JOURNAL OF CRUSTACEAN BIOLOGY, 30(2): 251-256, 2010 THE OLDEST BRACHYURA (DECAPODA: HOMOLODROMIOIDEA: GLAESSNEROPSOIDEA) KNOWN TO DATE (JURASSIC) Carrie E. Schweitzer and Rodney M. Feldmann (CES, [email protected]) Department of Geology, Kent State University Stark Campus, 6000 Frank Ave. NW, North Canton, Ohio 44720; (RMF, [email protected]) Department of Geology, Kent State University, Kent, Ohio 44242 ABSTRACT Downloaded from https://academic.oup.com/jcb/article/30/2/251/2419233 by guest on 02 October 2021 The oldest confirmed brachyurans, referable to the Homolodromioidea and Glaessneropsoidea, are Early to Middle Jurassic in age. Geographically, all of the occurrences are in Europe, with one exception, in Tanzania. KEY WORDS: Brachyura, Decapoda, Homolodromioidea, Jurassic, origins DOI: 10.1651/09-3231.1 INTRODUCTION tidae Beurlen, 1932; Prosopidae Von Meyer, 1860; The search for the oldest brachyuran has had many false starts. Tanidromitidae Schweitzer and Feldmann, 2008 [imprint As better and more complete diagnoses for the various 2007]. anomuran and brachyuran groups have become available, and as more fossils and more complete fossils are known, it has Homolodromioidea incertae sedis become possible to determine which organisms belong within Eoprosopon Fo¨rster, 1986 Brachyura and which are most likely anomurans or some other type of decapod. Recent workers have removed several Type and Sole Species.—Eoprosopon klugi Fo¨rster, 1986, candidates for the oldest brachyuran from the group (Schweitzer by monotypy. and Feldmann, 2005; Feldmann and Schweitzer, 2010). Herein we report on the oldest confirmed members of Brachyura, those occurrences that fit the definition of Brachyura without doubt. Eoprosopon klugi Fo¨rster, 1986 Krobicki and Zaton´ (2008) provided an overview of the Fig. 1 Jurassic roots of crabs, but that work did not take into account recent advances in the systematics and phylogeny Material Examined.—Cast of BSP 1986 I 19, holotype. of these animals. An important implication for our findings Discussion.—Fo¨rster (1986) described Eoprosopon from is that it permits the constraint of molecular studies of the Pliensbachian of Germany. He referred it to Prosopidae, Brachyura and other decapod taxa, with an earliest possible and Guinot (1995) later removed it to Homolodromiidae divergence time based upon actual evidence for these based upon its possession of a deep cervical groove, long groups. These findings also permit paleontologists to focus dactyls of pereiopods 2 and 3, reduced pereiopods 4 and 5, on an age horizon and therefore to target localities in the and a flattened pleon with epimeres. All of these features search for ancestral Decapoda within the anomuran and are present, but the nature of the pleon suggests that brachyuran lineages; these two clades are shown to be placement into Homolodromiidae is not warranted. Exam- sisters in most recent analyses (Ahyong and O’Meally, ination of the pleonites indicates the presence of discrete, 2004; Ahyong et al., 2007). Recognition of an Early to obvious swellings in the area of the epimeres (Fig. 1), Middle Jurassic origin for Brachyura can allow workers to which are not present on any extant homolodromiid. Each begin investigating the dynamics of the origin and radiation of this ecologically important group of animals. individual pleonite 5-7 is rectangular and long, they fit Institutional abbreviations: BMNH, The Natural History closely together, and they maintain their length laterally. Museum, London, UK; BSP, Bayerische Staatsammlung Pleonite 7 has clear articulating rings. The telson is slightly fu¨r Pala¨ontologie, Mu¨nchen, Germany; OUM, Geological longer than wide. In extant homolodromiids, both male and Collections, Oxford University Museum. female, the telson is very long and longer than wide, e.g., illustrations in Guinot (1995). Pleonites 5-7 are short in both males and females, and they narrow laterally into SYSTEMATICS triangular tips. Weak articulating rings may be present if Infraorder Brachyura Linnaeus, 1758 present at all. Section Dromiacea De Haan, 1833 Thus, the pleon of Eoprosopon is unlike that of Superfamily Homolodromioidea Alcock, 1900 Homolodromiidae. However, the features of Eoprosopon seem to suggest clearly that it is a brachyuran. It possesses Included Families.—Homolodromiidae Alcock, 1900; Buc- one pair of chelae; the walking legs are long and achelate; culentidae Schweitzer and Feldmann, 2009a; Goniodromi- the last pair of pereiopods and possibly the fourth pair also 251 252 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 30, NO. 2, 2010 Downloaded from https://academic.oup.com/jcb/article/30/2/251/2419233 by guest on 02 October 2021 Fig. 1. Eoprosopon klugi Fo¨rster, 1986, cast of BSP 1986 I 19, holotype. Scale bar 5 1 cm. are reduced; the dorsal carapace has well-developed long lateral rostral spines and downturned central rostral regions and cervical and branchiocardiac grooves; and the spine; augenrests large, directed forward, protected by three carapace is dorsoventrally compressed. The pleon extends spines; dorsal carapace ornamented by long spines; cervical posteriorly from the carapace, and does have epimeres, groove deep, nearly straight; mesogastric region well- both of which are atypical of the brachyurans. However, ornamented by spines and muscle scars. these features are seen in the extant homolodromiids. Thus, Eoprosopon is the earliest known specimen that can be Discussion.—Eudes-Deslongschamps (1835) named the referred to Brachyura with reasonable confidence, probably genus and its type and sole species in 1835. In 1865, best referred to the Homolodromioidea incertae sedis. Woodward and Salter referred a species to Protocarcinus, Examination of the type specimen, perhaps with some P. longipes, and then later Woodward (1866) placed that additional preparation, or recovery of more material might species within the new genus Palaeinachus. He did not use permit referral of the specimen to a family with confidence. the name Protocarcinus because: 1) the specimen in his To date, the appendages and pleon of the Jurassic opinion bore no relation to Carcinus,and2)‘‘the prefix homolodromioids are unknown. Garassino et al. (2005) ‘Proto’ is objectionable in Palaeontology’’ (Woodward, 1866, had referred a specimen with preserved appendages to p. 493). Examination of specimens in the Natural History Pithonoton sp.; however, Feldmann et al. (2006) have Museum, London, suggests that Homolus auduini and already suggested that it is probably a porcellanid. Palaeinachus longipes are synonymous as suggested by Unfortunately, Eoprosopon lacks well-preserved front and Glaessner (1929) and Guinot (1995). The two species (H. lateral views that could permit determination of the auduini and P. longipes) appear to be conspecific based upon possession of three large nodes on the anterior half of presence and size of augenrests or orbital structures carapace (one on anterior extension of mesogastric, and one necessary to place it within a family. each on protogastric), a pair of nodes on the posterior end of the mesogastric region, a straight path of the cervical groove, Homolodromiidae Alcock, 1900 and possession of pseudorostral spines. All three generic Homolus Eudes-Deslongschamps, 1835 names are available according to Nomenclator Zoologicus. However, Homolus is the earlier name, was fully described Protocarcinus Woodward and Salter, 1865, fig. 15. and illustrated, and was designated with an associated Palaeinachus Woodward, 1866, p. 493, pl. 24, fig. 1. species; therefore, we use it here. Wehner (1988) referred Type and Sole Species.—Homolus auduini Eudes-Deslong- the species to Foersteria, but that name has been replaced by champs, 1835, by monotypy. Gabriella Collins et al., 2005, and the species is not referable to that genus (Schweitzer and Feldmann, 2009b). Diagnosis.—Carapace longer than wide, widest about 70% Homolus is referable to Homolodromiidae based upon the distance posteriorly in branchial region; rostrum with possession of large, forward-directed augenrests protected SCHWEITZER AND FELDMANN: THE OLDEST CRABS 253 by spines; lateral rostral spines that are very long and a International Code of Zoological Nomenclature (1999: downturned central rostral spine; a forward-directed outer- Articles 11 and 12), the name does appear to be available. orbital spine; a well-defined branchiocardiac and a All of the species referred to it were originally referred to moderately defined postcervical groove; an inflated sub- other genera, and they have usually been treated as hepatic region; a well-defined, straight cervical groove; belonging to the genus to which they were originally lateral margins that are subparallel and diverge posteriorly; referred in generic revisions, especially because Van relatively short first pereiopods; a dorsal carapace that has Straelen did not designate a type species. Thus, most of high flanks that become markedly narrower posteriorly, the species referred to Avihomola have since been referred possibly indicating a poorly calcified posterior portion; and to other genera. The remaining species that Van Straelen an abdomen that is partially visible in dorsal view. In fact, referred to Avihomola are known from poor, incomplete Homolus is remarkably similar to extant homolodromiids. material. At this time, it appears, therefore, that Avihomola Bouvier (1896) had already noted this remarkable similar- is a nomen nudum. ity. No other decapod family exhibits this combination of Beurlen (1932) reported Early Jurassic