Ostenocaris ribeti ( Secrétan, 1985 ) Laville & Forel & Charbonnier, 2023

Laville, Thomas, Forel, Marie-Béatrice & Charbonnier, Sylvain, 2023, Re-appraisal of thylacocephalans (Euarthropoda, Thylacocephala) from the Jurassic La Voulte-sur-Rhône Lagerstätte, European Journal of Taxonomy 898 (1), pp. 1-61 : 25-29

Ostenocaris ribeti ( Secrétan, 1985) comb. nov.

Figs 5C , 11–12

Clausia ribeti Secrétan, 1985: 388 , fig. 5d, i–j.

Clausia sp. – Secrétan & Riou 1983: 85, pl. IV figs 2, 4–5.

Clausia ribeti – Arduini 1992: 266.

Clausocaris ribeti – Schram et al. 1999: 772. — Charbonnier 2009: 205–206. — Charbonnier et al. 2010: 115, 117, 121, tab. 1, fig. 5d. — Schram 2014: 351. — Rak et al. 2018: 268, tab. 1.

Original diagnosis ( Secrétan 1985)

The new species differs from the type species Clausia lithographica Oppenheim in its much stronger anterior appendages. Like the type species, C. ribeti is small, has a rounded carapace and large eye with oval lateral aspect. The appendages, however, have very fine paintbrush like extremities and are very much shorter and very much more massive than these of C. lithographica .

Emended diagnosis

Ostenocaris with a tuberculate, club-shaped dorso-lateral carina and hook-shaped first raptorial appendages.

Material examined

Holotype FRANCE • La Boissine, La Voulte-sur-Rhône, Ardèche , France; Callovian, Middle Jurassic; MNHN.F. R50981 .

Additional material

FRANCE • 11 specs; same collection data as for holotype; MNHN.F. A29349 , A29365 , A29368 , A29372 , A84023 to 84028 , R50970 .

Measurements

See Table 2 .

Description

Shield morphology

Sub-oval shield in lateral view with a sinuous and proverse anterior margin, a sharp, antero-dorsal corner, a convex dorsal midline with a short flattening near the rounded postero-dorsal corner, a straight and vertical posterior margin, a rounded postero-ventral corner a ventral margin consists of a posterior half steeply descending antero-ventrally and of a horizontal anterior one ( Figs 11–12 ). Rim of all free margins delimiting a marginal fold.

Shield macro-ornamentation

Shield adorned by two large, tuberculate carinae on its lateral sides: a club-shaped horizontal dorso-lateral carina and a sinuous medio-lateral carina ( Figs 11A , 12A–C, G ).

Sensory structures

Two oval, sessile compound eyes protruding from the optic notch, occupying most ( Fig. 11 ).

Cephalic appendages

One elongate structure, consisting of at least three elements, lying in front of the raptorial appendages ( Fig. 12A–C ).

Raptorial appendages

Presence of three pairs of raptorial appendages recognised by preservation of muscles, protruding from the ventral margin, posterior to the presumed cephalic appendages ( Fig. 12A–D ). Raptorial appendages consisting of two parts: a proximal postero-ventrally oriented and a distal one, antero-ventrally oriented. Articulation between both parts forming an elbow.

First pair of raptorial appendages, the shortest, having a hook-shaped morphology ( Fig. 12D ). Proximal part consisting of two rectangular podomeres: podomere 1 being 2.5 times as long as wide; podomere 2 being 8 times as long as wide. Distal part formed by three rectangular podomeres: podomere 3 being 3.4 times as long as wide; podomere 4 being 3.4 times as long as wide; podomere 5 being 1.7 times as long as wide and ending in a spiny tip.

Second pair of raptorial appendages consisting of a proximal part with two rectangular podomeres ( Fig. 12D ): podomere 1 not being well preserved; podomere 2 being 4.6 times as long as wide. Distal part of the appendage consisting of three podomeres: podomere 3 being 3.8 times as long as wide; podomere 4 being 5 times as long as wide; podomere 5 being 2.7 times as long as wide and ending in a spiny tip.

Third pair of raptorial appendages, the longest, consisting of a proximal part with two long rectangular podomeres ( Fig. 12D ): podomere 1 being 1.8 times as long as wide; podomeres 2 being 5.2 times as long as wide. Distal part consisting of three podomeres: podomere 3 being 4.7 times as long as wide; podomere 4 being 5 times as long as wide and bearing a row of spine on its medial and lateral sides; podomere 5 being slightly curved and 2.9 times as long as wide.

Posterior trunk

Presence of seven rectangular posterior trunk segments, with only three rectangular pleural parts preserved ( Fig. 11F–G ). Trunk appendages not preserved.

Gills

Six oval, short gills located below the median part of the shield ( Fig. 11F–G ).

Remarks

New observations of the holotype of Clausocaris ribeti indicate that this species has the general morphology of Ostenocaris : a proverse anterior margin with a sharp antero-dorsal angle, a convex dorsal midline, a straight posterior margin, a marginal fold on all free margins, presence of rows of spines on the medial and lateral end of podomere 4 of the third pair of raptorial appendages. This species also displays major differences with Clausocaris : absence of an optic notch and of serrations on the dorsal margin, presence of a posterior margin, differentiation of the ventral margin into a posterior and an anterior parts, short and stout raptorial appendages. Thus, we propose the new combination Ostenocaris ribeti ( Secrétan, 1985) comb. nov.

Here, it is also important to note that the paratype of C. ribeti (MNHN.F.R50978) is no longer considered to be a representative of this species. Based on its morphology, it is now considered to belong to Paraostenia voultensis . As pointed out by Laville et al. (2021c), there has been a lot of confusion between Paraostenia Secrétan, 1985 and Ostenocaris due the wrong assignement of specimens of Paraostenia from the Osteno Lagerstätte, Italy, to Ostenocaris (see Arduini et al. 1980). Both taxa differ from each other based on the morphology of their anterior margin (sinuous for Ostenocaris vs large optic notch for Paraostenia ), of their dorsal margin (presence of carina in Paraostenia ) and of their posterior margin (straight vs concave).

Ostenocaris ribeti is the second species of Ostenocaris described. The main difference with the type species is the size: O. ribeti is only 1.7 cm long while O. cypriformis can reach a length of 20 cm. Ostenocaris ribeti also differs from O. cypriformis by its large, tuberculate and elongate dorso-lateral and medio-lateral carinae and its hook-shaped first raptorial appendages. Its dorsal midline also appears less convex.

Figures

Fig. 5. Scatter plots of length of the shield against maximal height of the shield. A. Kilianicaris lerichei Van Straelen, 1923. B. Ostenocaris ribeti (Secrétan, 1985) comb. nov. C. Paraclausocaris harpa gen. et sp. nov. D. Paraostenia voultensis Secrétan, 1985. Abbreviations: Hmax = maximal shield height; Ls = length of the shield.

Fig. 11. Ostenocaris ribeti (Secrétan, 1985) comb. nov. A. Holotype MNHN.F.R50981. B. Specimen MNHN.F.A29349. C. Specimen MNHN.F.A29368. D–E. Specimen MNHN.F.A84023. D. General view. E. Drawing. F–G. Specimen MNHN.F.A29365. F. General view. G. Drawing. Abbreviations: am = anterior margin; b = body; ce = compound eye; dlc = dorso-lateral carina; dm = dorsal midline; g = gills; mlc = medio-lateral carina; p 1–5 = podomeres of raptorial appendages; ra 2–3 = raptorial appendages; tp1–7 = pleural part of the posterior trunk; vm = ventral margin. Scale bars: 5 mm. Photos: A: J. Falconnet (RECOLNAT – MNHN); B–C, F: T. Laville; D: L. Cazes.

Fig. 12. Ostenocaris ribeti (Secrétan, 1985) comb. nov. A–D. Specimen MNHN.F.A29372. A. General view. B. General view (colour-marked). C. Drawing. D. Close-up of raptorial appendages. E. Specimen MNHN.F.R50970. F. Specimen MNHN.F.A84024. G–H. Specimen MNHN.F.A84027. G. General view. H. Drawing. Abbreviations: am = anterior margin: ca = cephalic appendage; ce = compound eye; dlc = dorso-lateral carina; dm = dorsal midline; mlc = medio-lateral carina; p 2–5 = podomeres of raptorial appendages; pm = posterior margin; ra1–3 = raptorial appendages; vm = ventral margin. Scale bars: A–C, E–H = 5 mm; D = 1 mm. Photos: A–B, G: L. Cazes; D–F: T. Laville.