Eodollocaris keithflinti

urn:lsid:zoobank.org:act: 1D59F66C-0B63-460B-94AB-F177841EC908

ETYMOLOGY. –— In honor of the late Keith Flint from the band ‘The Prodigy’. As the specimen exhibits a certain “aggressiveness” and the fossil is preserved in bright red colour reminding of fire, we herewith tribute to the song ‘Firestarter’ of the band.

TYPE MATERIAL. — Holotype: ROMIP61586.

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. — Trapezoidal shield displaying a rounded rostrum, a tube-shaped dorsal midline, a finely tuberculated ventral margin, a dorsal midline ending posteriorly with a spine and a posterior margin with a notch in its dorsal part. Third prominent appendage bearing seven spines on its most distal element.

DESCRIPTION

Body

Largely enveloped by prominent folded shield (Figs 6 A-D; 7A, B). Few structures protruding from it. Exact origin of shield unclear, most likely relatively far anterior, possibly from segments of head. Shield trapezoidal in lateral view; longer than high, about 2.8× (h = 6.13 mm, hp = 4.97 mm; ls = 16.93 mm; lw = 15.23 mm). Antero-dorsal corner drawn out anteriorly into a rostrum with a rounded tip (l = 1.70 mm). The anterior margin is straight, oriented postero-ventrally (c. 30°). No clear optical notch distinguishable. Dorsal midline is slightly convex. Postero-dorsal corner drawn out posteriorly into small spine. Ventral margin is convex. Rim in this region is bearing numerous fine tubercles and appears slightly bent in its central part (Fig. 6F). Posterior margin further dorsally concave forming distinct notch, ventral part straight and oriented antero-ventrally (c. 30°). Dorsal midline connecting the two valves tube-shaped, with numerous pores along the entire dorsal line, terminated by the rostrum (Fig. 6E).

Large eyes

Anteriorly protruding from under the shield, prominent (l = 0.94 mm; Fig. 6 A-D). Appear to arise from massive proximal stalks. Posterior to eyes, a massive oval structure emerging ventrally from the shield. Exact origin unclear.

Two prominent appendages

Protruding from under the shield (Figs 6G, H; 7C, D), interpreted as raptorial appendages 2 and 3 (possibly appendages of post-ocular segments 5 and 6 = maxillae? and maxillipeds?). Proximal parts concealed by a tube-shaped structure, probably a part of the body.

Raptorial appendage 2

Differentiated into four elements. Element 1 rectangular in lateral view, longer than wide, 1.2× (l = 1.53 mm; w =

1.33 mm). Element 2 rectangular, longer than wide, about 2.1× (l = 1.85 mm; w = 0.87 mm), angled in the specimen at 51° downward from the first article. Element 3 not wellpreserved.Width appears similar to that of preceding element. Element 4 longest (Fig. 6G), thin, longer than wide, 5.6× (l = 3.92 mm; w = 0.70 mm), angled at 50° inward from element 3. Distally bearing two spines on its median margin. First spine smaller (l = 0.79 mm). Second spine very distal. Spine longer than the first one and directed anteriorly.

Raptorial appendage 3

Better preserved, longer than raptorial appendage 2. Differentiated into four elements (Fig. 7C, D). Element 1 rectangular, almost square-shaped (l = 2.13 mm; w = 2.03 mm); bearing a stout spine in the central part of the lateral margin (l = 0.38 mm). Element 2 hexagonal in outline, slightly rounded, wider in its central part than at its extremities (wmax = 2.19 mm; wmin = 1.05 mm; l = 1.79 mm). Bearing a spine on central part of lateral and median margin. Lateral spine longer (l = 0.28 mm) than median one (l = 0.16 mm). Element 3 rectangular, longer than wide, about 3.6× (l = 4.27 mm; w = 1.18 mm), angled at 24.3° from element 2. One spine on median margin (l = 0.37 mm). Element 4, distal one, also the longest one (l = 6.32 mm); rectangular, thin and elongated, longer than wide, about 7.2× (w = 0.88 mm), angled at 56.6° anteriorly from element 3. Bears seven spines on median margin (Fig. 7C, D). First one is thin (l = 0.65 mm); second one also thin (l = 0.58 mm), third also thin, longest of the series (l = 0.68 mm); fourth spine shorter (l = 0.44 mm); fifth similar to fourth (l = 0.45 mm); sixth spine shortest (l = 0.20 mm), close to seventh spine. Seventh spine corresponds to distal end of appendage; slightly curved, longer than the previous spine, about 3.2× (l = 0.64 mm).

Posterior part of trunk

Only the posterior part of trunk is visible. Carrying three appendages. First posterior trunk appendage paddle-like. Second one not well preserved but bearing a spine distally. Third, also paddle-like, arising from end of trunk.

REMARKS

The new specimen possesses a so far unique combination of characters. The rostrum with a rounded tip seems to be a common feature in Palaeozoic species of Thylacocephala, especially in species of Concavicaris Rolfe, 1961. The presence of such a rostrum has also been suggested to be a diagnostic feature of representatives of the group Ankitokazocaris Arduini, 1990 of the Triassic.

However, it differs from known representatives of both groups, Concavicaris and Ankitokazocaris, by the absence of a well-developed optic notch. Also, the new specimen possesses a posterior spine and a posterior notch, both structures are absent in species of Concavicaris and Ankitokazocaris. Yet, they are well known in specimens from younger deposits, such as representatives of Atropicaris Arduini & Brasca, 1984, Dollocaris Van Straelen, 1923 or Mayrocaris Polz, 1994. In these species, the posterior notch occupies the entire posterior margin whereas in the new specimen it is restricted to the dorsal part of the margin.

Among the species from younger deposits, especially Paradollocaris vannieri Charbonnier, 2017 shows many similarities concerning shield shape with the new specimen. The shield is trapezoidal with a convex dorsal midline. Anteriorly it is drawn out into a rostrum, which is rounded distally. The posterior end is also drawn out into a spine. The posterior margin also has a pronounced notch. Similar to other species from the Mesozoic, the notch occupies the entire posterior margin. The ventral margin of the new specimen seems unique with the anterior part being tuberculated.

Concerning the raptorial appendages, the new specimen exhibits particular differences in form and armature compared to other thylacocephalans from Mazon Creek. Convexicaris mazonensis and Concavicaris georgeorum have less robust appendages, and they do not display any spines. Due to the significant differences of the new specimen we interpret it as a representative of a new, so far unrecognised species: Eodollocaris keithflinti n. gen., n. sp.