GEOL 331/BSCI 333 Principles of Paleontology

Fall Semester 2022
Deuterostomia II: Crown group echinoderms


Key Points:
•Hypotheses of the phylogeny of living echinoderms are currently in a state of upheaval. This review assumes that both Pelmatozoa and Eleutherozoa are monophyletic, following the emerging majority view of the last decade.
•Crinoid morphology is distinct from that of other stalked echinoderms.
•Of the major crinoid groups, only Articulata survived into the Mesozoic.
•The Post-Paleozoic world has been hard on crinoids, who have either retreated to marginal habitats or significantly altered their morphotype.
•Although we think of Eleutherozoa as the "modern" echinoderms because of their success in the Post-Paleozoic oceans, they are roughly the same age as Pelmatozoa.
•Eleutherozoa contains Asterozoa (starfish and brittle stars) and Echinozoa (echinoids and holothurians) and their fossil relatives.
•Ophiuroidea (brittle-stars) are scavengers on particulate and larger food fragments, very common in the deep oceans.
•Asteroidea include predatory forms.
•Somasteroids - stem asterozoans - are flattened, five-armed deposit/suspension feeders.
•In contrast to other Eleutherozoans, echinoids have a robust fossil record.
•Echinoid evolution presents a series of interested paraphyletic grade groups leading up to Clypeasteroida - sand dollars.
•Holothurians have an especially poor record, as their skeletons are not integrated. Encompass a range of shallow and deep marine deposit feeders.
•Ophiocystioideans represent stem echinozoans with features suggestive of echinoids and holothurians.

"Never ask a starfish for directions."

We now explore the crown group of Echinodermata, containing: Remember, the realization that Pelmatozoa is monophyletic only came during the last decade. (See Ausich et al. 2015, Sumrall, 2015, and Sumrall, 2017.)

Pelmatozoa: Stalked echinoderms

Pentremites pyriformis from Humboldt State University Natural History Museum

Deep water crinoid from David Liddell
Department of Geology - Utah State University


(Sea lilies and feather stars) (Ord. - Rec.) Finally, a group with living representatives.

The crinoid body plan:

Suspension feeders: The crinoid points its oral surface downstream then arches its arms into the current. Crinoids can quickly orient themselves on their stems using a bizarre trick: The connective tissue running down their stems can be partially emulsified by nervous impulses, thus rapidly altering stiffness. Wow.

from SUNY Cortland
Calyx plate terminology:

Actinocrinites gibsoni
What is your assessment of the morphology of the crinoid at right:

Crinoid systematics:

Phylogenetic systematists have only just begun their exploration of crinoids, so we offer no synopsis of phylogeny here. Interested students can refer to Ausich et al. 2015. Traditionally, crinoids break down into three major groups that we present for descriptive purposes:


(Ord. - Rec.) Eleutherozoa 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 Eleutherozoan 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 Eleutherozoans being "new" is heightened by the poor preservation potential of all groups except for echinoids.

Eleutherozoan phylogeny:

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

Eleutherozoa characteristics:

Not necessarily synapomorphies:

Eleutherozoan systematics: The cladogram at right represents a decent consensus of current analyses of Eleutherozoan phylogenies with one huge caveat: Two reasonable hypotheses exist for the placement of Ophiuroidea:

Here we default to the Asterozoa hypothesis, but the issue is unsettled.

Somasteroid in oral view

Somasteroidea - (Ord. Dev.) These appeared slightly prior to the appearance of ophiuroids and asteroids, and were extinguished by the Devonian extinctions.


Somasteroids are argued by some to be the plesiomorphic morphotype for modern asterozoans.

from British Chalk Fossils
Ophiuroidea (brittle stars) - (Ord. Rec.)

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


Hudsonaster matutinus (Ord.)
Fossil Record: Although common today, aspects of starfish biology limit their preservation potential: And yet we have fossils of Ordovician starfish that are similar to living ones in anatomy. (E.G. Hudsonaster right). The Devonian, Permian, and Triassic extinction events each trimmed their diversity, but after each, they rebounded with a proliferation of new groups.

Galápagos green sea urchin Lytechinus semituberculatus
Echinoidea (sea urchins, heart urchins, sand dollars) (Ord. - Rec.)

Interior of Aristotle's lantern from
  • Feeding:

    Evolution and fossil record: Unless otherwise noted, these are probably paraphyletic grade-groups.

    Holothurian shows four of five ambulacra.
    Holothuroidea: (sea cucumbers) (Sil. (maybe Ord.?) - Rec.) Suspension and deposit feeders whose body plan resembles that of a regular echinoid that has been: The result is an animal with a mouth in front, an anus in back, two ambulacra on top and three underneath (on which it walks).


    Thelonota anamas

    Sollasina cthulhu from PhysOrg/a>
    Ophiocistioidea - (Ord. - P.)

    What would an animal close to the close to the ancestry of Echinozoa look like? Arguably like an ophiocystioid. Considered by Rahman et al., 2019 to be a paraphyletic grade of stem holothuroideans.

    Additional reading: