GEOL 331 Invertebrate Paleontology

Fall Semester 2008
Echinodermata II

Eleuthrozoa: (Cam. - Rec.) Eleuthrozoa contains the living groups of non-stalked echinoderms. These became the dominant echinoderms of the post-Paleozoic world, and continue to diversify. Do not think, however, that they are of recent origin. The four major eleuthrozoan groups are roughly as old as blastozoans and crinoids. Thus, although relative abundances and diversities have changed, echinoderms diversified quickly into their modern groups during the Ordovician. The illusion of eleuthrozoans being "new" is heightened by the poor preservation potential of all groups except for echinoids.

Eleuthrozoan phylogeny:

More recent cladistic analyses support the following tree, however in the last twenty years, enough been suggested that an Adam's consensus would be a whisk-broom polytomy:

Eleuthrozoan characteristics

: Not necessarily synapomorphies:

Ophiuroidea (brittle stars) - (Ord. Rec.)

  • Bodies with distinct central disks and five arms.
  • Arms
    • Lack coelomic extension or ambulacral grooves
    • Made up of a single row of large calcite plates termed vertebrae or vertebral plates. (Confusing??)
    • Although tube feet are present but lack ampullae (in this respect, similar to crinoids). They are used strictly for feeding. Unlike in other echinoderms, movement of the arms is effected directly by muscles. Thus, ophiuroids walk/climb around on their five arms, making them the fastest moving echinoderms (woo hoo!).
    • Commonly called "brittle-stars" because of their tendency to shed arms when molested. these are regenerated in a matter of months.
  • Central disks:
    • Mouth is ventral in the center of the oral surface.
    • Anus is absent. Waste is expelled through mouth.
    • There may be several madreporites that open on the oral surface, near the mouth.
    • Oral surface covered with small clacite scales, but aboral surface bears five pairs of large shields.

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  • Poor fossil record as they tend to break apart after death. Nevertheless, some trends are visible:

  • Ophiuroid ecology:

    Want to see a live one on campus? Go to the south elevator lobby of Cumberland Hall and look in their aquarium. It's usually hiding in the pile of rocks on the right side.

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    Asteroidea (starfish or sea stars) - (Ord. Rec.) Taken as "standard" echinoderms for illustrations of tube-feet, the WVS, etc. only because they are familiar. Actually quite idiosyncratic.

    • As in ophiuroids, a central disk and arms are present, but in this case poorly demarcated. Anatomy:

    • Arms
      • Contain coelomic extension and ambulacral grooves
      • Made up of a multiple small plates.
      • Although tube feet sport ampullae and are used strictly for locomotion/grasping. Asteroids rely on tube-feet and WVS for locomotion.
      • Capable of autotomizing (voluntarily severing) arms to sidetract predators. Prodigious powers of regeneration.
    • Central disks:
      • Mouth is ventral in the center of the oral surface.
      • Anus is very small, owing to characteristic feeding mode (see bleow)
      • Single madreporite apparant on aboral surface.

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  • Peculiarities of the Body surface:

    • The asteroid skeleton consists of small ossicles that are not tightly joined.
    • Small movable spines may serve to remove detritus or discourage larvae of sessile organisms from attaching. (Cf. bryozoan vibracia)
    • Papulae (AKA dermal branchia) - thin non-calcified outpouchings of the coelom that protrude between ossicles. Thought to facilitate gas-exchange
    • Pedicellaria - Small appendages with two to three valves used to clean, discourage parasites, and capture small prey. (Cf. bryozoan avicularia)

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  • Asteroid ecology:
    • Feeding:
      • Do not suspension feed.
      • Have two distinct stomachs, a cardiac stomach above the mouth, and a pyloric stomach above the cardiac stomach. Branches of the pyloric stomach, the pyloric caeca (sing. caecum) radiate into each arm.
      • Predators, preying on largish critters. How this is done depends on the size of the prey item:
        • Smallest items ingested directly at the mouth.
        • For larger items, the cardiac stomach can be everted to engulf the prey then pulled back inside.
        • For large bivalves, the valves are pulled apart slightly, the cardiac stomach everted into the shell, and the bivalve digested aby the asteroid and its nutrients absorbed inside its own shell. When the meal is over, the asteroid zips up its cardiac stomach and is good to go. In this case, waste elimination isn't an issue, so the asteroid anus can be small because it doesn't get much of a workout.
    • Present in a wide range of marine environments.

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    Somasteroidea - (Ord. Dev.)

    • Resemble asteroids in having Bodies with indistinct central disks and five arms, however they are significantly flattened.
    • The skeleton of the aboral surface is simple and not rigidly integrated, consisting of simple papillae.
    • The skeleton of the oral surface consists of ambulacral ossicles and radiating branches of virgals giving the skeleton a feather-like profile.
    • Speculated to have been benthic suspension or detritus feeders.

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    Echinoidea (sea urchins, heart urchins, sand dollars) (Ord. - Rec.)

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  • Feeding:
    • Ancestrally grazers, feeding by means of Aristotle's Lantern, a complex set of ossicles lining the esophagous.
    • In derived irregular echinoids, deposit feeding replaces grazing, so Aristotle's lantern is reduced or absent.

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  • Taxonomy: Unless otherwise noted, these are probably paraphyletic grade-groups.

    Holothuroidea (sea cucumbers) (Cam (?) - Rec.)

    • Morphology:
      • With the exception of the periesophageal ring elements encircling the mouth, there is no rigid test, only calcareous sclerites suspended in fleshy tissue. Thus fossil record is poor. We do have one, Redoubtia from the Burgess Shale.
      • Picture a fleshy echinoid laid on its side and pulled into a sauasge-shape and you've got it.
      • Tube feet (with ampullae) provide locomotion across sea floor.
      • Burrowing in sediment is achieved by contraction of body wall muscles against hydrostatic skeleton.
      • Around the mouth (which is anterior) specialized tube feet are developed into suspension feeding tentacles. in some. Others are deposit feeders.

      • The madreporite is internalized, thus the WVS loses its connection to the outside.
      • Internally, holothurians have significant intestines, and complex respiratory trees - breathing structures that form as outpouchings of the cloaca.
      • In contrast, the gonads discharge gametes near the mouth.
      • Coolest of all, holothurians respond to threats by partial or total evisceration of the intestines and respiratory tree through the anus. In some types, this is limited to the expulsion of special thread-like sticky/toxic cuvierian tubules, that seem to serve no other purpose. Eviscerated organs are eventually regenerated.

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    Ecology:
    • Motile, mostly infaunal deposit feeders that ingest sediment and digest the good parts.
    • Suspension feeders
    • Range roughly parallels that of ophiuroids - present, even common in reefs and shallow water (see image rt.) but fabulously abundant on the deep ocean floor.