Platyacrodus unicus Ameghino, 1935

Bogan, Sergio, Agnolin, Federico & Ezcurra, Martín D., 2019, Review of the enigmatic ‘ shark’, Platyacrodus unicus Ameghino, 1935, from the Paleocene of Patagonia, Argentina: a history of palaeontologists, sharks and crabs, Zootaxa 4646 (2), pp. 293-300 : 295-298

publication ID	https://doi.org/ 10.11646/zootaxa.4646.2.5
publication LSID	lsid:zoobank.org:pub:4D88FC66-FCD6-4059-BBB8-E542EEB9997B
persistent identifier	https://treatment.plazi.org/id/038B878D-FFBB-141D-FF11-FC0CFCC306F1
treatment provided by	Plazi
scientific name	Platyacrodus unicus Ameghino, 1935
status

Treatment

Platyacrodus unicus Ameghino, 1935 View in CoL

( Figs. 2 View FIGURE 2 , 3A View FIGURE 3 )

Material examined. Holotype. MACN-A 12701 , nearly complete dorsal carapace ( Figure 2 View FIGURE 2 , 3A View FIGURE 3 ).

Locality and horizon. The original label indicates “Salamancan, Western Río Chico”. This fossil area is located in Chubut province, Patagonia, Argentina. No more details about the exact provenance of the specimen are available (see Simpson 1933). Ameghino (1906, 1935) described a large number of shark taxa from the “Piso Salamanquense” (Salamancan Stage). The Salamancan Stage is dated as late Danian−early Thanetian ( Arratia & Cione 1996; Foix et al. 2012, 2015; Clyde et al. 2014) and palaeomagnetic evidence in the northern part of the basin correlates it with chron 27r (63.3-61.9 Ma, late Danian). The shallow marine Salamanca Formation is the stratigraphically lowest Cenozoic geological unit in the San Jorge Basin of central Patagonia ( Argentina) and has a rich fossil record of marine microorganisms ( Frenguelli 1936; Camacho 1954; 1967; Méndez 1966; Masiuk 1967; Bertels 1975; Matheos et al. 2005) and invertebrates ( Ihering 1903; Feruglio 1949; Camacho 1967; Chebli & Serraiotto 1974; Andreis 1977; Parma & Casadío 2005). This unit represents the first Atlantic transgression in the San Jorge Basin ( Camacho 1967).

Description. At first sight, the specimen does not belong to the tooth plate of a vertebrate because it lacks dentine, enamel or cement. Indeed, the external surface possesses a complex pattern, with inflated areas, ridges and grooves, which closely resembles that of decapodan dorsal carapaces ( Figure 2 View FIGURE 2 , 3 View FIGURE 3 B-C). The external surface of the carapace is slightly weathered and, as a result, its fine ornamentation cannot be distinctly observed, but it is generally ornamented by very small subcircular bulges. The carapace is relatively small ( Table 1 View TABLE 1 ), its surface is distinctly dorsoventrally flattened, and it possesses two convex, elevated median areas. The anterior elevation is approximately centred in the external surface of the carapace and corresponds to the metagastric region. The posterior elevation is lateromedially broader and corresponds to the cardiac region. Both regions are separated from each other by the posterior extension of the cervical groove that delimits the metagastric region laterally. There are three pairs of mainly lateromedially-oriented ridges. The anterior and median ridges are slightly anterolaterally oriented and converge at the median line and with the anterior portion of the metagastric region. The median ridges are sharper than the anterior ones and extend posteromedially up to the level of the groove separating the metagastric and cardiac regions. The posterior ridges are very slightly posterolaterally oriented and do not intersect the median ridge. The fronto-orbital margin of the carapace seems to be damaged and, as a result, a rostrum or anterior teeth (if they were present) are not preserved. The posterior margin possesses a deep median notch for articulation with the first pleonal somite. The lateral flanks of the carapace form a slightly acute angle with the dorsal surface. The internal surface of the carapace is covered with matrix.

Remarks. The revision of the Ameghino fossil collection has allowed the rediscovery of a specimen that completely matched the original description of Platyacrodus unicus by Ameghino (1935). However, when it was originally rediscovered, the specimen lacked a catalogue number, and a label indicating its taxonomic assignment. The only available information in the label was its locality and horizon ( Figure 4E View FIGURE 4 ). The location of the unpublished drawings of the specimen drawn by F. Ameghino ( Figure 4 View FIGURE 4 ) was also an important key for the identification of this specimen as the holotype of Platyacrodus unicus .

The morphology observed in this specimen, MACN-A 12701, indicates that it probably should be referred to the genus Costacopluma Collins & Morris, 1975 , of the brachyuran family Retroplumidae , because it matches the following diagnosis of the genus: wider than long carapace with a distinctly flattened surface crossed by three elevated ridges, the anteriormost complete and the medial and posterior ridges converging mesially to define a depressed, triangular mesobranchial region ( Feldmann & Portell 2007; see also Vega & Feldmann 1992; Feldmann et al. 1995; 2014). Furthermore, MACN-A 12701 resembles Costacopluma salamanca Feldmann, Rodríguez, Martinez & Aguirre-Urreta, 1997 , in the presence of a combination of a subrectangular carapace with a narrow front and moderately wide posterior margin ( Feldmann et al. 1997; Figure 3 View FIGURE 3 ). It is worth mentioning that C. salamanca comes from approximately the same horizon and age than MACN-A 12701. In sum, it is possible that Platyacrodus unicus may represent a senior synonym of Costacopluma salamanca , and may be a valid taxon. However, this taxonomic interpretation should be tested by a more detailed taxonomic revision of fossil and recent taxa of the Costacopluma genus, which goes beyond the scope of the present contribution.