Concavicaris georgeorum Schram, 1990

Laville, Thomas, Haug, Joachim T. & Haug, Carolin, 2021, New species of Thylacocephala, Eodollocaris keithflinti n. gen., n. sp., from the Mazon Creek Lagerstätte, Illinois, United States (c. 307 Ma) and redescription of other Mazon Creek thylacocephalans, Geodiversitas 43 (10), pp. 295-310 : 298-300

Concavicaris georgeorum Schram, 1990 ( Fig 3 ; 4 )

TYPE MATERIAL. — Holotype: SDSNH 36777 . — Paratypes: SDSNH 36759 , 36762 , 36793 , 36764 , 36768 , 36769 , 36774 ; PE 10853, 10916, 11020, 14105 15346, 15356, 23144, 23145, 23146, 24566, 24589, 24590, 24591, 29466, 29469, 30556, 30575, 31049, 31752, 32957, 40107, 45969, 51937 ; MCP 591 , 595 , 596 .

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 .

DIAGNOSIS (repeated from Schram 1990). — Carapace suboval in outline with a short rostrum, an optic notch prominently occupying half of the anterior aspect, a ventral margin marked by a notch anterior of its midpoint, a pointed postero-dorsal aspect and a single, dorsally situated longitudinal ridge.

EXAMINED MATERIAL. — ROMIP61586, 61587, 61591 (lateral view).

DESCRIPTION

Prominent appendage 2 (possible appendage of post-ocular segment 5, maxilla?) emerged from an anterior concavity of the shield ( Fig. 3 A-C). Proximal region not accessible, only the more distal parts recognisable. Six elements apparent. Proximal three not fully accessible, as they are partly concealed by the shield and only recognisable as compressed-through structures. Element 4 short, rectangular in lateral view, longer than wide, about 1.6× (l = 0.94 mm; w = 0.58 mm); oriented posteroventrally. Element 5 elongated, longer than wide, about 2× (l = 1.50 mm; w = 0.75 mm); oriented posteriorly. Element 6, distal element, not clearly visible, appears quite thin.

Prominent appendage 3 (possible appendage of post-ocular segment 6, maxilliped?) with ten elements ( Figs 3 D-F; 4A-H). Curved, with the proximal part directed antero-ventrally and the distal part directed anteriorly. Most proximal element, element 1, rectangular in lateral view, longer than wide, 1.6- 1.8× (l = 1.10-1.22 mm; w = 0.65-0.76 mm), with a spine in the latero-distal angle. Element 2 appears square-shaped, almost as wide as long (l = 0.91 mm; w = 0.81 mm). Element 3 is wider than long, 1.2× (l = 0.8 mm; w = 0.98 mm). The fourth element is trapezoidal, wider proximally than distally (l = 1.43-1.90 mm; wmax = 2.56 mm; wmin = 1.34 mm). It is directed ventrally. The fifth element is longer than wide, 1.2× (l = 1.29 mm; w = 1.06 mm). The sixth element is rectangular, longer than wide, 1.5× (l = 1.32 mm; w = 1.05 mm). The seventh element is also rectangular and is the shortest; it is longer than wide, 1.5× (l = 1.19 mm; w = 0.78 mm). The eighth element is massive and rectangular. It is longer than wide, 1.76× (l = 2.13 mm; w = 1.21 mm) and is angled at 44.3° anteriorly from the seventh one. The ninth element is elongate, longer than wide, 5× (l = 2.88-3.84 mm; w = 0.64-0.77 mm). It is angled at 50° anteriorly from the eighth element. The tenth element is thinner, longer than wide, 5.9× (l = 2.89 mm; w = 0.49 mm). It becomes thinner distally, forming a tip.

The trunk consists of seven visible segments. The first of the posterior trunk appendages seems to end with a spiny tip. The trunk terminates in a specialised terminal structure(furca?; Fig.4 ). The terminal structure becomes thinner distally and ends with three spines.The ventral one is longer than the other spines,1.3×.

R EMARKS

Schram (1990) interpreted a rectangular structure in the ventral part of the shield as the midgut. A similar structure is present in one of the new fossils ( Fig. 4C, D ). Yet it is probably not the midgut. Instead it seems more likely to be the proximal part of the third prominent appendage. This interpretation would explain the apparent subdivision into discrete units already recognised bySchram (1990). Most likely, these units correspond to the proximal three elements, the most proximal one most likely representing the basipod. If this interpretation is correct, the insertion of the appendage is further posterior than so far assumed.

The new specimens of C. georgeorum possess a specialised terminal structure that could represent a furca. So far, a comparable terminal structure was only known in Convexicaris mazonensis . The terminal structure in C. georgeorum differs from that of Co. mazonensis by bearing three spines distally, while the structure in Co. mazonesis is long paddle-shaped. Also it seems that the terminal structure in C. georgeorum might not be paired; if this is the case, an identity as furca is unlikely.

Figures

FIG. 3. — Concavicaris georgeorum Schram, 1990, ROMIP61587: A, general view; B, general view (color-marked); C, line drawing of general view; D, close-up of the third raptorial appendage; E, close-up of the third raptorial appendage (color-marked); F, line drawing of the third raptorial appendage. Abbreviations: a2, a3, second, third raptorial appendages; e, eyes; e4-e10, element 4-10; r, rostrum; s, shield; sp, posterior spine; st, stalk. Photographs: C. Haug and J. T. Haug. Line drawings: T. Laville. Scale bars: A-C, 5 mm; D-F: 1 mm.

FIG. 4. — Concavicaris georgeorum Schram, 1990: A-D, ROMIP61591, part; A, general view; B, close-up of the third raptorial appendage;C, general view (colormarked); D, line drawing; E, F, ROMIP61588; E, general view; F, close-up of the third raptorial appendage; G, H, ROMIP61591, counter-part; G, general view; H, line drawing. Abbreviations: a2, a3, second, third raptorial appendages; e, eyes; e1-e5, element 1-5; f, furca?; r, rostrum; s, shield; t, trunk. Arrows indicate the spines of the possible furca. Photographs: C. Haug and J. T. Haug. Line drawings: T. Laville. Scale bars: A, C-E, G-H, 5 mm; B,F, 1 mm.