Dromilites belli, van Bakel & Robin & Charbonnier & Saward, 2017

van Bakel, Barry W. M., Robin, Ninon, Charbonnier, Sylvain & Saward, Jeff, 2017, Revision of Dromilites bucklandii (Crustacea, Decapoda, Brachyura): Type material revealing its real identity, a junior synonym, and a new species, Palaeontologia Electronica 20 (3), pp. 1-12 : 5-9

Dromilites belli n. sp.

Figures 2–3 , Table and Appendix zoobank.org/ 951EDF0C-6E01-445F-9E6F-45EB3E3B0D75

v. 1858 Dromilites bucklandi H. Milne Edwards ; Bell, p. 31-32, pl. VI, figs. 1-11.

v. 1898 Dromilites bucklandi H. Milne Edwards ; Carter, p. 18.

v. 1906 Dromilites bucklandi H. Milne Edwards ; Schröder, p. 7.

v. 1925 Dromilites bucklandi H. Milne Edwards ; Gripp, p. 129.

v. 1928 Dromilites bucklandi H. Milne Edwards ; Beurlen, p. 164.

v. 1929 Dromilites bucklandi H. Milne Edwards ; Glaessner, p. 139.

v. 1945 Dromilites bucklandi H. Milne Edwards ; Wrigley, p. 217.

v. 1966 Dromilites bucklandi H. Milne Edwards ; Davidson, p. 211.

v. 1969 Dromilites bucklandi H. Milne Edwards ; Glaessner, p. R487, fig. 297.3.

v. 1977 Dromilites bucklandi H. Milne Edwards ; Cooper, p. 172.

v. 1982 Dromilites bucklandi H. Milne Edwards ; Förster and Mundlos, p. 155.

v. 2006 Dromilites bucklandi H. Milne Edwards ; Collins and Saward, p. 68-69, table 1.

v. 2009 Dromilites bucklandi H. Milne Edwards ; Rayner et al., p. 56.

v. 2010 Dromilites bucklandi H. Milne Edwards ; Schweitzer et al., p. 64.

v. 2010 Dromilites bucklandi H. Milne Edwards ; Schweitzer and Feldmann, p. 422-432, fig. 2.

Etymology. The species name is dedicated to the

British paleontologist Thomas Bell, who erected in

1858 the genus Dromilites previously proposed by

H. Milne Edwards (in 1837) by assigning species

Dromia bucklandi and Dromia lamarckii to the genus.

Type material. Holotype: NHMUK PI OR 59089

(fig. 2.1) and 6 paratypes: NHMUK PI OR 59091

(fig. 2.2), MAB.k 3583-3587 (fig. 3).

Additional examined material. MNHN.F.R03853-

4 and MNHN.F.A66911 (casts of specimens figured in Bell, 1858, Plate VI, figs. 8-10) (London Clay,

Copenhagen Fields, London), ‘specimen 1’ figured in Rayner et al., 2009, p. 56 (London Clay, Division

B1, Seasalter foreshore exposure, Kent, United

Kingdom, 51°20'36.77"N / 1°0'2.76"E) (see Table).

Description. Carapace subcircular, length and width subequal (holotype dimensions: CL = 35.5

mm, CW= 37 mm, FM= 18.5 mm, OFM= 16 mm,

PM= 20.5 mm), maximum width at midlength to in posterior half, strongly convex in longitudinal and transverse cross sections; orbitofrontal margin prominent, wide, 43% maximum carapace width.

Rostrum projected beyond orbits, broadly triangular, bilobed, axially notched, frontal margin with four triangular sharp teeth, median two from rostrum advanced; orbits forwardly, outwardly directed, upper orbital margin concave, outer orbital corner pointed. Lateral margin prominent;

anterolateral margin weakly arched, with four strong, triangular, broad-based teeth; two anterior, two posterior the cervical notch; third lateral spine largest, flattened, more anteriorly directed. Posterolateral margin rounded in cross section, conspicuously short, strongly curved towards posterior margin, anteriorly bearing single prominent tooth. Posterior margin narrower than orbitofrontal margin, straight, distinctly rimmed.

Carapace regions strongly swollen, distinctly marked by large, rounded tubercles, deep, wide groove system; small epigastric swellings bounding anterior mesogastric process, mesogastric region narrowly triangular, posterior mesogastric region large, wide, strongly vaulted, with two large horizontally lined prominent tubercles, divided by short distinct median groove. Protogastric regions with two low tubercles, one anteriorly, one posteriorly, wider spaced; hepatic region flat, inclined. Metagastric region trapezoidal, wide anteriorly, posterior margin concave, arched, surface wrinkled. Urogastric region undefined. Cardiac region distinctly defined, large, strongly inflated, diamond-shaped, apex pointed posteriorly, large central tubercle bearing two horizontally lined pits; intestinal region small, depressed. Branchial region large, subdivided by oblique postcervical, branchial grooves; epibranchial, mesobranchial regions bearing single large tubercle, mesobranchial tubercle largest, wider spaced, laterally marked by large pit; metabranchial region wide, short, conspicuously swollen, bearing numerous pits.

Cervical groove widely V-shaped, deep, distinct on medial carapace, interrupted at axis by gastric pits, faint laterally, weakly notching carapace margin; branchial grooves subparallel to cervical groove, wider, distinctly notching lateral margin; post-cervical grooves shallow, indistinct, separating branchial tubercles; branchiocardiac grooves conspicuously deep, anteriorly longitudinally directed, weakly concave, posteriorly converging to posterior margin.

Carapace epicuticle with numerous evenly spaced setal pits. Internal mold smooth.

Third maxillipeds long, pediform, coxae large, flabelliform, endopod robust, exopod slender, elongate. Chelipeds well developed, sub-homochelous, merus subtriangular in cross section; manus outer surface smooth, bulbous, spine on proximal upper surface; pollex scoop-shaped, pointed teeth on occlusal surface; dactylus curved. Pereiopods two, three moderately sized, pereiopod four, five reduced, carried subdorsally. Distal ends not preserved, most probably subchelate.

Male sternum narrow, sternite four elongate, rounded, posterior sternites strongly curved. Male pleon narrow, telson longer than wide, reaching base of chelipeds; somite six long, uropods dorsally visible, small; somites four and five short, wide. Female pleon moderately wide; telson conspicuously large, longer than wide, reaching anterior end of coxae of chelipeds; somite five rectangular; somite six with large, triangular uropods.

Range. London Clay, Ypresian, lower Eocene.

Occurrence. London basin.

Remarks. This new species is assigned to Dromilites based on the following morphological characters: a carapace slightly wider than long; a bidentate rostrum; the anterolateral margins with widened teeth, an entire cervical groove, the carapace regions well defined, with sharp tubercles, and a prominent cardiac region. Dromilites belli n. sp. is morphologically closest to the species Dromilites montenati Robin, Van Bakel, Pacaud, and Charbonnier, 2016 in showing comparable highly defined carapace regions with marked tubercles, but differs in displaying four anterolateral (instead of five) and one posterolateral teeth (instead of none), prominent tubercles on the posterior mesogastric region, a less defined urogastric region, a more carved cardiac region with a large central tubercle bearing two horizontally lined pits, epibranchial and mesobranchial regions bearing a single large tubercle (instead of two); and in having cervical and branchial grooves notching the lateral carapace margins less and more distinctly respectively. Dromilites belli n. sp. differs from D. bucklandii , D. pastoris and? D. cardwelli in having less distinct grooves, in particular a much weaker cervical groove, and distinctly vaulted carapace regions with strong tubercles (weakly vaulted carapace regions, without distinct tubercles in D. bucklandii , D. pastoris and? D. cardwelli ) in all size ranges. D. belli n. sp. differs from D. vicensis in having a relatively shorter carapace (however, ratio length/width varies considerably; see Table), a well-defined mesogastric region, a distinctly vaulted cardiac region, and presence of strong tubercles.

The London Clay dromiid crabs are morphologically very similar. There are a number of differences by which D. belli n. sp. and D. bucklandii are separated. The anterolateral margin in D. bucklandii has three teeth instead of four in D. belli n. sp., in which only a single tooth is present between the cervical and branchial grooves. This third anterolateral tooth in D. bucklandii is flattened posteriorly, giving a flanged appearance. The posterolateral margin in D. belli is strongly curved, whereas it is anteriorly straight in D. bucklandii . This results in a relatively wider posterior margin in D. bucklandii . The single posterolateral tooth, just posterior the branchial groove, is subtle in D. bucklandii , while it is well-developed in the second species. The carapace regions in D. bucklandii are weakly vaulted, whereas they are strongly inflated in D. belli n. sp. Also the mesogastric region is undefined in D. bucklandii while it is visible in D. belli n. sp. Large, coarse tubercles mark the gastric and branchial regions in D. belli , while these are absent in D. bucklandii . Finally, there are two weak bumps with a pit on the cardiac region of D. bucklandii ; these are merged into one strongly inflated tubercle in D. belli n. sp.

Figures

FIGURE 2. Dromilites belli n. sp. type series from the Ypresian of London Clay (Copenhagen Fields, London, United Kingdom). 1. Holotype NHMUK PI OR 59089 (Wetherell coll.) coated with ammonium chloride (female individual): 1a. carapace in dorsal view, 1b. carapace in frontal view, 1c. carapace in ventral view, 1d. carapace in left-lateral view; 2. Paratype NHMUK PI OR 59091 (Wetherell coll.) coated with ammonium chloride (male individual): 2a. carapace in dorsal view, 2b. carapace in frontal view, 2c. carapace in ventral view, 2d. carapace in left-lateral view. Scale bars equal 10 mm. Photographs: Philip Hurst, Natural History Museum, London.

FIGURE 3. Dromilites belli n. sp. type series from the Ypresian London Clay (Division B1, Kent, United Kingdom). 1. Paratype MAB.k 3583 (Seasalter, foreshore exposure) coated with ammonium chloride: 1a. carapace in dorsal view, 1b. carapace in frontal view; 2. Paratype MAB.k 3584 (Seasalter, foreshore exposure): claws; 3. Paratype MAB.k 3585 (Seasalter, foreshore exposure) (female individual): abdomen in ventral view; 4. Paratype MAB.k 3587 (Seasalter, foreshore exposure) (female individual): abdomen in ventral view; 5. Paratype MAB.k 3586 (Tankerton, foreshore exposure) (male individual): abdomen in ventral view. Scale bars equal 1cm. Photographs. Lilian Cazes (1) and Barry van Bakel (2-5).