Palaeobiology of Invertebrates

Three evolutionary faunas

Cambrian:

• Trilobita

• Mono- & Polyplacophora

• Inarticulate Brachiopoda

• Archaeocyatha

Palaeozoic:

• Articulate Brachiopoda

• Crinoidea

• Cephalopoda

• Anthozoa

• Ostracoda

Modern:

• Gastropoda

• Bivalvia

• Malacostraca

• Echinoidea

• Demospongiae

Extinction events

• Ordovician/Silurian - 444 Ma

• End-Devonian - 372-359 Ma

• Permian/Triassic - 252 Ma

• Triassic/Jurassic - 201 Ma

• Cretaceous/Palaeogene - 66Ma

Great Ordovician Biodiversification Event (GOBE)

• Strong increase in diversity

• Establishment of many major taxa

• Might be continuous with the Cambrian Explosion

Mesozoic Marine Revolution (MMR)

• 252-66 Ma

• Evolution of shell-crushing (durophagous) behaviour

• Vagile organisms become important and outcompete sedentary ones

• Decline of Palaeozoic fauna and rise of modern fauna

Sponges

• Sister group to all other metazoans

• Solitary to reef-building, use to be mostly reef-building

• Major subtaxa: Archaeocyatha, Stromatoporoidea, Demospongiae, Hexactinellida, and Calcarea

Sponge morphology

• No tissue or organs

• Water flows through ostia into spongocoel and expelled through osculum

• Choanocytes for water current and filtering

• Mesohyl as endoskeleton

• Spicules

• Spongin

Sponges body plans

• Asconid → thin and small, not too efficient

• Syconid → extra cavities, bigger

• Leuconid → thick and bigger, efficient   

Fossil record of sponges

• Before Cambrian

• Pretty consistant over time

Archaeocyatha

• Only Early Cambrian

• First reef-builders

• Very high diversity

• No spicules, contiguous skeleton, good fossil record

• Almost extinct in end-Botomian extinction

Stromatoporoidea

• Early Ordovician to end-Devonian

• Most important group of reef-builders in Palaeozoic

• Big, compact, heavily calcified

• Some dendroid (tree-like)

• Sudden extinction in end-Devonian

Sphinctozoa

• Polyphyletic assemblage of similar looking species

• Chambered, external rigid skeleton

• Cambrian to present

Demospongiae

• Largest extant group

• Freshwater and deep-sea

• Spicules and spongin, fossils identified by spicules

• Extinct groups heavily calcified and reef-building

• Cambrian to present

Hexactinellida (glass sponges)

• Marine, deep water

• Large

• Spicules

• Six rayed

• No spongin

• Diverse and common in Jurassic and Cretaceous, reef-building

• Cambrian to present

Calcarea (calcareous sponges)

• Marine, shallow water (CCD)

• Small, syconid type

• Only group with calcium carbonate spicules

• Peak in Late Triassic

• Cambrian to present

Ctenophora (comb jellies)

• Marine, mostly plankton, carnivorous

• Gelatinous body

• Similar to jellyfish, no stinging cells

• Sister group to Cnidaria + Bilateria or to all metazoans (including sponges)

• No good fossil record

Cnidaria (nettle bearers)

• Sister group to Bilateria

• Including jellyfish, hydroids, corals, etc.

• Marine, some freshwater species

• Polymorphism, alternation of generations

• Sexual and asexual reproduction (strobilation)

• Planktonic, nektonic, benthic, interstitial or parasitic

• Usually predators, sometimes with symbionts → can photosynthesise

Cnidaria morphology

• Diploblastic → epidermis and gastrodermis

• Gelatinous mesoglea (hydrostatic skeleton)

• Mouth surrounded by tentacles

• Body cavity with only one opening

• Cnidocytes (nettle cells)

Phylogeny of Cnidaria

• 2 clades: Medusozoa and Anthozoa

• Split of the main groups before Cambrian

• Medusozoa:

  • Free swimming medusoid stage (jellifish-like)

  • Medusoid stage dominant (Scyphozoa and Cubozoa)

  • Sessile polypoid stage dominant in Hydrozoa and Staurozoa

  • Poor fossil record

Conulariida

• Ediacaran to Triassic

• Ice-cone exoskeleton of calcium phosphate

• Classified as stem-group Medusozoa

• Only group of Medusozoa with exoskeleton and benthic

Anthozoa

• Cnidarians without medusoid stage (polypoid stage dominant)

• Exoskeleton of calcium carbonate

• Symmetry

• Skeleton shared in colonies

• Octocorallia → sessile forms without exoskeleton

• Hexacorallia: Tabulata, Rugosa, Scleractinia, Actiniaria, and Zoantharia

Octocorallia

• Polyps always colonial

• Only internal skeletal elements

• Fossil record poor

• Silurian to present

Hexacorallia (Actiniaria and Zoanthiniaria)/Sea anemones

• Solitary, large polyps

• No skeleton

• Silurian to present

Tabulata

• Ordovician to Permian/Triassic

• Important reef-builders in Palaeozoic

• Exoskeleton of calcite

• Colonial, polyps small

• Corallites separated by horizontal tubulae

• Septae reduced

• In or in front of reef core

Rugosa

• Ordovician to Permnian/Triassic

• Exoskeleton of calcite

• Corallites large

• Solitary or colonial

• Corallites with tubulae and septae, four-fold symmetry (Tetracorallia)

• Colonial forms in reef core, solitary forms further away

Tabulata and Rugosa

• Suggested symbiosis with microscopic algae in all Tabulata and some colonial Rugosa

• Absent in some colonial and all solitary Rugosa 

Scleractinia (stony corals)

• Appear in the Middle Triassic, probably from anemone-like animals

• Exoskeleton of aragonite

• Corallites small or large

• Solitary or colonial

• Corallites cup-like, with septae

• Many forms have symbiosis with photosynthetising dinoflagellates → zooxanthellae

Coral bleaching

• Zooxanthellae expelled due to environmental stress → coral dies

• Factors: increasing water temperature, UV radiation, pollution, infections, ocean acidification

Reefs

• Biogenic underwater structures

• Composed of skeletal material

• Today most diverse and productive marine ecosystems

• Host 2 million species

• Mostly tropical regions with low influx of nutrients

History of reefs

• Exist throughout Phanerozoic in various forms

• Reef-building organisms change over time

• Maxima of reef development in Middle to Late Devonian, Late Jurassic, and Miocene

Reef types through time

• Early Palaeozoic → Archaeocyatha and Stromatolites

• Middle Palaeozoic → Stromatoporoidea and rugose and tubulate corals

• Late Palaeozoic → Bryozoa, but mostly alage and microbes

• Late Permian and Triassic → Scleractinia and calcified sponges

• Jurassic → Scleractinia and Hexactinellida

• Upper Jurassic:

  • Larger than today, also in areas with higher nutrient input and in deeper water

  • Scleractinian corals, hexactinellids, calcified demosponges, bivalves, gastropods, brachiopods, echinoids, crinoids, etc.

  • Mostly modern evolutionary fauna

• Cretaceous → Scleractinia and Rudists (Bivalvia)

• Cenozoic:

  • Scleractinia and coralline algae

  • Tropical reefs dominated by corals with photosymbiosis and coralline algae → more susceptible to global warming

  • Various cnidarians, bivalves, gastropods, crustaceans, echinoids, fish, etc.

  • Also cold water reefs with non-zooxanthellate corals, bivalves, and/or polychaetes

7 reef crises

• Decline in the production of reef carbonate

• Cambrian → collapse of archaeocyath reefs

• End-Devonian → collapse of Middle Palaeozoic reefs

• Permian/Triassic → disappearance of all reefs

• Triassic/Jurassic

• Early Jurassic → Toarcian Ocean Anoxic Event

• Cretaceous/Palaeocene Thermal Maximum Causes:

  • Rapid climate change

  • Ocean acidification

  • Oceanic anoxia

  • Poisoning of the water column

Current reef crisis

• Global warming and ocean acidification

• Coral bleaching events corresponding with extreme heat waves

• 2.0 degrees warming will result in more than 99% reduction of tropical reefs

• Pollution, fishing, perturbations of the existing ecosystems

• Reef important for their biodiversity, for food, and protection from weather events

Mollusca

• Mostly marine, also freshwater (Gastropoda and Bivalvia) and on land (Gastropoda)

• Synapomorphies:

  • Mantle, some species secrete calcium carbonate

  • Mantle cavity → includes gills

  • Paired nerve chords

  • Radula → reduced or lost in some groups

  • Life cycle involving trochophora larva

  • Head with sensory organs and mouth

• Problems for interpretation of fossil:

  • Many modifications to the basic mollusc body plan

  • Distinguish characters are in soft tissue → rarely preserved

Mollusca phylogeny

• Morphological vs molecular

• Aculifera/Conchifera

• Testaria

• Dorsoconcha/Variopoda

Aculifera/Conchifera

• Aculifera → worm-like without true shell:

  • Solenogastres

  • Caudofoveata

  • Polyplacophora

• Conchifera → true shell (contiguous, may be reduced):

  • Monoplacophora

  • Cephalopoda

  • Scaphopoda

  • Gastropoda

  • Bivalvia

Testaria

• All molluscs that secrete partial or true shells:

  • Mono- and Poyplacophora

  • Bivalvia

  • Scaphopoda

  • Cephalopoda

  • Gastropoda

• Solenogastres and Caudofoveata as sister groups of Testaria

Dorsoconchia/Variopoda

• Rely on genomics → limitations on fossils

• Dorsoconchia:

  • Gastropoda

  • Bivalvia

  • Mono- and Polyplacophora

• Variopoda:

  • Scaphopoda

  • Caudofoveata

  • Solenogastres

  • Cephalopoda

Kimberella

• Oldest mollus or close relative

• Radula, soft body, no skeleton

Solenogastres and Caudofoveata

• Classified as Aplacophora

• Solenogastres → worm-like body with calcareous sclerites

• Caudofoveata → worm-like body, chitinous cuticle with calcareous scales

Polyplacophora (Chitons)

• Single row of eight aragonite plates (valves) along the back

• Valves surrounded by girdle, with spicules or scales

• Body similar to gastropods

• Well-developed radula

• Ordovician to present

• Multiplacophora → Palaeozoic stem-group with more than eight valves

Monoplacophora

• Simple molluscs with a single shell

• Late Cambrian to present

• Today deep sea, fossils in shallow waters

• Lack of fossil record due to shift to deep sea

• No record after Late Devonian

• Rarer now than in the past

Bivalves (Lamellibranchiata/Pelecypoda)

• No head

• Secondary loss of radula due to change of feeding mode → benthic filter-feeders

• Synapomorphies:

  • Exoskeleton of two valves with hinge joint

  • Teeth to lock the two valves together (hinge)

  • Calcite or calcite and aragonite

  • Secondary loss of radula

  • Paired ctenidia (gills for breathing and filter-feeding)

Bivalve biology

• Burrows in soft sediment

• Siphons to inhale and exhale water

• Gills for filter-feeding

• Boring valves chemically bore themselves → live there their whole lives

• Some epibenthic, attaching to substrate with byssus fibres

• Banks → clusters of bivalves

• Oysters cement themselves to the substrate

• Giant clams → reef-building species using photosymbiosis

• Scallop → capable of swimming

• Many secrete nacre (mother-of-pearl) to neutralise debris or parasites

Bivalve phylogeny and history

• Earlier models focused on dentition type or gill structure

• Modern approaches integrate morphological and molecular data

• Elements of modern evolutionary fauna

• Increase of diversity and disparity with time

• Affected by extinction events, but quick recoveries

• Long-lasting competition with brachiopods

Fossil bivalves

• Appeared in Early Cambrian

• Rare in Cambrian, diversification in Ordovician

• Megalodontida → Early Devonian to Mid-Cretaceous

• Trigoniida → Early Devonian to present

• Ostreida: Appear in Triassic, important in Mesozoic, extant Ostreidae Gryphaeidae → on soft sediment, non-identical valves

• Inoceramidae:

  • Permian to Cretaceous Common in Jurassic and Cretaceous

  • Gigantism

• Hippuritida:

  • Late Jurassic to Late Cretaceous

  • Unequal valves, one valve conical and one flat

  • Soft body small → big calcareous skeleton with body only on external-most part Important reef-builders in Cretaceous

  • Probably photosymbiosis

Scaphopoda (tusk shells)

• Opening on both ends of the shell

• Filter-feeding with tentacles (captacula)

• Early Carboniferous to present

• Youngest class of molluscs

• Likely descend from Rostroconchia

Rostroconchia

• Cambrian to Permian → throughout Palaeozoic

• Superficially bivalve-like, many convergent adaptations

Gastropoda

• Second most diverse class of organisms

• Marine, limnic, and terrestrial

• Ordovician to present

• Synapomorphies:

  • Single shell, typically coiled, conchiolin and calcium carbonate

  • Torsion of the body

  • Head with sensory organs

  • Ventral foot

Gastropoda eye types

• Simple pit eye

• Pinhole camera eye

• Lense eye

Bellerophontida (drilling snails)

• Cambrian to Early triassic

• Gastropod-like with symmetrically coiled shell

• Early gastropods or distinct class of molluscs

Drivers of gastropod evolution

• Predator/prey interactions

• Environmental change

Gastropods as index fossils

• Cross sections through the whorls can be used for identification

• Useful to understand palaeoclimate

Cephalopoda

• Typically nectonic

• Usually predators

• Synapomorphies:

  • Appendages (arms and tentacles) growing around the mouth

  • Hyponome (funnel) for jet propulsion

  • Complex nervous system and eyes

Cephalopod morphology

• Anterior mouth with beaks and radula

• Mouth surrounded by arms and tentacles

• Mantle cavity in ventral position, with gills and hyponome

• External or internal shell in posterior or dorsal position

• Shells with chambers with gas, regulated by siphuncle

• Soft body in living chamber

Cephalopod taphonomy

• Shells composed of aragonite, high preservation potential

• Dissolution of shell results in preservation of internal moulds

• Under dysoxic conditions, only periostracum (outer layer of organic substance) preserved

• Calcitic parts may be preserved under certain chemical conditions, but aragonitic parts don't

• Soft tissue preservation extremely rare due to buoyancy → float

• In belemnites → rostra, onychites, and hooks

Origin of Cephalopoda

• Cambrian

• Evolution from monoplacophoran or gastropod

• Development of chambered shell with septae and siphuncle enables nectonic lifestyle

• Radiation in Ordovician

• Complex evolution of early shell forms → straight shells (orthocone) → nautiloids

• Endogastric vs ectogastric coiling

Nautilida

• Siphuncle central

• Suture lines simple

• Large number of arms

Bactritida

• Considered ancestral to ammonoids and coleoids

• Straight or loosely coiled shells

• Devonian to Mid-Triassic

Ammonoidea

• Largest group of fossil cephalopods

• Devonian to K/Pg

• Shell usually coiled

• Siphuncle usually ventral

• Suture lines compex

• Soft body unknown

• Ammonoids with more developed sutures → better and faster buoyancy

Ammonoidea main groups

• Agoniatitida → Devonian to P/T

• Clymeniida → Devonian to P/T

• Goniatida → Devonian to P/T

• Prolecantida → Devonian to P/T

• Ceratitida → Late Permian to End of

• Triassic Ammonitida → Jurassic to K/Pg

Ammonitida

• Heteromorphic ammonites → deviations from the standard shapes

Ammonite jaws

• Lower and upper jaws modified into aptychi, sometimes upper jaw lost

Ammonite anatomy

• Soft tissue unknown

• Arm number unknown, probably ten

• Length of body equal to length of living chamber

• Aptychi functioning as jaws and/or lids

• Gigantism

Belemnoidea

• Important in Jurassic and Cretaceous

• Internal skeleton with reduced phragmocone (aragonite) and long rostrum/guard (calcite)

• Ten arms of equal length with small hooks

• Sometimes large hooks (onychites) → mating

• Early Devonian to K/Pg

Decabrachia (true squids)

• Ten arms, two typically modified into tentacles

• Highly developed nervous system and sensory organs

• Fossil record poor

• Mid-Jurassic to present

Vampyropoda

• Cephalopods with eight arms

• Vampyromorphida → might be closely related to extinct shallow-water forms

• Octopoda

• Late Triassic to present

Arthropoda

• Largest biodiversity of any group of organisms

• Larges biomass of any animal group

• Extreme diversity of body plans and life modes

• First animals on land

• First animals to develop flight

• Integral component of almost all ecosystems on earth

Arthropod crown group

• Jointed (arthropodised) appendages

• Segmentation of the body

• Exoskeleton (cuticle) of chitin

• Moulting (ecdysis)

• Compound eyes

• Ladder-like nervous system

Systematics of Arthropoda

• Traditionally considered sister taxon to Annelida based on body segmentation → Articulata hypothesis

• Newer evidence, relationship with other moulting animals → Ecdysozoa hypothesis

Main groups of crown-group arthropods

• Arachnomorpha:

  • Artiopoda → trilobites

  • Chelicerata → arachnida

• Mandibulata:

  • Myriapoda

  • Crustacea

  • Hexapoda

Oldest arthropods/small shelly fauna

• Lower Cambrian

• Oldest unequivocal arthropod fossils

• Mostly bivalved forms → close to the crown group

Lobopodia

• Extinct panarthropods with unsegmented legs

• Cambrian to Early Permian

• Phylogenetic position(s) unclear

• Includes various basal panarthropods

• Some lobopodians are stem arthropods, others closer to tardigrades or onycophorans

Opabinia

• Flaps along the body

• Five eyes

• Proboscis with grasping claw

• Cambrian only

Radiodonta

• Mostly Cambrian to Early Devonian

• Important part of Cambrian ecosystems

• Active predators, also suspension feeders and deposit feeders

• Highly complex compound eyes

• Large frontal appendages for grasping

• Circular oral cone for chewing

Fuxianhuiida

• Early cambrian

• Biramous appendages

• Segmented thorax and abdomen

• Either early deuteropods (derived stem-lineage arthropods) or early mandibulates

Isoxyida

• Cambrian

• Anterior grasping appendages and biramous trunk appendages

• Body covered by bivalved carapace

Megacheira

• Cambrian to Early Devonian

• Large, specialised anterior appendages (great appendages)

• Either basal deuteropods or early chelicerates, likely paraphyletic

Marellomorpha

• Walking legs and various antennae-like frontal appendages

• Interpreted as crown-group arthropods, either arthropods or mandibulates

• Cambrian to early Devonian

Hymenocarina

• Cambrian only

• Diverse group

• Almost certainly members of Mandibulata

Thylacocephala

• Ordovician to Cretaceous

• Large grasping appendages and often large eyes

Euthycarcinoidea

• Cambrian to Triassic

• Elongated body with walking legs

• Possibly amphibious

• Probably originator of the earliest terrestrial trackways

• Interpreted as mandibulates

Arthropods evolutionary trends

• Arthroidsation of appendages

• Acquisition of biramous limbs with gnathobases

• Arthrodisation of trunk segments

• Shifting of the mouth from anterior (terminal) to ventral

• Reduction or modification of the first (protocerebral) appendages

• Increase in the number of segments composing the head

• Complex modification and specialisation of head appendages

• Sclerotisation of the exoskeleton

The arthropod head problem

• Homologies between head segments/somites and appendages of extant and fossil arthropod groups are highly controversial

• Depends on interpretations of embryology and fossil evidence

Biramous appendages

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Arthropod traditional phylogeny

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Antennata hypothesis

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Trilobita

• Dominant group in Palaeozoic

• Important index fossils, especially in Cambrian

• Always marine

• Cambrian to End of Permian

• Cephalon with facial sutures for moulting

• Compound eyes

Trilobita compound eyes

• Holochroal:

  • Plesiomorphic

  • Varying number of lenses (up to more than 15,000)

  • Lenses in direct contact to each other

  • Single cornea covers all lenses

• Schizochroal:

  • Only Phacopida

  • Few, larger lenses (up to 700)

  • Lenses separated from each other

  • Each lense has their own cornea

• Abathochroal:

  • Only Eodiscina

  • Few, small lenses (up to 70)

  • Lenses separated from each other

  • Each lense has their own cornea

Trilobite timeline

• Cambrian Explosion → appearance of many trilobite groups

• Cambrian/Ordovician → highest diversity

• Ordovician/Silurian extinction event

• Late Devonian → biotic crises

• Permian/Triassic → extinction of the last trilobites

Trilobite phylogeny

• Redlichiids and olenellids considered basal/ancestral

• Assignment of Agnostida to trilobita is controversial

• Facies-dependent trilobites

Trilobite palaeobiology/behaviour

• Predators and scavengers:

  • Plesiomorphic condition

  • Small spines at the gnathobases

  • Epibenthic crawlers

• Filter feeders:

  • Large cephalon with long spines

  • Filtering chambers with pores

• Nectonic feeders:

  • Streamlined body

  • Very large eyes

  • Cosmopolitan

  • Nectonic mode evolved convergently

• Agnostida:

  • Small body

  • Cephalon and pygidium similar, thorax reduced

  • Secondarily blind

  • Benthic or planctonic

  • Assignment uncertain

  • Possibly derived from early trilobites through paedomorphosis

• Enrolling

• Social behaviour

• Cannibalism

• Sexual competition

• Mating behaviour → claspers for mating

• Ichnofossils

Morphology of Chelicerata

• Two body sections (tagmata):

  • Prosoma

  • Opisthosoma

  • Often fused

• One or two pairs of specialised cephalic appendages:

  • Chelicerae → to handle and chew food

  • Pedipalps → different functions

• No antennae

• Typically four pairs of walking legs

Chelicerata models

• Traditional model

• Alternate hypothesis → Arachnida paraphyletic → independently evolved terrestrialisation

Pycnogonida (sea spiders)

• Classification as chelicerates controversial

• Basal Chelicerata

• Middle Silurian to present

Pycnogonida morphology

• Small body and long legs

• Some internal organs shifted into the legs

• Proboscis incorporates chelicerae and palps, used for hunting

• Large size range

Synziphosurina

• Palaeozoic group

• Early Euchelicerata or Merostomata

• Late Cambrian to Middle Carboniferous

Xiphosura

• Aquatic, plesiomorphic euchelicerares

• Late Ordovician to present

• Large carapace on prosoma and opisthosoma

• Telson

Xiphosura morphology

• Chelicerate characters:

  • Prosoma and opisthosoma

  • Chelicerae

  • Book lungs

• Plesiomorphic characters:

  • Gnathobases

  • Compound eyes

  • No pedipalps

  • 5 pairs of walking legs → not 4

Chasmataspidida

• Merostomata

• Late Cambrian to Middle Devonian

• Similar to Eurypterida

Eurypterida (sea scorpions)

• Most important fossil group of aquatic chelicerates

• Apex predators in Silurian and Devonian

• Includes the largest arthropods ever found

• Earlier forms marine, moved to brackish/limnic in Devonian

• Among the first animals on land, never fully terrestrial

• Middle Ordovician to end of Permian