Laeviprosopon gignouxi ( Van Straelen, 1928 )

Charbonnier, Sylvain, Garassino, Alessandro & Brochet, Richard, 2025, A new furry lobster (Achelata, Synaxidae) and a forgotten crab (Brachyura, Homolidae) from the Early Cretaceous of France, Geodiversitas 47 (21), pp. 739-747 : 743-746

Laeviprosopon gignouxi ( Van Straelen, 1928) , n. comb.

( Fig. 3)

Prosopon gignouxi Van Straelen, 1928: 606 , figs 1, 2; 1936: 26. — Withers 1951: 184.

TYPE MATERIAL. — Holotype. France • 1 specimen (carapace; adult); Auvergne-Rhône-Alpes, Drôme, Loriol-sur-Drôme ; Early Cretaceous, Hauterivian; Maurice Gignoux leg.; UJF-ID1826 .

TYPE LOCALITY. — Loriol-sur-Drôme, Drôme, Auvergne-Rhône-Alpes, France.

TYPE AGE. — Early Cretaceous, Hauterivian.

ORIGINAL DIAGNOSIS BY Van Straelen (1928: 607). — Carapace convex transversally, inflected anteriorly, wider posteriorly, widest at level of posterior margin; half-circle-shaped cervical groove; gastro-antennal grove marking a median gastric region bearing two crests posteriorly at level of cervical groove; postcervical groove deep medially, originating a groove marking the mesobranchial region; branchiocardiac groove extending posteriorly, originating a forward-directed short groove marking the epibranchial region; hepatic regions with a groove; marked cardiac regions narrowing posteriorly bearing two tubercles posteriorly; grooved branchial regions on their anterior median part (literal translation from the original French text).

SYSTEMATIC REMARKS

According to Van Bakel et al. (2021: 7), Laeviprosopon has generally been included in the Prosopidae Meyer, 1860 by several authors (see Schweitzer & Feldmann 2008; Guinot 2019; Klompmaker et al. 2020; Starzyk 2020). However, Van Bakel et al. (2021) excluded it from this family, because it possesses a linea homolica (see Patrulius 1966; Collins & Wierzbowski 1985), a weakly projecting trifurcate front, a non-sinuous, arched concave posterior margin, and undefined orbits (the cornea resting on the dorsal carapace surface). In addition, Laeviprosopon usually has a complete oblique groove that subdivides the hepatic regions. The genus is interpreted as an early representative of the superfamily Homoloidea , and thus excluded from the Homolodromioidea Alcock, 1900, adopting the views expressed by several previous authors ( Patrulius 1966; Collins & Wierzbowski 1985), and recently by Starzyk et al. (2023). However, the presence or not of a molting line similar to the linea homolica in Laeviprosopon is not well clarified yet though three species of the genus show this character. In the extant representatives of the Homolidae , the linea homolica is always present in all individuals, not in the molting specimens only (A.G., pers. obs. 2023). Anyway, the discussion of the systematics of Laeviprosopon is not the aim of the present note.

In conclusion, we follow herein the most recent updated systematics proposed byStarzyk et al. (2023) for Laeviprosopon based on the presence of a molting line in the type species and other species of the genus.

EMENDED DESCRIPTION

Carapace

Small carapace uniformly granulated, longer than wide moderately vaulted transversally and longitudinally; nearly straight frontal margin poorly preserved; rostrum not preserved; poorly preserved?small orbits; anterolateral margins slightly narrowing frontally; posterolateral margins slightly rounded weakly narrowing posteriorly: posterior margin not preserved; very narrow epigastric region; slightly swollen hepatic regions; slightly swollen protogastric regions; elongate, narrow anterior metagastric process; mesogastric region with a pair of swollen distal tubercles and a pair of small mesogastric groove tubercles; small gastric pits between meso- and urogastric regions; narrow urogastric region; diamond-shaped cardiac region with a pair of aligned tubercles medially and one tubercle distally; wide intestinal region; slightly swollen epi-, meta-, and mesobranchial regions; deep cervical groove reaching lateral margins; deep branchiocardiac grooves, delimiting cardiac region and reaching posterior margin; lateral transverse groove joining cervical to branchiocardiac grooves; molting line not present.

Thoracic appendages

Elongate, subcylindrical P2-P4 partially preserved, flattened laterally, ovoidal in transverse section; elongate P2-P4 meri with smooth dorsal and ventral margins; narrow subtriangular P2-P4 carpi shorter than meri.

DISCUSSION

Van Straelen (1928: 606) assigned the studied specimen to “ the tribe Dromiacea , subtribe Dromiidea and the family Prosoponidae ”, but he did not provide a detailed discussion to justify his assignment, limiting to comparisons with several other Jurassic and Cretaceous dromiacean crabs known at that time. Prosopon gignouxi was not reported or discussed in any systematic study after Withers (1951).

Our examination of the holotype leads us to highlight major affinities with Laeviprosopon , which was recently revised by Starzyk et al. (2023). According to these authors, the diagnostic characters of Laeviprosopon are as follows: carapace rectangular, longer than wide, often narrowing slightly anteriorly; regions well defined by deep grooves; rostrum, projected well beyond orbits; orbits shallow, rimmed, directed forward, located at base of rostrum; augenrest poorly formed to nearly absent; subhepatic region inflated, sometimes greatly; epigastric, protogastric, and hepatic regions separated by transverse grooves into three anteriorly to posteriorly successive regions; cervical groove deep, strongly concave forward to sinuous; branchiocardiac groove shallower than cervical groove; postcervical groove short, interrupted axially; posterolateral portions of carapace often broken as if less calcified than other portions of carapace. Although the holotype does not preserve a complete frontal margin, rostrum, and orbits, and despite the lack of a molting line ( linea homolica), the morphological characters listed in our updated description are diagnostic of Laeviprosopon , and we therefore propose the new combination Laeviprosopon gignouxi ( Van Straelen, 1928) , n. comb.

We add that the holotype is a corpse and is furthermore the only specimen partially preserving P2-P4, previously unknown in all representatives of the family.

According to Starzyk et al. (2023), Laeviprosopon icaunensis ( Van Straelen, 1936) from the Hauterivian of Yonne (Bourgogne-Franche-Comté) is the only other species of the genus reported to date in France ( Fig. 4). Although most of the central dorsal carapace is not preserved in the holotype of L. icaunensis , it differs from L. gignouxi n. comb. by a carapace bearing hepatic regions with two grooves, a cervical groove not rimmed, and the median part of branchial regions without groove. Moreover, L. icaunensis has a smooth carapace (vs granulated in L. gignouxi n. comb.), and is more convex antero-posteriorly with a strongly downward projected rostrum. Its frontal margin is more rounded. There are two shallow flattened epibranchial tubercles/bulges (vs smooth epibrachial regions in L. gignouxi n. comb.), and a shorter epigastric region (as preserved).

In conclusion we consider L. gignouxi n. comb. as a valid distinct species within the genus as actually regarded ( Starzyk et al. 2023).

Van Straelen (1928) noted that Laeviprosopon gignouxi n. comb. was collected in the deep-water sediments of the Vocontian Trough, and thus considered it as the only bathyal species known to date within the genus. Another interpretation could be that the crab specimen – as most other known species – likely did not live in this deep-water setting but was transported from the neighbouring shallow-carbonate platforms. The transport may have been brief as attested by the completeness of the specimen. It is not possible to decide which of these two hypotheses is the most consistent one.