GEOL 331 Invertebrate Paleontology

Fall Semester 2008
Echinodermata I - Basal echinoderms and relatives

Echinodermata: After molluscs and arthropods, the most commonly fossilized bilatrian taxon. Indeed, disarticulated echinoderm plates are major constituents of Phanerozoic carbonates, especially from the Mississippian. Deterostomes - thus closely related to hemichordates and chordates.

Potential synapomorphies of Echinoderms and Chordates:

  • Deuterostomous development
  • Hollow nerve cord (only a short segement in echinoderms, hollow for whole length in chordates).

Recent molecular phylogenies indicate sister taxon relationship between Echinodermata and Hemichordata, united as Ambularcraria.

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Newly discovered fossil group: Vetulicolia :

  • From Early Cambrian Chengjiang Fm. lagerstätte in China
  • Looks like what we might expect of something close to the ancestry of Deuterostomia.
  • Front end is a pharynx (filter feeding organ) with five pharygeal slits
  • Authors interpret ventral structure in pharynx to be endostyle (gland rich groove of basal chordates; homolog of thryoid gland of vertebrates), but lacks any kind of internal skeleton.
  • The anterior segment is armored by plates of cuticle, giving it an arthropod-like aspect.
  • The rear is a laterally compressed segmented tail.
  • may include Nectocaris from the Burgess Shale.

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  • Hemichordata:

    Characteristics:

    • Like echinoderms, these are suspension and deposit feeders, however food capture is by means of ciliary action rather than tube feet.
      • Bodies divided into proboscis, collar, and trunk:
        • Cilia move food particles to mouth.
        • Collar encircles mouth and proboscis. Developed into lophophore-like arms containing extension of coelom in pterobranchs.
        • Trunk contains a large pharynx with pharyngeal openings (gill slits)
        • Open circulatory system present, with blood propelled by contractions of primary blood vessels. No hemoglobin.
      • Dorsal nerve cord, hollow in some parts of collar.

      Potential synapomorphies of hemichordates and echinoderms:

      • Molecular characters
      • Arms (in this case of proboscis) containing coelomic extensions
      • Torneria larvae of enteropneusts resemble auricularia larvae of holothurians.

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    Diversity:
    • Enteropneusta: "Acorn worms": (No record)

      • Largish (5 CM - >2m) worm like burrowing suspension or deposit feeders.
      • Pharynxes large and perforated by many pairs of gill openings.
      • Acorn-shaped proboscis gives common name
      • Invariably solitary.
      • Infaunal, inhabiting U-shaped burrows. Probably responsible for many U-shaped domichnia.

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    Pterobranchia: (Spotty record, Cambrian - Rec, excluding graptolites)

    Characteristics:

    • Tiny (<2 mm.) colonial critters that secrete branching colony consisting of proteinaceous tubes.
    • The proboscis is developed into a cephalic shield, used to secrete the proteinaceous material of the colony structure.
    • The collar is developed into one to five pairs of arms, each housing an extension of the coelom. Arms sport cilia-covered tentacles that transport food particles to mouth.
    • Only one pair of pharyngeal openings.
    • The gut is U-shaped.
    • Each individual zooid lives in a cylindrical zooidal tube
    • Unlike bryozoans, zooids are able to move around on the outside of the colony, attached by a strand of contractile tissue, the peduncle to a common tissue thread, the stolon. When the colony is alarmed, individuals are quickly "reeled in." Cute.
    • Pterobranch growth. The stolon grows from the peduncle of the ancestral zooid. New zooids bud off from the stolon. As the stolon grows, it is encased in a creeping tube secreted by a specialized zooid, the terminal bud. Other, regular zooids bud from the stolon and secrete their own zooidal tubes.

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    Graptolites (Cambrian - Miss.)

    • Enigmatic fossils of the Early Paleozoic. Typically compressed into two dimensions.
    • Very common and diverse in Ord - Sil with pelagic global distribution. The premier index fossils of these periods.
    • Discovery of three-dimensionally preserved specimens, and subsequent study of pterobranchs led to realization that graptolites were colonies of zooids that secreted a proteinaceous hard structure very similar to that of pterobranchs. Implies very close relationship.
    • BUT, because they had long been studied and used as index fossile prior to being biologically understood, they have their own set of terminology. Geologists note: You may actually have to use these some day.
      • the colonial structure is the rhabdosome
      • Individual zooids inhabit cylindrical or conical thecae
      • The first theca is the sicula. It is conical.
      • From its tip, a thread-like nema extends. This attaches the rhabdosome to the substrate or to floating objects.
      • As new zooids bud from the original, they form branches of the colony called stipes. These may branch many times, once, or not at all.

    Graptolite diversity: There are two major groups. (We spare you the minor groups.)
    • Dendroidea: (Cam. - Penn)
      • Stipes branch many times in sequence, yielding a dendritic rhabdosome.
      • The nema attaches the rhabdosome to the substrate.
      • Thecae are dimorphic, with adjacent pairs of large autothecae and small bithecae (probably female and male zooids respectively.)
    • Graptoloidea: (Latest Cam. - Dev.)
      • Stipes branch at most once. See lab for rhabdosome morphology terminology based on stipe angle to nema. BUT NOTE, in some, the stipes are scandent, - they grow up the sides of the nema.
      • The nema attaches the rhabdosome to a floating object. (Some genera seem to have secreted siphonophore-style floats from which numerous rhabdosomes hung.)
      • Thecae are uniform.
      • Monograptid morphology an unbranched stipe growing up one side of the nema. Such rhabdosomes often assume spiral shapes. Perhaps they did not attach and depended on a slow sinking rate to remain in the photic zone.
      • Biostratigrapher's delight: Peak abundance and diversity in Ord. - Sil. Key index fossils for this interval.

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    Echinodermata

    Echinoderm characteristics:

    • Exclusively marine: Echinoderms lack osmoregulatory mechanisms that might allow them to live in brackish or fresh water.
    • Skeleton is internal test comprised of individuals plates of porous high-Mg calcite. In life, the pores are occupied by a protein matrix and dermal cells. Such skeletal tissue is known as stereom. Carbonate petrologists typically call the pores meat holes. an individual birefringent element
    • One portion of the coelom develops into water vascular system. (Dissection)
      • This system is passively involved in gas exchange, maintainance of posture, and locomotion.
      • The latter is effected by outpouchings of the WVS that penetrate the body wall to form podia or tube feet that can be employed in suspension feeding or in locomotion, depending on the critter.
      • Tube feet are arranged into five double-rows termed ambulacra. Typically, these converge on the mouth amd/or anus.
      • Muscles are used to pump water around the WVS, and each tube foot is equipped with small longitudinal muscles to help aim it, yet the the hydrolic force of the WVS is what primarily effects movement in most echinoderms.
      • the WVS obtains water from the outside. It is connnected by a calcite-reinforced stone canal that opens to the exterior in the hydropore. The hydropore is covered by a seive-like plate, the madreporite, that strains incoming water.
      • The lining of the WVS is ciliated, allowing circulation of its fluid. thus, tube-feet function as gas exchange organs.
      • The coelom (including the WVS) contains coelomocytes which attack foreign material and, in some cases, carry oxygen and CO2.
      • Despite this weirdness, they are proper bilateralians with a mouth, flow-through gut, and anus.
      • NOTE: In addition to the WVS, echinoderms also retain a normal enterocoelic coelom.
    • All but most primitive have strong pentameral symmetry.
    • Mutable collagen: A form of collagen that can be partially emulsified byt eh application of nervous action potentials.
    • Ancestrally suspension-feeders. Living crinoids still suspension feed. Tube feet snare food particles that are then conveyed down ambulacral grooves by cilia to the mouth.

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    Echinoderm Systematics

    Traditionally all stemmed echinoderms were lumped into "Pelmatozoa", and all non-stemmed to "Eleutherozoa". Only the latter is monophyletic. A more cladistic phylogeny of Echinodermata:

    Helicoplacoidea (Early Cam.)

    • Body is helically-built (hence name)
    • Sessile suspension feeders: Among the last "hangers-on" to the Ediacaran algal mat environment.
    • Ambulacral groove spiralling along body. If, in your mind, you "unroll" a helicoplacoid, you find three ambulacra converging on the mouth, which is located on the side of the body, about two-thirds of the way up. (The location of the anus is a mystery.)

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    Edrioasteroid: (Cam. - Penn)

    • Sessile, benthic, attached to hard substrates like the surfaces of brachiopods, mollusks, etc.
    • The body took the form of a lens-like blister or a bulb sitting on a short broad stalk.
    • Have five ambulacra, like more derived forms. Close examination show that two pairs of these actually converge some distance from the mouth. Thus, only three ambulacra actually converge at the mouth. Edrios, therefore, provide a morphological bridge between the triradiate helicoplacoids and the proper, pentamerally symmetrical later echinoderms. NOTE: A line bisecting the edrio mouth and anus shows the primordial plane of bilateral symmetry that can be hard to recognize in other echinoderms.

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    "Eocrinoids": (Cam - Sil.)

    • Paraphyletic group of basal stalked echinoderms
    • Ones with brachioles (tiny tentacles surrounded by calcitic rings, only rarely preserved) may actually be basal blastozoans
    • Others, with simple arms (which house extensions of the coelomic cavity), may be sister taxa to Crinozoa + Eleutherozoa

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    Blastozoa

    The final group, Eleuthrozoa will require their own lecture.

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    Echinoderm origins: Who are the closest relatives or ancestors of echinoderms? Hard to say for fossils that lack echinoderm like calcitic skeletal elements. Some possibilities: Your call.

    To Syllabus.