TYPE SPECIES. — Convexicaris mazonensis Schram, 1990 from Carbondale Formation, Francis Creek Shale Member, Mazon Creek area, Illinois, United States.

DIAGNOSIS (repeated fromSchram 1990). — Sinuously shaped carapace displaying a rounded antero-dorsal part, a convex antero-lateral margin, and a blunt postero-ventral aspect. Elongate subchelate raptorial limbs composed of narrow articles. Body ending by long caudal rami.

Convexicaris mazonensis Schram, 1990 (Fig. 5)

TYPE MATERIAL. — Holotype:PE 32958.— Paratypes:PE 11255, 23525, 38169, 39350, 40076, 45692, 45695; SDSNH 36781; MCP 594.

TYPE HORIZON. — Carbondale Formation, Francis Creek Shale Member, c. 307 Ma, Westphalian D, Middle Pennsylvanian, Carboniferous

TYPE LOCALITY. — Mazon Creek area, Pit 11, Illinois, United States.

EXAMINED MATERIAL. — ROMIP 47987 (lateral view).

DIAGNOSIS. — Same as for genus.

DESCRIPTION

The new specimen provides important details about the trunk, especially about the appendages (Fig. 5 A-D). The shield is longer than high, 2.1× (ls = 16.7 mm; h = 7.85 mm). The posterior trunk is obviously organised into eight discrete segments, especially indicated by eight rectangular appendages emerging from under the shield. The distal end of the appendages appears bifid. The appendages are 1.69 mm to 1.89 mm long.

REMARKS

Trunk appendages have been known for several species of Thylacocephala (Arduini et al. 1980; Secretan 1985; Arduini 1992; Polz 1990, 1994, 2001; Lange et al. 2001; Haug et al. 2014; Ji et al. 2017; Braig et al. 2019; Jobbins et al. 2020). In most cases they appear to be either paddle-like or multiannulated. The morphology observed here, paddle-like, but with a bifid distal end, appears new. Yet, in fact our knowledge of the posterior trunk appendages is rather incomplete. Therefore, this may in fact be a more common feature.

Although Schram (1990) suggested that Co. mazonensis should possess eight posterior trunk segments with swimming appendages, this was not really apparent in his specimens. The new specimen now clearly shows eight distinct such appendages and hence supports the assumption by Schram (1990).