Bajoprosopon piardi, van Bakel & Maerten & Jagt & Fraaije, 2021

van Bakel, Barry W. M., Maerten, Lionel, Jagt, John W. M. & Fraaije, René H. B., 2021, Bajoprosopon piardi n. gen. and sp. from the Middle Jurassic of France, with a revised diagnosis of the family Prosopidae von Meyer, 1860 (Brachyura, Podotremata) and notes on the availability of names introduced by Hermann von Meyer (1835, 1857), Palaeontologia Electronica (a 26) 24 (2), pp. 1-15 : 8-9

Bajoprosopon piardi n. sp.

Figure 4

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Material. A single dorsal carapace, MAB k.3764 (maximum carapace length, excluding rostrum = 12.2 mm, maximum carapace width = 8.7 mm) from the upper Bajocian ‘Oolithe ferrugineuse de Bayeux’ Formation (upper garantiana ammonite Zone, dichotoma ammonite Subzone), collected by one of us (LM) at the ‘Les Fours à Chaux’ outcrop between Croisilles and Les Moutiers-en-Cinglais (Calvados, France). It is preserved for the most part as an internal mould, but portions of cuticle remain.

Derivation of name. The species is named after the late Serge Piard, researcher and fossil collector, and good friend of one of us (LM).

Description. Carapace elongate, longer than wide, L/W approximately 1.40 (excluding rostrum), widest in branchial regions about one third of carapace length from posterior; convex in longitudinal and transverse cross-sections. Dorsal carapace regions well marked by grooves, with large tubercles, cuticle with granules. Rostrum partially preserved, only proximal portion remaining and appearing wide. Orbitofrontal margin occupying entire anterior carapace width, orbital fossae conspicuously large, directed anteriorly, suboval, well delineated, with interior arched rim; outer orbital corner sharp. Lateral carapace margins arched, notched at cervical and branchial grooves. Posterior margin wide, but poorly preserved. Epigastric regions as small nodes; protogastric regions with three tubercles parallel to mesogastric lateral margin, median tubercle largest; mesogastric region with narrow and pointed anterior process, bearing small central tubercle, posterior mesogastric portion large, widely subtriangular, with two large, horizontally lined tubercles. Urogastic region weakly delineated, rather low, with two widened tubercles at same level as tubercles on posterior mesogastric region, and two weaker tubercles laterally. Epibranchial regions oblique, with large tubercle halfway, near branchial groove, and two smaller tubercles near lateral margin. Cardiac region relatively small, diamond shaped, apex directed posteriorly, with large, low central swelling. Cervical groove complete, composed of three portions; lateral portions rather short, weakly arched; central portion as wide V, medially interrupted by shallow gastric pits. Branchial grooves distinct, slightly arched, subparallel to cervical groove; branchiocardiac grooves strongly arched. Posterior branchial region large, undifferentiated, weakly inflated. Remarks. In view of its superficially similar appearance, Bajoprosopon piardi n. gen. and sp. can be compared with Homolus auduini from the Bathonian at Ranville (Calvados, France). The most obvious differences are the branchial condensation and undefined/weakly defined orbits in Homolus , as described above. Of ornamentation, there is only a single protogastric spiniform tubercle in Homolus auduini , while there is a row with three blunt protogastric tubercles in Bajoprosopon piardi n. gen. and sp. Also, there are distinct epibranchial tubercles in Bajoprosopon piardi n. gen. and sp. , which are missing from the reduced epibranchial region in Homolus auduini .

Figures

FIGURE 4. Holotype of Bajoprosopon piardi n. gen. and sp. (MAB k.3764) from the upper Bajocian ‘Oolithe ferrugineuse de Bayeux’ Formation (upper garantiana ammonite Zone, dichotoma ammonite Subzone) at the ‘Les Fours à Chaux’ outcrop between Croisilles and Les Moutiers-en-Cinglais (Calvados, France; Figure 1). A, dorsal view of carapace. B, frontal view of carapace. C, left lateral view of carapace. Scale bar equals 5 mm.