Dynomenidae

Guinot, Danièle, 2008, A re-evaluation of the Dynomenidae Ortmann, 1892 (Crustacea, Decapoda, Brachyura, Podotremata), with the recognition of four subfamilies, Zootaxa 1850, pp. 1-26 : 20-22

Fossil Dynomenidae (preliminary remarks)

Ancestral dynomenids are believed to have been Tethyan crabs ( McLay 1999). Extant dynomenids are survivors from the Jurassic, having endured Cretaceous/Paleogene perturbations ( Wright & Wright 1950; Schweitzer & Feldmann 2005). It is evident that the diversity of their morphological features, exhibiting different states of transformation, is linked to early diversification and a long evolutionary history. Ten fossil dynomenid genera were listed by McLay (1999: 434), who suggested that extinct species traditionally assigned to Dynomene could in fact belong to other genera. Schweitzer et al. (2003: 20, 21) recognised 13 fossil dynomenid genera. The assignment of some fossil genera to the Dynomenidae needs to be corroborated, however. Fortunately, dorsal carapace features tentatively suffice to assign dynomenid species to a particular subfamily. The Acanthodromiinae n. subfam. perhaps is an exception because of its spinose ornament which masks the grooves and sutures.

Diaulax Bell, 1863 (type species: D. carteriana Bell, 1863 , Lower Cretaceous), alternatively considered to be a prosopid ( Wright & Wright 1950: 24) or a dynomenid ( Glaessner 1969: R488), is now the type genus of the Diaulacidae Wright & Collins, 1972 (see Bishop 1983: 45; 1986: 133), and thus affiliated to the Podotremata ( Guinot & Tavares 2001). The Diaulacidae was synonymised with the Dynomenidae by Schweitzer et al. (2003). Bell (1863: 7, pl. 1, figs. 14–16) described the basal portions of the pereopods preserved in D. carteriana and indicated that “the last pair are placed on a much higher level than the others”. Wright & Collins (1972: 58, pl. 9, figs. 6–8, pl. 10, fig. 1a, b) described the sternum as twice as long as wide, anteriorly with “an acute point”, the abdominal somites as “short, flat and rectangular”, and the telson as “bluntly triangular, wider than long (in a supposed male)”. According to Bell’s original and subsequent descriptions of the type material ( Carter 1898: 19; Wright & Wright 1950: 24, pl. 1, fig. 9a, b; Wright & Collins 1972: 57), there are two parallel transversal grooves that cross the dorsal carapace. The cervical one is continuous, with “a slight interruption on the middle line, on either side of which is a small pit” [i.e., herein gastric pit] ( Wright & Collins 1972: 57), and probably notches the lateral margin; the branchial one is not so pronounced. In the absence of any significant illustration, it is difficult to assign Diaulax carteriana with certainty. This observation is of importance because the Diaulacinae could admittedly constitute a senior synonym of one of the subfamilies erected herein. Synonymy with the Acanthodromiinae n. subfam. or the Paradynomeninae n. subfam. may be reasonably excluded as they are so different morphologically. A possible relationship with the Metadynomeninae n. subfam. is also questionable. According to Glaessner (1931: 4; 1969: R488), Diaulax originated during the Late Jurassic (see also Wright & Wright 1950: fig. 13; Wright & Collins 1972: 56; Fraaye 1996: fig. 3), and this could imply (in the case of a genuinely close relationship between Diaulax and the Metadynomeninae n. subfam.) an early appearance of the metadynomenine lineage, together with the diaulacine one.

Some fossil genera appear to conform to the new diagnostic subfamilial definitions and may be more easily assigned. A good example is the Paleocene Kierionopsis Davidson, 1966 (type species. K. nodosa Davidson, 1966: 211 –213, figs. 1, 2), which was originally ( Davidson, 1966: 211) and subsequently ( Schweitzer & Feldmann 2008: 122) attributed to the Dromiidae , as possibly related to Dromilites americana Rathbun, 1935 , or regarded as “enigmatic” ( Schweitzer et al. 2002: 41, fig. 29, table 4) within the Dynomenidae ( Schweitzer et al. 2003: 21; Schweitzer & Feldmann 2005: 22), and clearly belongs to the Paradynomeninae n. subfam. as defined here. In K. nodosa , the carapace is subrectangular, the dorsal surface is ornamented by 12 elevated granular bosses, the posterolateral border bears an elongated subdistal tooth, the intestinal region is recessed, and there is an orbital eave, all characters found in the extant Paradynomeninae n. subfam. Apart from the front, slightly bidentate in K. nodosa , distinctly bidentate or tridentate in extant Paradynomene , the resemblance is amazing.

The Paleocene Dromilites americana Rathbun not only “differs significantly from the type of the genus” Dromilites H. Milne Edwards, 1837 , D. bucklandii (H. Milne Edwards, 1837) (see Schweitzer et al. 2003: 21) but, moreover, the thoracic sternum figured by Rathbun (1935: pl. 17, fig. 2) does not correspond to that of a dromiacean crab and probably does not represent a podotreme condition. Consequently, the assignment of Dromilites americana to Dromilites and its attribution to the Dynomenidae , with a possible link to Kierionopsis , are not recognised here. Dromilites should be assigned to the Dromiinae (Dromiidae) .

Similar to Kierionopsis , Kromtitis Müller, 1984 (type species: Dromilites koberi Bachmayer & Tollmann, 1953 ), assigned to the Dromiidae ( Müller 1984: 64, pl. 31, figs. 1–4; Müller & Collins 1991: 63, fig. 3e, pl. 3, figs. 4, 5, 8; Portell & Collins 2004: 111; Beschin et al. 2002: 12; Donovan et al. 2003: 106) or to the Dynomenidae in close proximity of Paradynomene ( Beschin et al. 2007: 26, 27; see also Beschin et al. 2004), can be confidently included in the Paradynomeninae n. subfam. In Kromtitis , as in Kierionopsis , the subrectangular carapace, the pronounced subdistal posterolateral teeth, the lobate and ornamented dorsal surface closely matches those of species of Paradynomene species recently described by McLay & Ng (2004). At least, in dorsal carapace features, there has scarcely been any divergence between these fossil genera and extant Paradynomeninae n. subfam. All species of Kromtitis , from the Eocene to the Miocene, are coral associates ( Beschin et al. 2007: 27), as are extant species of Paradynomene . Modern Paradynomeninae n. subfam. clearly are barely modified relicts.

A number of fossils, known solely from their dorsal carapaces and which conform to the general pattern of the Dynomenidae , appear as possible metadynomenines. Such is the case, for instance, of species of Dromiopsis Reuss, 1859 (type species: Brachyurites rugosus von Schlotheim, 1820) as D. elegans Reuss, 1859 , with a rounded carapace, complete and deep cervical groove and lateral branchial groove, both grooves reaching and forming notches with the lateral border of the carapace. However, the genus Dromiopsis , supposed to be a dynomenid genus, is probably not monophyletic.

It is premature and beyond the scope of the present paper to assign podotreme fossils to a dynomenid subfamily, the preliminary task being to attempt to include extinct genera in their appropriate family, i.e., either in the Dromiidae (and possibly in its constituent subfamilies) or Homolodromiidae or Dynomenidae , without excluding extinct families such as the Diaulacidae and also the Prosopidae which are the ancestors of the Homolodromioidea. Any affiliation based on the carapace shows only similarities and thus remains speculative at best. The nature of fossil dromiacean genera, even those that are known from a number of characters, remains questionable. For example the familial status of Basinotopus M’Coy, 1849 (type species: Inachus lamarckii Desmarest, 1822 ), traditionally assigned to the Dromiidae (P4 and P5 reduced and dorsal), is puzzling, despite the availability of both male and female abdomens with their uropods (M’Coy 1849; Bell 1858; see Guinot & Tavares 2001). Recent discoveries of more complete specimens of the Eocene Basinotopus tricornis Collins & Jakobsen, 2004 , which in particular reveal sternal characters ( Collins & Jakobsen 2004: 69, fig. 3, pl. 2, figs. 1–7), require a new interpretation based on all known data in accordance with phylogeny.