Not all clades of organisms produce preservable hard parts. Soft bodied organisms CAN produce fossils under special (konservat-lagersätten) situations, but these are vanishingly rare.
Review of the Major Animal Fossil Makers
Main hard substances found in organisms:
- Calcite and/or aragonite - CaCO3 (calcareous hard parts dominate the fossil record)
- Silica - SiO2 (mostly in the form of cryptocrystalline quartz or Opal - hydrated silica - SiO2- nH2O (mollusk radulae)
- Calcium phosphates such as apatite - Ca5(PO4)3 (vertebrate bone and enamel)
- Complex organics, such as:
- Chitin (a polysaccharide - arthropods)
- Spongin (a protein - demosponges)
List of some of the major clades producing common body fossils: (Classifications used here largerly follow these on the UMCP website)
Animalia ("Metazoa" under one definition of that term): Will list these groups in more detail!
Porifera (sponges): (Cambrian - rec.)
"Parazoan"-grade organisms, benthic sessile epifaunal suspension feeders
Hard parts include spicules made of aragonite, silica, and spongin
Spongin doesn't fossilize readily. Various important fossil clades of sponges are all calcareous (except for hexactinellids).
Volumetrically significant in reef environments.
Probably ancient sponges:
Cnidaria (corals, hydroids, jellyfish, anemones) (Ediacaran - rec.) The majority are soft bodied with practically no fossil record. (But not the occasional trace fossil left by a resting anemone or jellyfish.)
Benthic epifaunal sessile suspension feeders
Cnidarians with hard calcareous skeletons commonly called "corals."
Dominant groups in the fossil record:
- Rugosa, "horn corals" or "tetracorals", (Ordovician - Permian), predominantly solitary
- Tabulata, "tabulate corals", (Ordovician - Permian) exclusively colonial, major reef formers
- Scleractinia, "hexacorals", (Triassic - Recent), mostly colonial, major reef formers of Jurassic Period and Cenozoic Era
Bryozoa (AKA "Ectoprocts": (Ordovician - rec.)
Exclusively colonial, benthic, epifaunal, sessile suspension feeders
Some forms lack hard skeleton; those that have them are calcareous
Minor components of many reef systems
Brachiopoda: (Cambrian - rec.)
Solitary benthic suspension feeders, strictly marine
Two hard valves surrounding most body tissue
Hard parts are calcium phosphates in lingulates, calcite in craniids and articulates
Phenomenal fossil record; modern diversity much lower than Paleozoic diversity
E. Mollusca: (Cambrian - rec.)
Extremely significant, extremely diverse clade of (almost all motile) animals
Most forms have shells (single, double, or eight) made of aragonite (i.e. calcareous)
Major clades are:
- Polyplacophora, or "chitons": eight-valved epifaunal benthic grazers. Probably a decent approximation of the ancestral mollusk, especially in terms of soft tissue.
- Monoplacophora: (Lower Paleozoic - Recent but with no post-paleozoic fossil record) Single-shelled but with soft tissue resembling that of polyplacophorans.
- Gastropoda, or "snails": (Ordovician - Recent) univalved primarily epifaunal benthic grazers and predators
- Cephalopoda, (Ordovician - Recent) single-shelled swimming predators, with three major fossil-forming groups:
- Nautiloids: Ordovician - Recent. Paraphyletic grouping of primitive shelled cephalopods
- Ammonoidea: (Devonian - Cretaceous). Remarkably abundant, high species turnover rate, probable suspension feeders rather than predators
- Belemnoidea: (Triassic - Paleocene) Early and common representatives of Coleoidea - the cephalopod group containing squids, octopuses, and cuttlefish.
- Bivalvia, "bivalves" (also called Pelecypoda): (Cambrian - Recent) diverse bivalved clade of mostly suspension feeders, including infaunal and epifaunal, motile and sessile forms. During the Cretaceous, the major reef-formers were bivalve molluscs.
- Rostroconchia: (Cambrian - Permian) Superficially resembling bivalves but with immobile pseudobivalve shell. Openings at either end for flow-through current from which food could be filtered.
F. Arthropoda: (Cambrian - rec.)
Extremely common, diverse clade of (predominantly) motile animals
Majority have exoskeleton of chitin; some forms have calcified skeletons
Grow by ecydysis, so a single individual produces many potential fossils
Major clades include:
- Trilobita: (Cambrian-Permian) Paleozoic marine forms with calcified skeleton, extraordinarily diverse and abundant
- Chelicerata: (Cambrian-Recent) includes marine Eurypterida ("water scorpions") of the Paleozoic, Xiphosura ("horseshoe crabs") of Phanerozoic range, and terrestrial Arachnida
- Crustacea: (Cambrian-Recent) stupendously diverse marine clade (may be paraphyletic with regards to Hexapoda). Main fossil formers are heavily calcified forms:
- Hexapoda: (Mississippian-Recent) insects and their allies, terrestrial or freshwater forms, very common as later Paleozoic fossils onward
G. Echinodermata: (Cambrian - rec.)
Calcitic skeleton, unique "water vascular system" to control feeding tube feet
Strictly marine, macroscopic, almost all epifaunal
Five-fold body symmetry in all but most primitive forms
Ancestrally suspension feeders; later forms include some grazers and predators
Several major groups with substantial fossil record:
- Asteroidea, or "starfish": (Ordovician - Recent) motile benthic predators
- Ophiuroidea, or "brittle stars": motile benthic suspension feeders
- Echinoidea, or "sea urchins" and "sand dollars": motile grazers, ancestrally epifaunal but derived forms include infaunal taxa
- Homalozoa (Cambrian - Ordivician), Helicoplacoidea (Cambrian), Edrioasteroidea (Cambrian - Pennsylvanian): non-pentameral (indeed, asymmetric) early Paleozoic forms
- Blastoidea (Ordivician - Permian), Rhombifera (Ordivician - Devonian), Diploporita (Ordivician - Devonian): Paleozoic sessile stalked forms with diverse complex internal breathing apparatuses
- Crinoidea, or "sea lilies" and "feather stars": (Ordovician - Recent), most stalked sessile suspension feeders but a few motile. Very significant sediment generators of the late Paleozoic.
Graptolithina, or "graptolites": (Cambrian - Pennsylvanian)
Colonial benthic or planktonic suspension feeders
Hemichordates related to acorn worms and pterobranchs (may actually be a clade of pterobranchs)
Strictly mid-Paleozoic, important index fossils
Hard parts are collagen
Conodonta: (Cambrian - Triassic)
Known as exceptionally valuable Paleozoic and early Mesozoic index fossils, but until late 20th century, no one knew what they were. (I.e. neither what kind of animal nor what part.)
Hard parts (apparatus) are calcium phosphate, formed mouth parts
Craniata - deuterostomes with heads: (Cambrian - Recent)
Basalmost forms have few hard parts
Primitive forms have exoskeleton of bone, endoskeleton of cartilage
Derived forms ossify the endoskeleton and deossify the exoskeleton
Many separate bones in the body
The fossil record contains diverse jawless craniates in addition to hagfish and lampreys (whose record is very poor). Traditionally lumped together as "ostracoderms" (paraphyletic):
Gnathostomata - craniates with heads: (Silurian - Recent) include:
- Placodermi (Silurian - Devonian) Jawed vertebrates with bony plates in place of teeth. A flash in the pan, but very abundant and diverse in their day.
- Chondrichthyes (Silurian - Recent) "Cartlagenous fish" - sharks, chimaeras, and their kin.
- Osteichthyes (Devonian - Recent) "bony fish" but also includes land vertebrates.
Osteichthyan survey: The diversity of "fish" breaks down into two groups:
- Actinopterygii: Ray-finned fish. Most living fish. These have a substantial fossil record.
- Sarcopterygii: Lobe-finned fish. (Devonian - Recent) In today's fauna - land vertebrates and a few aquatic weirdos. In the Paleozoic, however, aquatic sarcopterygians were common and diverse.
This course regards the terrestrial sarcopterygians and transitional forms with particular interest.
Tetrapoda: (Mississippian - Recent) Sarcopterygians with fingers and toes.
Within Tetrapoda, there is a rich record for:
- Basal tetrapods: Creatures on the stems leading to:
- Amphibia: frogs, salamanders, and caecilians; and their close extinct relatives. (Triassic - Recent). Note: In phylogenetic terminology, monophyletic Amphibia is restricted to a rather small group within the non-amniote tetrapods, leaving out many creatures that were traditionally referred to as "amphibians."
- Amniota: vertebrates that lay an air-breathing egg or its derivative. (Pennsylvanian - Recent)
Amniota - the land vertebrates
Proliferation of land vertebrates in two major groups:
- Synapsida: Mammals and their fossil relatives.
- Dominated land faunas before the age of dinosaurs and after, but not during
- Dominated land faunas during the age of dinosaurs and an enduring presence since