depleted in dissolved oxygen (anoxic) and are sulphidic. The sulphide is primarily dissolved H2S. Euxinia can occur in highly stratified water bodies, such as lakes and enclosed seas where there may be an the anoxic layer occurs beneath shallower waters with varying amounts of dissolved oxygen. However, euxinia may also have occurred in larger oceanic water masses in the geological past.
Exoskeleton: The outer shell or hardened carapace that many organisms produce for structural coherence, protection and other life activities. They may be composed of chitin as is the case for many arthropods (e.g., crabs, shrimp), or mineralized structures such as the calcareous shells of molluscs or tests of protists (e.g., foraminifera). Arthropod exoskeletons must be moulted as the animal grows; for contrast the shells of bivalves and gastropods increase in size as the animal grows. Cf. endoskeleton.
Extant: Means still living or surviving. It usually applies to biological forms, as in extant species.
Extinction event: A term applied to global mass die-off events, or rapid declines in biodiversity; also called biotic crisis or mass extinction. Most events, like the end-Permian or end-Cretaceous are caused by external factors such as bolide impacts or widespread volcanism that introduces more CO2 and SO2 into the atmosphere inducing anoxia, particularly in the oceans. Major events include the end-Ediacaran, Ordovician, Late Devonian, end-Permian and Permian-Triassic, and end-Cretaceous events where up to 90% of Earth’s life forms became extinct. The modern era is also witnessing widespread extinction of species (e.g. Cowie et al., 2022, OA).
Facial suture (Trilobite) The suture on the cephalon separating the free cheeks from the fixed cheeks. During molting the free cheeks would break along the suture and separate from the cephalon.
Feeding trails: (Trace fossils) (Fodichnia) These trails are constructed by deposit feeders on or beneath the sediment surface. They tend to reflect regular patterns of the search for food. Zoophycus is an excellent example expressed as a corkscrew-like, arcuate pattern of spreiten around a central cylindrical burrow.
Fixed – free cheeks (Trilobite) Solid plates on either side of the glabella and separated by the facial suture. The free cheeks could separate from the cephalon during molting, leaving the fixed cheeks and the rest of the cephalon intact.
Foliated structure (Bivalve), Layers in molluscan shells where micron-sized calcite crystal plates create a fabric that approximately parallels the shell surface.
Fossula: (Corals; Plural Fossulae) Narrow gaps between the four quadrants of septa in Rugose corals; they are not always discernible, particularly if preservation is poor or there has been significant calcite recrystallization or neomorphism.
Fusiform: In paleontology, a reference to a form that is tapered, spindle, or leaf-shaped. An example is the graptolite Phyllograptus.
Gastropoda: The most abundant modern class of mollusc, that most commonly construct a single, helically coiled shell, or less commonly internally coiled or uncoiled shells (e.g. limpets). There are at least 60,000 modern representatives, mostly marine, but with fresh water and terrestrial representatives. They are herbivores or carnivores. Some species use their radula to cut through the shells of other molluscs. They first appear in the Cambrian.
Genal spine (Trilobite) A large spine extending from the outer, posterior, left and right margins of the cephalon.
Glabella (Trilobite) A longitudinal lobe that separates the cephalon bilaterally. It is flanked by the fixed cheeks and compound eyes.
Graptolite: An extinct subclass of the phylum Hemichordata, thought to be the earliest hemichordates that eventually gave rise to full chordates (including vertebrates). They appear in the Middle Cambrian and died out in the latter part of the Carboniferous. There are two main orders – the Dendroids and Graptoloids. Graptolites were composed of collagen and chitinous compounds. They are preserved mostly in shale lithologies. Graptoloids provide high resolution biostratigraphic subdivision of the Ordovician and Silurian.
Graptoloid: One of the main graptolite orders appearing early in the Ordovician. They were entirely marine planktic and probably the earliest metazoan representatives in the planktonic realm. They evolved rapidly in the Ordovician and Silurian. Representatives have one to four stipes on which individual zooids secreted collagen-like thecae (tubes), interconnected by a thread-like structure called a nema. Their closest living relatives are worm-like pterobranchs (a class in the hemichordate phylum). Graptoloid taxonomy is based primarily on the number, organisation and morphology of stipes, thecae, and sicula.
Grazing traces: (Trace fossils) (Pasichnia) Tracks of benthic animals searching for food; the traces . commonly reflect systematic patterns of search. Helminthoida is an iconic example where the animal behaviour is expressed as repeated, highly sinuous meandering or spiral trails across the sediment surface.
Growth lines-rings (Bivalve): Concentric bands or ridges that represent changes in growth and calcium carbonate secretion, centred about the umbo. Depending on the species, rings may be delicate lines, low-relief ridges, or contain more complicated ornamentation.
Growth lines (Brachiopod) Faint lines or bands that are concentric about the beaks of both valves, that represent stages of shell growth. Similar to bivalve and gastropod growth lines.
Growth lines: (Corals) These are most prominent on solitary Rugose corals where they are expressed as raised concentric ridges on the epitheca. Their wrinkled appearance gave rise to the name of this order – from the Latin ruga. It is hypothesized that each line represents a day’s growth of the corallum.
Growth lines:(Gastropod) Fine threads or raised ridges oriented axially on a whorl surface. They are oriented at a high angle to ornamental spiral threads and ribs.
Gymnosperm: An important group of plants that produce seeds not enclosed in a flower ovary of fruit (unlike angiosperms). From the Greek gymnos meaning naked, and sperma meaning seed. The group includes conifers, cycads, and Gingko.
Hemichordate: (Graptolite) A phylum of marine, bilaterally symmetrical, worm-like animals that have a relationship to echinoderms and to chordates, but lack a true notochord that in chordates always appears at embryonic stages of life and in vertebrates eventually gives rise to a backbone. From a geological perspective, the Hemichordate class Pterobranchia is the most important because it probably relates to graptolites.
Heterozoan: Heterotrophs are organisms that consumes plants or animals for energy and nutrients. The group includes a huge variety of invertebrate and vertebrate organisms. Invertebrate heterozoans are important contributors to tropical and temperate carbonates, particularly skeletal grainstones-packstones and reef structures. C.f.. autotrophs.
Hinge (Brachiopod) A narrow zone of articulation between the two valves. It may be straight (strophic hinge) or curved (astrophic hinge).
Hinge line (Bivalve): A curved or straight line located below the beak, that contains elements of dentition and ligament. If a resilifer pit is present, then the hinge line is divided into anterior and posterior segments.
Horizontal: (Graptolites) Graptoloids where thecae have been added to laterally, or horizontally from the sicula. These forms usually have two stipes.
Holothurian: One of the five classes of Echinoderm, holothurians have no exoskeleton but do have five-fold symmetry that is internal. Also called Sea Cucumbers. They graze soft sediment and hard substrates. They have very low preservation potential.
Horn corals: The common name for solitary Rugose corals having a horn-shaped corallum. They were the most common taxa of the Paleozoic Rugose order.
Ichnocoenosis: (plural Ichnocoenoses) A trace fossil assemblage that represents a single benthic ecological community and reflecting/recording a set of physical, biological, and chemical conditions (e.g., (e.g., Curran and White 1991)). For example, a sandy open-sea high-energy beach may contain a specific set of filter-feeding bivalves in the low tide zone, and crab burrows higher up the beach. Lower energy beaches associated with lagoons will present a different set of benthic organisms that create their own set of traces and burrows.
Ichnofacies: Assemblages of trace fossils that recur in space and time, each assemblage representing a set of hydrologic, sedimentologic, and climatic conditions. For example deep water basin plain and base-of-slope environments commonly contain assemblages of grazing and feeding traces that reflect low hydrologic energy and long periods of reduced or non-deposition. In this case, a typical depositional setting is submarine fans. The most common ichnofacies have been given names that reflect the dominant kind of trace fossil; marine lithofacies include Trypanites, Glossifungites, Skolithos, Cruziana, Zoophycus, Nereites, and Teredolites (wood hardground). Each ichnofacies corresponds to a bathymetric range. Non-marine – continental examples are, Scoyenia, Mermia, and Coprinisphaera ichnofacies. The ichnofacies concept was championed by Adolf Seilacher, e.g., 1967.
Ichnology: The study of animal traces and trace fossils in terms of their environment, behaviour (ethology), and physical activity. A particular trace will reflect what the animal was doing at the time, how it was doing it, substrate composition (mud, sand), environmental conditions such as wave and current energy (water and wind), or salinity. Successions of trace fossils can also reflect changing environments (e.g., oxygenation, sediment flux, nutrients), substrate changes such as sea floor carbonate cementation, and evolving benthic communities.
Inarticulates (Brachiopod) Bivalved brachiopods composed of calcium phosphate, where the valves lack a well defined hinge, teeth or sockets. Lingula is the best known modern representative of this group.
Infauna: Marine and non-marine benthic organisms that live or feed within sediment, usually the upper few centimetres below the sediment-water interface. Common examples include molluscs and crustaceans. Infaunal activity produces bioturbation.
Interambulacral plates (Echinoderm) Polygonal-shaped plates, each a single calcite crystal, arranged petal-like as part of the five-fold symmetry, located between the ambulacral regions. These plates do not contain tube feet, but they do have tubercles to which the spines are attached.
Lebensspuren: German for “traces of life”, used primarily for the physical structures left from animal interaction with sediment. Basically it is synonymous with traces and trace fossils, although some reserve the term for modern structures.
Ligament (Bivalve): A tough, elastic, complex protein (called conchiolin) secreted by the animal that, in combination with the adductor muscles, keeps the two valves attached, and assists in articulation. When the valves are closed the ligament is under tension; when open the ligament is relaxed. The ligament itself has very low preservation potential.
Lophophore (Brachiopod) The soft organs that support respiration and feeding, that are held in place by the brachidium. Rarely preserved.
Lunule (Bivalve): A small heart-shaped depression immediately below and anterior to the beak. The size of the lunule varies considerably among species.
Macroalgae: A loose term that generally applies to large brown, red, and green algae, otherwise called seaweeds.
Mangrove: Flowering shrubs and small trees that are salt tolerant, living on sandy and muddy tidal flats and salt marshes. Most common between the subtropics but do extend beyond these latitudinal limits. They deal with salt uptake by excreting it from their leaves. They have complex root systems that help stability under conditions of shifting sediment and tides. Mangroves provide important habitats and breeding grounds, and also help protect coasts from storm wave surges and erosion. See also paralic, marsh.
Meniscus structure: (Trace fossils) Flat to curved structures formed as an animal back-fills sediment and fecal pellets in the burrow as it moves through the sediment. Menisci are usually repeated through the burrow.
Mollusca: One of the most numerous and diverse of all invertebrates, they are characterised by a mantle that protects the viscera (it may be external as in shell-less forms, or internal), breathing apparatus, and a radula (a saw-like structure that scrapes and cuts food, and is capable of cutting through the shells of other molluscs – only the bivalves lack one of these tools). There are six Classes: the limpet-like Monoplacophora, now found only in deep water; Amphineura that includes the common chitons; Gastropoda – marine, fresh water and terrestrial snails; marine Scaphopoda – that have the well-known tusk shape; Bivalvia – two-valved, marine and fresh water shells; and marine Cephalopoda, that includes the modern squid, octopus, cuttlefish, and Nautilus, and extinct ammonites.
Nacreous structure: (Gastropod-Bivalve) Layers in molluscan shells where patterns of micron-sized aragonite plates arranged parallel to the surface – where this layer occurs on the inside of a shell, it is easily recognized by the lustrous, rainbow-like colours e.g., abalone/Paua, mussels, oysters.
Native (species): A species that is indigenous to or has evolved in a particular region, but it may occur naturally in other regions – as opposed to ‘introduced’ or exotic’ which imply human intervention (cf. endemic). Native species can migrate or be distributed naturally by animals, air, water, or the longer-term plate tectonic rearrangements of oceans and landmasses.
Nautiloids (Cephalopod) Some of the oldest cephalopods, appearing in the Cambrian and surviving today as the genus Nautilus. They are characterised by chambers separated by simple flat to slightly curved septa, and a siphuncle that tracks through the centre of each septum. cf. Ammonoids.
Nema: (Graptolites) A thin collagen tube that connects the sicula to zooids in successive thecae through the centre of a graptolite stipe. The nema thread commonly extends from the latest nema along a stipe, or directly from the sicula to a float.
Nematocysts: Stinging cells, one of the defining structures of the phylum Cnidaria, are present in soft and hard coral polyps, sea anemones, jelly fish, and siphonophores like Bluebottles and Portuguese Man of War. Their primary uses are to stun prey and for defense, but are also used for locomotion.
Niche: More correctly ecological niche applied to an organism, defines the physical (including location), chemical, and biological conditions in which an organism lives and multiplies. How the organism responds to the system in which it lives, and what it does to accomplish this, are of equal importance. Some organisms thrive in a very specific niche with a narrow range of biotic and abiotic conditions (e.g., Darwin’s finches on specific islands of the Galapagos; or reef corals that exist within a narrow seawater temperature range, light intensity, and symbiotic algae); others exist in a broad range of conditions (e.g., wide-ranging wolves, or blue whales).
Nuclear whorls:(Gastropod) The initial whorl or whorls secreted by the animal. They have low preservation potential. Together they constitute the protoconch.
Omission surface: Depositional surfaces swept bare by erosion or starved of sediment. Omission surfaces are important components of condensed stratigraphic sections. They are commonly modified by encrusting and boring organisms.
Operculum: (Gastropod) The tough chitinous or solid calcareous, moveable cap that covers the aperture and is connected to and protects the animal.
Ophiuroids Also called brittle stars, this group is superficially similar to star fish in having 5 arms and a very small test containing discs and plates. Each arm contains a calcite tube. Arms frequently break off.
Oral surface (Echinoderm) Defined where there is an opening for the mouth, the term is used to orient specimens – there is no dorsal-ventral or anterior-posterior position because of the radial symmetry of most echinoderms. The oral surface is down for species that graze, and up for most filter feeders.
Orientation (Bivalve): Valves are identified as left and right. To determine this, orient the shell upright with the beak pointing away from you. The valve on the left is left valve. The shell margin closest to you is posterior; away from you is anterior.
Ornamentation Bivalve): Most commonly manifested as raised ribs that radiate from the beak towards the ventral margin. Depending on the species, they can be presented as narrow, low-relief ridges to more prominent corrugations like those commonly seen in Pectens (scallops). The intersection of growth lines and radial ribs in some species can produce some spectacular nodular or spiny growths (e.g., Spondylus)
Orthocones (Cephalopod) Straight shelled cephalopods, including early nautiloids and ammonoids, and belemnites.
Ossicles (Echinoderm) Echinoderm tests consist of interlocking plates and discs bound by connective tissue. Many tests also bear spines. All these components are made of calcite – collectively they constitute the ossicles. Individual ossicles are composed of a single calcite crystal.
Pallial line – pallial sinus (Bivalve): The pallial line marks the edge of the mantle that in life covers the animal viscera. It is commonly etched into the shell and occurs on both valves. The pallial sinus is an indentation in the pallial line that marks the position of the animal’s siphons; it is usually posterior.
Pedicle (Brachiopod) A fleshy, stalk like structure that extends from the posterior end of the pedicle or ventral valve, that in some species is used for attachment to a hard substrate. Rarely preserved.
Pedicle foramen (Brachiopod) A round opening at the posterior end of the pedicle-ventral valve through which the pedicle stalk extends. It is analogous to the delthyrium.
Pendant: (Graptolites): Graptoloid constructions where thecae are added downward (almost vertically) from the sicula on the inside margin of the stipe. The sicula is located on the outside margin of the stipes. An example is Didymograptus murchisoni.
Pentaradial symmetry (Echinoderm) Ambulacral and interambulacral plates are arranged in 5-fold symmetry. Star fish (asteroids) are one of the more obvious examples of this arrangement, with 5, or multiples of 5 arms. All echinoderms except the holothurians (sea slugs) display the symmetry externally in their tests and arms; in the sea slugs it is internal.
Periostracum: (Gastropod) An organic layer secreted by many mollusc species on the outer layer of a shell, that helps protect the shell from abrasion – it has low preservation potential.
Periproct (Echinoderm) The opening for the anus on the aboral surface; usually opposite the peristome. It is on the upper surface for grazing echinoderms, and on the lower surface for filter feeders.
Peristome (Echinoderm) The opening for the mouth on the oral surface. In most forms it is on the side opposite the anus.
Photic zone: The uppermost layer of the oceans and lakes where light penetrates. About 45% of incident light penetrates to one metre depth; at 30 m depth the light intensity is about 16%. The base of the zone is at about 1% of incident sunlight. On average it is about 200 m deep. It is the layer where more than 95% of photosynthesis by marine organisms takes place.
Phototrophism: The growth of an organism in response to light – light intensity can be a limiting factor. Common in plants. In aquatic environments this includes algae, seagrasses, and heterozoan organisms that rely on photosynthesis – macro- brown algae, calcareous red and green algae, and various protozoa.
Phragmocone (Cephalopod) Cone-shaped depression in a belemnite rostrum (guard) that contains septa and body chamber. The septa were probably aragonitic.
Phytoplankton: Single-cell microalgae that contain chlorophyll and therefore require sunlight to generate photosynthesis reactions. They exist primarily within the photic zone of water bodies. The two main groups are diatoms and dinoflagellates, that occur in marine and non-marine waters. Both are critical components of the ocean/lacustrine food webs. Diatoms produce microscopic shell-like structures called frustules composed of amorphous silica. Dinoflagellates are mobile, using a whip-like structure called a flagellum. Dinoflagellates do not produce a mineralized shell. Phytoplankton produce nearly 50% of the oxygen in Earth’s atmosphere.
Pleistocene: From the Greek pleistos meaning much or more, and the word element -cene meaning recent – the most recent geological period. The name was introduced by Charles Lyell in 1839 to describe the shelly fauna in Sicily that includes modern species; he initially referred to the interval as Newer Pliocene. It extends from 2.58 – 0.0017 Ma, and includes several major glaciations. It is preceded by the Pliocene. It is also the period when Homo sapiens evolved.
Pleural lobes (Trilobite) These are located either side of the axial lobe on the thorax. Each is segments, and each had two pairs of legs attached. Left and right pleura are determined with the dorsal surface facing and pygidium down (or towards you).
Plications (Brachiopod) Folds and corresponding sulci that are structural elements involving the whole shell – cf. superficial ribs on the outer surfaces of the shells.
Pliocene: Initially defined by Lyell in 1833, with two divisions – the Older and Newer Pliocene – the Newer is now the Pleistocene. Hence its etymology is similar to that of Pleistocene – from the Greek pleistos meaning much or more, and the word element -cene meaning recent. The Latin form of -cene is recens from which we also get recent. The Pliocene spans 5.333 – 2.58 Ma. It is the ultimate Series in the Neogene System.
Pneumatophore: On some mangrove genera (e.g., Avicennia) spongy growths extend from the tree roots vertically a few 10s of centimetres above the sediment surface; their primary function is to enhance gas exchange when exposed to air and water exchange when submerged. The pneumatophores also provide a substrate for epiphytic algae and diverse invertebrate species.
Polyp: (Corals) The living animal resides within the calice of a corallum or corallite, and responsible for secretion of the calcium carbonate in the construction of solitary and colonial corals. Each polyp has a central mouth surrounded by tentacles that contain nematocysts (stinging cells). Coral polyps live symbiotically with photosynthetic zooxanthellae (algae).
Posterior (Cephalopod) With the shell upright and aperture downward and facing the observer, the posterior margin is on the opposite side of the shell. The opposite margin is anterior.
Preservation potential: A nebulous expression that is generally used to express the relative potential for preservation of sedimentary structures and fossils. Thus, the soft part of an animal has very low potential because it degrades rapidly or is consumed by other critters; the shelly exoskeletons, shells, test, and internal skeletons have significantly higher potential. Likewise, plant leaves, flowers, fruits and seeds have relatively low potential – although pollen, because of its composition, are commonly well preserved.
Prismatic structure: (Gastropod-Bivalve) Layers in molluscan shells where high- or low-magnesium calcite or aragonite prisms are oriented normal to the shell surface. This is commonly the outer layer of a shell.
Prokaryote: Organisms containing cells that do not contain organelles, such as a nucleus, but have circular links of DNA (rather than strands as in the Eukaryotes) that is contained within a nucleoid that lacks a membrane. This primitive group includes bacteria, cyanobacteria, and archaea.
Propagule: Any part of a plant that can be used to propagate (seeds, nuts, cut stems, leaves, root sections). Mangroves are angiosperms and produce flowers and seeds within a fruit – commonly referred to as propagules. Seed distribution and germination have evolved, like other mangrove traits, to survive saline conditions where dispersal is primarily by wind and tidal currents. Propagules can survive many months in transit to locations where they can put down roots.
Protoconch (Cephalopod) The first chamber of a fledgling cephalopod. It is located at the apex of coiled forms, and at the pointy posterior end of phragmocones in belemnites. cf. gastropods.
Pterobranch: (Graptolite) One of the living classes of the Hemichordate phylum that includes worm-like colonies constructed by filter-feeding zooids – considered to be the modern analogue to graptolite colonies.
Pygidium (Trilobite) The tail (posterior) that is divided bilaterally and segmented like the axial lobe. The suture dividing the thorax and pygidium allowed flexibility.
Radula: (Gastropod) A ribbon-like structure with attached teeth, is part of the mouth in most molluscs except the bivalves. It is used for feeding. Some species of gastropod use it, in conjunction with ab acidic secretion, to bore though the shells of other molluscs – this action commonly leaves a small circular hole in the shell of the intended dinner partner. It is an important criterion for species identification. It has low preservation potential.
Reclined: (Graptolites): Graptoloids where thecae are added obliquely upwards from the sicula on the outside margin of the stipe. The sicula is located on the inside margin of the U- or V-shape stipes. An example is of Isograptus.
Resilifer pit (Bivalve): A triangular-shaped depression just below the beak and present in both valves. In life it is occupied by the resilifer, a fibrous tendon-like structure that participates in valve articulation.
Resting traces: (Trace fossils) (Cubichnia) The impressions of animals taking a break (or perhaps dead). They tend to reflect animal shapes such as starfish, or arthropods like Trilobites. They occur on bedding planes.
Rhabdosome: (Graptolites) Refers to the entire graptolite colony. It may consist of 1, 2, 3, or 4 stipes in graptoloids, or multiple stipes in dendroids.
Rhizome: Fibrous or woody plant structures that grow within a soil, from which stems, leaves, and roots extend. In plants such as seagrasses they can develop dense mats just below the sediment-water interface. They are one of the main mechanisms for expansion of plant growth.
Rostrum – Guard (Cephalopod) The solid calcite, bullet-shaped structure that is the most commonly preserved part of a belemnite. Rostrum cross-sections range from circular to oval. It was an internal structure that probably functioned to assist buoyancy.
Rugose corals: An important hard coral Order (Subclass Hexacorallia), that appeared during the Mid-Ordovician, becoming extinct at the end of the Permian. They are most commonly found as solitary, horn-shaped coralla, but colonial forms also occur. They have bilaterally symmetrical radial septa, unlike the Scleractinians. Calices are separated by tabulae and dissepiments.
Scandent: (Graptolites): Graptoloids wherein successive thecae were added vertically upward from the sicula, on the outside of the stipes. An examples is Climacograptus.
Scleractinian corals: The most important, extant, hard coral Order (Subclass Hexacorallia), primarily as tropical reef builders but also as solitary corals in colder environments. Polyp growth depends on symbiotic, photosynthetic zooxanthellate algae. Polys and the corallites they secrete have 6-fold radial symmetry, mostly expressed in 6 or 12 primary, radial septa with higher order septa inserted in between. Successive calices are separated by dissepiments. They first appear in the fossil record during the Early Triassic.
Seagrass: Seagrasses are monocotyledons, the group of angiosperms that evolved a tolerance to saline conditions from their Late Cretaceous terrestrial ancestors. They inhabit low to moderate energy, intertidal and shallow subtidal environments, and develop extensive root systems, produce flowers, and are pollinated while submerged. They are one of the most productive marine ecosystems, act as nurseries and habitats to many infaunal-epifaunal invertebrate and vertebrate species and dampen waves and tidal currents. Seagrass communities frequently coexist with mangrove forests, salt marshes, and coral reefs.
Septa (Cephalopod) (singular septum) Internal walls of (probable) aragonite separating the chambers. Their geometries range from simple flat surfaces, to surfaces containing highly intricate, three dimensional lobes and saddles, particularly in Mesozoic ammonites. The complexity of the septa is reflected in the shape of the corresponding sutures.
Septa: (Corals) Vertical plates that radiate from the wall to the centre of a coral tube. They are secreted by the polyp as it grows from one calice to the next. In detail, septa may be laminated, perforated or spinose. Septa are prominent in Rugose and Scleractinian corals, but either absent or weakly developed in the Tabulates. In Scleractinian corals the septa are arranged in 6-fold symmetry, with primary septa the thickest and largest, and higher-order septa in sets of 12, 24 and so on in between. In Rugose corals the septa are arranged in quadrants separated by narrow gaps, or fossula; fossula are indistinct or poorly preserved in many species. The arrangement of septa in Rugose corals creates a bilateral symmetry.
Septal neck (Cephalopod) A small aragonite tube extending from the convex side of the septum, through which the soft siphuncle tracks successive chambers.
Sicula: (Graptolites) The first cone-like tube secreted by a zooid on a graptolite stipe. All successive thecae along the stipe are constructed from this initial growth.
Siphonal canal:(Gastropod) When viewed upright, the opening in the aperture margin that provides access to and protects then siphons used for water intake. In some species, this structure is as long or longer than the remainder of the shell.
Siphuncle (Cephalopod) A soft tube that connects abandoned chambers and provides the animal with a means for regulating chamber buoyancy. In the ammonoids it is located along the out margin of coils. In nautiloids it passes through the centre of successive septa.
Spines (Echinoderm) Slender rods extending from tubercles on the interambulacral plates. Each rod is a single crystal of calcite. A system of pores extend along the length of the spines; in cross section they appear as circular to oval openings. Spines are very common in Echinoid species, particularly the sea urchins. Some spines are poisonous.
Spines (Trilobite) Spines of varying length and complexity are common outgrowths of pleural segments on the thorax and pygidium. In a few species, spines also grew on the upper surface of the thorax.
Spiral threads:(Gastropod) Ornamental fine or coarse lines and ridges that extend around whorls; they are generally parallel to sutures. The intersection of threads and growth lines is frequently the locus of ornamental spines and nodes.
Spire:(Gastropod) The pointy end of the shell containing all whorls extending from the body whorl. The spire tip contains the nuclear whorls, or protoconch.
Sponge spicules: The internal scaffolding-like skeleton of sponges consisting of solid rodlike structures (spicules or sclerites), that are bound together to form a semi-rigid framework, around which the soft spongin grows (spongin is an insoluble protein). The spicules are most commonly composed of amorphous silica (opal-like), but also high-magnesium calcite. Spicules act to focus light in much the same way as optic fibres. The spicules disaggregate when the sponge dies.
Spreite: (Trace fossils) Plural spreiten. Curved or U-shaped surfaces that reflect the back-and-forth action of the burrowing, deposit-feeding animal as it moves progressively through sediment. The surfaces are stacked, and in 2-dimensional exposures appear as a series of grooves and ridges. Spreite stacks can have any orientation. For example, Diplocraterion stacks tend to be vertical, and Zoophycus stacks corkscrew or spiral. Spreite have greater three-dimensionality than meniscus structures. They also represent different animal behaviour.
Stipe: (Graptolites) The branch-like structure that defines graptolites. It commonly appears saw-toothed. Stipes grow as successive zooids secrete a theca (tube). Dendroids develop multiple stipes; graptoloids construct 1, 2, 3, or 4 stipes.
Stony coral: The name normally reserved for Scleractinian corals, although it is sometimes used for other hard corals like the Rugosa and Tabulates.
Sulcus (sulci) (Brachiopod) The negative, or depressed structure on the pedicle valve that sits opposite a corresponding fold on the brachial valve.
Sutures (Cephalopod) These are seen on the outer surface of shells and delineate the intersection of septa with the shell wall. They are relatively simple straight or curved lines in the nautiloids and early ammonoids, but became increasingly complicated with the addition of lobes and saddles on the septa, particularly in Cretaceous ammonites.
Sutures:(Gastropod) The contact between whorls. There may be some overlap from one whorl to the next. C.f.. involute coiling.
Tabulae: (Corals; singular tabula) Flat or slightly curved horizontal plates secreted within the corallite or corallum by the growing polyp as it moves to a new calice – the tabula separates the polyp from the rest of the coral tube. Present in Rugose and a defining characteristic of Tabulate corals; they are not found in the Scleractinians. Cf. Dissepiments.
Tabulate corals: An important hard coral Order (Subclass Hexacorallia), that appeared during the Early Ordovician, becoming extinct at the end of the Permian. The Order is entirely colonial where corallites form chain-link and honeycomb structures. Most corallites lack septa; a few species have poorly developed septa. Successive calices are separated by tabulae.
Theca: (Corals) The solid calcareous outer wall of a corallite or solitary corallum, thickened in some species and thin or compressed in others. The theca may be covered by a thin outer wall, or epitheca.
Theca: (Graptolites) (plural thecae) The collagen or chitin-like tube secreted by zooids along a Graptolite stipe. Each theca and its zooid along a stipe is connected by a nema.
Thorax (Trilobite) Located between the cephalon and pygidium, it contains an axial lobe and the two pleural lobes. Each lobe contained movable segments that provided flexibility, including the protective action of curling into a ball. The trilobite legs were attached to the pleura (but rarely preserved).
Trace fossil: Traces and burrows that record the activity and behaviour (ethology) of animals on and within a sediment substrate. Borings into hard substrates such as rock, wood or shelly material are also included in the definition. Traces are regarded as both fossils and sedimentary structures. Body fossils are rarely found within the structures they create. A single critter can create different traces, and different animals can create similar traces. Synonymous with the term ichnofossil.
Transpiration: Natural water loss as vapour from the exposed parts of a plant (mostly from leaves, but also flowers, stems, and branches). The process is used to remove excess water and as a cooling mechanism. The rate of transpiration is inversely proportional to the relative humidity of air, proportional to water uptake by the roots, and increases/decreases with temperature. Some plants, like salinity-tolerant mangroves, have evolved to reduce transpiration.
Trilobita: One of the earliest and an important class of complex arthropods that appeared in the Early Cambrian and died out during the Permian extinction event. As arthropods they possessed bilateral, trilobed symmetry of their head (cephalon), thorax, and tail (pygidium) segments. They had a carapace of calcium carbonate that over the lifetime of an individual could have been shed (molted) several times. The carapace had high preservation potential.
Tube feet (Echinoderm) Small, fleshy, tube-like structures that the animal extends through pores in the ambulacral plates. They are part of the animal’s vascular system. Their primary functions are to pass food to the mouth, respiration, and locomotion across a substrate.
Tubercles (Echinoderm) Raised knob-like, calcareous structures on the interambulacral plates to which spines are attached.
Umbilicus: (Gastropod) A cone-shaped opening through the centre of the columella. It is only visible in the body whorl. However, it may also be completely or partially covered by a shelly callus that extends from the inner aperture margin.
Umbo (Bivalve): The dorsal part of a valve where curvature increases significantly; this region also contains the beak.
Uniserial: (Graptolites) Stipes on which there is a single row of thecae.
Virgella: (Graptolites) A spine-like structure that extends from the base of, and is part of the sicula.
Wetland: The region between terrestrial and fully aquatic systems, where the watertable is very shallow or at the surface for a significant period such that hydrophytic plants thrive. Wetlands may be tidal or non-tidal. Wetland waters may be fresh, brackish (riverine, lacustrine), or partly saline from marine derived salts (e.g. estuarine, coastal plain, delta plain).
Whorl: (Gastropod) One complete shell revolution about an imaginary axis that extends from the top of the spire to the base of the aperture. Whorls may be inflated rounded) or flat.
Zooid: (Graptolites) Soft-bodied, filter-feeding, colonial, marine animals that are the primary builders of graptolites, secreting collagen or chitin-like tubes (thecae). Each theca is connected by a thread-like tube, the nema, that extends the entire length of the graptolite stipe. Modern Pterobranch colonies are also constructed by zooids.
Zooxanthellae: (Corals): Photosynthesizing algae that live symbiotically with coral polyps. Zooxanthellae use CO2 produced by the polyps to produce sugars and other nutrients via photosynthesis, that in turn are used by the polyp to grow and construct the coral skeleton.
