292-7 Systematics and Paleoenvironmental Significance of a New Genus and Species of Irregular Echinoid from the Upper Cretaceous Bearpaw Formation, Alberta

Abstract:

Irregular echinoids are exceedingly rare in Upper Cretaceous marine deposits of Western Canada. The only previously documented species, Cassidulus taylori Warren, 1926, is known from seven poorly preserved endocasts from Birch Lake, Alberta, that lack most diagnostic test features. In 2023, new irregular echinoid specimens were collected from the upper Bearpaw Formation southeast of Drumheller, Alberta, on the Historic Graham Ranch between East Coulee and Dorothy. Unlike the C. taylori material, these specimens preserve much of the test, allowing detailed comparative morphological and cladistic analyses, which together support the erection of a new genus and species within the extinct family Faujasiidae (Neognathostomata, Clypeasteroida). The new echinoid closely resembles C. taylori, and the two taxa are interpreted as closely related, though differences in test morphology, including the position of the apical system, support recognition of a distinct taxon.

The new genus and species is the only North American faujasiid from farther north than Colorado, extending the family's known range within the Western Interior Seaway. Faujasiids are otherwise known almost exclusively from limestone and chalk deposits, so its recovery from fine- to medium-grained siliciclastics also expands the group's ecological range. Faujasiids have been proposed as the most primitive stem group of Clypeasteroida (sand dollars and relatives), derived from cassiduloid echinoids in the Late Cretaceous, though this transition remains poorly understood due to their scarcity and poor preservation. The new species has a highly derived morphology, with a suite of features intermediate between cassiduloids and clypeasteroids, including test flattening, reduced petals, and a more centrally positioned peristome, offering a rare data point for this transition.

The echinoid-bearing interval occurs approximately 3 m above the Dorothy Tongue and 34 m below the Zero Coal. Associated molluscs and trace fossils, together with the stenohaline physiology and functional morphology of the new species, indicate deposition in a moderate-energy, nearshore marine setting under stable euhaline conditions. This is the first detailed systematic study of an irregular echinoid from the upper Bearpaw Formation and documents a previously undescribed associated body-fossil assemblage. These results refine previous paleoenvironmental interpretations of the upper Bearpaw Formation, which relied largely on sedimentological and ichnological evidence, and demonstrate the value of integrating shelly fauna into reconstructions of the final stages of marine deposition in the study area.

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