Echinoderms underwent an initial radiation into marine shelf environments in the Early to Middle Cambrian Within about 45 million years, nine new classes of echinoderms (as recognized in an evolutionary classification) appeared suddenly in the fossil record, many of these without obvious ancestors This radiation then tapered off during the Late Cambrian, and no additional classes of echinoderms appeared in the record until the Early Ordovician, about 20 million years later, when a secondary radiation of echinoderms began

Radiation of Echinodermata

In defence of the calcichordates

Echinoderms underwent a major two-part radiation that produced all of the major groups found in the fossil record between the Early Cambrian and the Middle Ordovician. A small initial radiation in the Early and Middle Cambrian produced about nine classes containing low-diversity members of the Cambrian Evolutionary Fauna. These were characterized by primitive morphology, simple ambulacral feeding structures, and the early development of a multiplated stalk or stem for attachment to skeletal fragments on a soft substrate. Several groups became extinct at the end of the Middle Cambrian, leaving the Late Cambrian as a gap of very low diversity in the fossil record of echinoderms with only four classes preserved and very few occurrences of complete specimens, mostly associated with early hardgrounds. The survivors from this interval re-expanded in the Early Ordovician and were joined by many newly evolved groups to produce a much larger radiation of more advanced, diverse, and successful echinoderms representing the Paleozoic Evolutionary Fauna on both hard and soft substrates. At least 17 classes were present by the Middle Ordovician, the all-time high point for echinoderm class diversity, and nearly all of the major ways-of-life (except for deep infaunal burrowing) had been developed. With the rise to dominance of crinoids, many less successful or archaic groups did not survive the Middle Ordovician, and echinoderm class diversity dropped further because of the mass extinction at the end of the Ordovician. This weeding-out process of other less-successful echinoderm groups continued throughout the rest of the Paleozoic, and only five classes of echinoderms have survived to the Recent from this early Paleozoic radiation.

Early radiation of echinoderms

Echmatocrinus from the Middle Cambrian Burgess Shale of British Columbia was originally described as the earliest crinoid(?) known from the fossil record. Recently, Conway Morris and Ausich & Babcock have questioned whether Echmatocrinus is in fact an echinoderm, comparing it instead to cnidarians with a polyp-like body and pinnate tentacles, and other authors are beginning to use this reinterpretation. We studied the well-preserved holotype of Echmatocrinus brachiatus, two paratypes, and 18 new specimens recovered from different levels in the Burgess Shale sequence at three localities. All are preserved as pyrite films in dark shale with relatively little relief, suggesting a lightly skeletized body. Complete specimens have a long, slightly tapering, large-plated attachment stalk, a conical cup or calyx with numerous small to medium-sized irregular plates, and 7–10 short arms with heavier plating and (in the holotype) soft appendages alternating from opposite sides of several arms. Several morphologic features indicate that Echmatocrinus is an echinoderm and has crinoid affinities: (1) Sutured plates, shown by darker depressed sutures, slightly raised plate centers, and oriented plate ornament, cover all major parts of the body; (2) reticulate surface ornament in the pyrite film on the plates of all specimens matches the ornament in the Burgess Shale edrioasteroid Walcottidiscus, an undoubted echinoderm, but not the pyritized surfaces of other metazoans in the fauna; (3) this distinctive ornament may represent the surface expression of microporous stereom; (4) possible ligament or muscle pads are present between the arm ossicles to fold and unfurl the more heavily plated arms. Within the echinoderms, only crinoids commonly have a calyx attached by a stalk or stem to the substrate and bear erect, moveable, uniserial arms for feeding. Although Echmatocrinus shows some resemblance to octocorals in overall body shape as an attached suspension feeder, almost all the details are different, indicating that Echmatocrinus is most likely unrelated to this group. All complete specimens of Echmatocrinus are attached to hard substrates, either another fossil or skeletal debris. The new specimens indicate that Echmatocrinus was twice as common (about 0.02%) in the Burgess Shale fauna as previously recorded and represents one of the earliest attached, medium-level, skeletized, suspension feeders or microcarnivores in the fossil record.

Revision of Echmatocrinus from the Middle Cambrian Burgess Shale of British Columbia

This review is a presentation of progressive knowledge gained on the three subphyla Homalozoa (= Carpoidea), Blastozoa and Crinozoa during the las decade — i. e. after drafting of the manuscripts for the Treatise on Invertebrate Paleontology, vols. S 1(1) and S 1(2) of 1967. Concerning the Crinoidea, the Treatise volume of which has not appeared yet, the accounting period covers two decades.

Review of Recent Research on “Pelmatozoans”

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Review of Recent Research on “Pelmatozoans”