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Echinoids, or sea urchins, are rare in the Palaeozoic fossil record, and thus the details regarding the early diversification of crown group echinoids are unclear. Here we report on the earliest probable crown group echinoid from the fossil record, recovered from Permian (Roadian-Capitanian) rocks of west Texas, which has important implications for the timing of the divergence of crown group echinoids. The presence of apophyses and rigidly sutured interambulacral areas with two columns of plates indicates this species is a cidaroid echinoid. The species, Eotiaris guadalupensis, n. sp. is therefore the earliest stem group cidaroid. The occurrence of this species in Roadian strata pushes back the divergence of cidaroids and euechinoids, the clades that comprise all living echinoids, to at least 268.8 Ma, ten million years older than the previously oldest known cidaroid. Furthermore, the genomic regulation of development in echinoids is amongst the best known, and this new species informs the timing of large-scale reorganization in echinoid gene regulatory networks that occurred at the cidaroid-euechinoid divergence, indicating that these changes took place by the Roadian stage of the Permian.
Figure 1. Eotiaris guadalupensis n. sp.(A) Paratype USNM 610604. Interambulacral area fragment first mentioned in Kier24 from Roadian of the Glass mountains. Note two column interambulacral area structure indicative of crown group echinoids. (B) Holotype USNM 610600. Interambulacral area fragment and associated spine. Note crenulate tubercles. (C) USNM 610605a. displaying clavate, bulbous spine morphology. (D) Paratype USNM 610604. Internal view of interambulacral fragment showing apophyses at adoral end. (E) Paratype USNM 610601. Interambulacral fragment of larger specimen. Note at least six plates in ambulacral columns and crenulate tubercles with sunken areoles. Plates rigid at least below adapical plates. (F) Paratype USNM 610605b. Spine displaying less clavate morphotype and spinules. (G) Internal view of interambulacral area of paratype USNM 610602. Note apophyses, which identify this species as a cidaroid, and denticulate adambulacral plate margin indicative of beveling. (H) Close up of apophyses of USNM 610602. All scale bars represent 2.5 mm.
Figure 2. New divergence date of the divergence of cidaroid and euechinoid clades based on the Roadian occurrence of Eotiaris guadalupensis n. sp.Thick lines represent fossil range and thin lines represent inferred range based on phylogenetic relationships. The establishment of E. guadalupensis as the oldest known cidaroid in the fossil record also extends the inferred range of euechinoids, as the oldest known euechinoids, Diademopsis herberti, and Hemipedina hudsoni are first found in the fossil record in the Norian, 40 Ma years later. Phylogenetic relationships are from Kroh and Smith1 and Kroh35 modified with information regarding phylogenetic placement of E. guadalupensis from Supplementary Figure S2.
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