Cambrian edrioasteroid reveals new mechanism for secondary reduction of the skeleton in echinoderms

Zamora, Samuel; Rahman, Imran A.; Sumrall, Colin D.; Gibson, Adam; Thompson, Jeffrey R.

doi:10.1098/rspb.2021.2733

Cited by 5 publications

(1 citation statement)

References 60 publications

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“…However, there is some support for the idea that arms of crinoids and those of asterozoans are not homologous, as implied by the topology in Deline et al (2020). Although tests of these ideas were beyond the scope of the present work, such considerations are important in the context of recent discoveries such as Yorkicystis (Zamora et al, 2022), as well as the debate over asterozoan origins, as discussed in Hunter and Ortega-Hernández (2021) and Blake and Hotchkiss (2022). We agree with the latter authors that Hunter and Ortega-Hernández (2021) have misinterpreted the nature of virgals through a misapplication of the extraxial-axial theory.…”

Section: Discussionmentioning

confidence: 90%

Crinoid calyx origin from stem radial echinoderms

2023

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Evidence from the earliest-known crinoids (Tremadocian, Early Ordovician), called protocrinoids, is used to hypothesize initial steps by which elements of the calyx evolved. Protocrinoid calyces are composed of extraxial primary and surrounding secondary plates (both of which have epispires along their sutures) that are unlike those of more crownward fossil and extant crinoids in which equivalent calycinal plating is strongly organized. These reductions inspired several schemes by which to name the plates in these calyces. However, the primary-secondary systems seen in protocrinoids first appeared among Cambrian stem radial echinoderms, with primaries representing centers around which secondaries were sequentially added during ontogeny. Therefore, the protocrinoid calyx represents an intermediate condition between earliest echinoderms and crownward crinoids. Position and ontogeny indicate certain primaries remained as loss of secondaries occurred, resulting in abutting of primaries into the conjoined alternating circlets characteristic of crinoids. This transformative event included suppression of secondary plating and modification or, more commonly, elimination of respiratory structures. These data indicate subradial calyx plate terminology does not correspond with most common usage, but rather, supports an alternative redefinition of these traditional expressions. Extension and adoral growth of fixed rays during calyx ontogeny preceded conjoined primaries in earliest crinoids. Restriction with modification or elimination of calyx respiratory structures also accompanied this modification. Phylogenetic analyses strongly support crinoid origination from early pentaradiate echinoderms, separate from blastozoans. Accordingly, all Tremadocian crinoids express a distinctive aggregate of plesiomorphic and apomorphic commonalities; all branch early within the crinoid clade, separate from traditional subclass-level clades. Nevertheless, each taxon within this assemblage expresses at least one diagnostic apomorphy of camerate, cladid, or disparid clades.

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Section: Discussionmentioning

confidence: 90%

Crinoid calyx origin from stem radial echinoderms

2023

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A Review and Evaluation of Homology Hypotheses in Echinoderm Paleobiology

Sumrall

Sheffield

Bauer

et al. 2023

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The extraxial-axial theory (EAT) and universal elemental homology (UEH) are often portrayed as mutually exclusive hypotheses of homology within pentaradiate Echinodermata. EAT describes homology upon the echinoderm bauplan, interpreted through early post-metamorphic growth and growth zones, dividing it into axial regions generally associated with elements of the ambulacral system and extraxial regions that are not. UEH describes the detailed construction of the axial skeleton, dividing it into homologous plates and plate series based on symmetry, early growth, and function. These hypotheses are not in conflict; the latter is rooted in refinement of the former. Some interpretive differences arise because many of the morphologies described from eleutherozoan development are difficult to reconcile with Paleozoic forms. Conversely, many elements described for Paleozoic taxa by UEH, such as the peristomial border plates, are absent in eleutherozoans. This Element recommends these two hypotheses be used together to generate a better understanding of homology across Echinodermata.

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Cambrian edrioasteroid reveals new mechanism for secondary reduction of the skeleton in echinoderms

et al. 2022

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Echinoderms are characterized by a distinctive high-magnesium calcite endoskeleton as adults, but elements of this have been drastically reduced in some groups. Herein, we describe a new pentaradial echinoderm, Yorkicystis haefneri n. gen. n. sp., which provides, to our knowledge, the oldest evidence of secondary non-mineralization of the echinoderm skeleton. This material was collected from the Cambrian Kinzers Formation in York (Pennsylvania, USA) and is dated as ca 510 Ma. Detailed morphological observations demonstrate that the ambulacra (i.e. axial region) are composed of flooring and cover plates, but the rest of the body (i.e. extraxial region) is preserved as a dark film and lacks any evidence of skeletal plating. Moreover, X-ray fluorescence analysis reveals that the axial region is elevated in iron. Based on our morphological and chemical data and on taphonomic comparisons with other fossils from the Kinzers Formation, we infer that the axial region was originally calcified, while the extraxial region was non-mineralized. Phylogenetic analyses recover Yorkicystis as an edrioasteroid, indicating that this partial absence of skeleton resulted from a secondary reduction. We hypothesize that skeletal reduction resulted from lack of expression of the skeletogenic gene regulatory network in the extraxial body wall during development. Secondary reduction of the skeleton in Yorkicystis might have allowed for greater flexibility of the body wall.

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Cambrian edrioasteroid reveals new mechanism for secondary reduction of the skeleton in echinoderms

Cited by 5 publications

References 60 publications

Crinoid calyx origin from stem radial echinoderms

Crinoid calyx origin from stem radial echinoderms

A Review and Evaluation of Homology Hypotheses in Echinoderm Paleobiology

Cambrian edrioasteroid reveals new mechanism for secondary reduction of the skeleton in echinoderms

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