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Evolutionary significance of the blastozoan Eumorphocystis and its pseudo-arms

Thomas E. Guensburg, James Sprinkle, Rich Mooi and Bertrand Lefebvre
Evolutionary significance of the blastozoan Eumorphocystis and its pseudo-arms
Journal of Paleontology (March 2021) 95 (2): 327-343


Twelve specimens of Eumorphocystis Branson and Peck, 1940 provide the basis for new findings and a more informed assessment of whether this blastozoan (a group including eocrinoids, blastoids, diploporites, rhombiferans) constitutes the sister taxon to crinoids, as has been recently proposed. Both Eumorphocystis and earliest-known crinoid feeding appendages express longitudinal canals, a demonstrable trait exclusive to these taxa. However, the specimen series studied here shows that Eumorphocystis canals constrict proximally and travel within ambulacrals above the thecal cavity. This relationship is congruent with a documented blastozoan pattern but very unlike earliest crinoid topology. Earliest crinoid arm cavities lie fully beneath floor plates; these expand and merge directly with the main thecal coelomic cavity at thecal shoulders. Other associated anatomical features echo this contrasting comparison. Feeding appendages of Eumorphocystis lack two-tiered cover plates, podial basins/pores, and lateral arm plating, all features of earliest crinoid 'true arms'. Eumorphocystis feeding appendages are buttressed by solid block-like plates added during ontogeny at a generative zone below floor plates, a pattern with no known parallel among crinoids. Eumorphocystis feeding appendages express brachioles, erect extensions of floor plates, also unknown among crinoids. These several distinctions point to nonhomology of most feeding appendage anatomy, including longitudinal canals, removing Eumorphocystis and other blastozoans from exclusive relationship with crinoids. Eumorphocystis further differs from crinoids in that thecal plates express diplopores, respiratory structures not present among crinoids, but ubiquitous among certain groups of blastozoans. Phylogenetic analysis places Eumorphocystis as a crownward blastozoan, far removed from crinoids.

ISSN: 0022-3360
EISSN: 1937-2337
Serial Title: Journal of Paleontology
Serial Volume: 95
Serial Issue: 2
Title: Evolutionary significance of the blastozoan Eumorphocystis and its pseudo-arms
Affiliation: Field Museum of Natural History, Integrative Research Center, Chicago, IL, United States
Pages: 327-343
Published: 202103
Text Language: English
Publisher: Paleontological Society, Lawrence, KS, United States
References: 66
Accession Number: 2021-045741
Categories: Invertebrate paleontology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, 3 plates
N34°00'00" - N34°30'00", W97°30'00" - W96°45'00"
Secondary Affiliation: University of Texas at Austin, USA, United StatesCalifornia Academy of Sciences, USA, United StatesUniversite Claude Bernard-Lyon I, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, Copyright, The Paleontological Society. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2021
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