Saturday, 20 August 2022

Pagrus major: A Red Seabream from the Late Pleistocene of Taiwan.

Although the island of Taiwan has been extensively studied by geologists interested in its tectonic structure, it has been largely overlooked by palaeontologists. This is unfortunate, as the subduction of the South China Sea basin beneath the Philippine Sea Plate here has created an accretionary wedge on the Hengchun Peninsula on the south of the island that records the evolving marine faunas of the South China Sea biodiversity hotspot the Late Pleistocene.

In a paper published in the journal Zoological Studies on 5 May 2022, Chien-Hsiang Lin of the Biodiversity Research Center of Academia Sinica, Hsin-Yueh Ou of the Department of Life Science at Tunghai University, Chia-Yen Lin, also of the Biodiversity Research Center of Academia Sinica, and Hong-Ming Chen of the Department of Aquaculture at the National Taiwan Ocean University, describe a fossil Fish from a concretion assigned to the Late Pleistocene Szekou Formation of the Hengchun Peninsula.

The specimen comes from a sandstone block collected some decades ago by amateur collectors, and donated to the Biodiversity Research Museum at Academia Sinica by Cheng-Kai Chiang. Only half of the block is present, exposing a Fish skeleton, and this appears does not appear to have been broken open by Human intervention, making it likely that the counterpart was never found. Although the exact point of origin of the fossil is unknown, its collectors did record where it was found, and a visit to this site suggests that the Szekou Formation is the only lithological unit here which matches the rock type of the specimen. Since nodules from the Szekou Formation have produced a variety of other Late Pleistocene Fish, Crustaceans, and Molluscs, this does not seem an unreasonable assumption.

Pagrus major, ASIZF0100141, from the Late Pleistocene Szekou Formation, Hengchun Peninsula, southern Taiwan. (A) Lateral view of the nodule. (B) Dorsal view of the anterior part of the nodule, note that fragments of the right jaws are exposed. (C) Drawing of the recognised skull parts. Abbreviations:, alveolar process of the premaxilla; An, angular., articular process of the premaxilla;, ascending process of the premaxilla; Art, articular; Brr, branchiostegal ray; can., caniniform teeth; Ce, ceratohyal. con., conical teeth; cp, coronoid process of dentary; cr.s, superior crest on the articular; Den, dentary;, descending process of the articular; Enp, endopterygoid; Ino, interoperculum; mdc, dorsal crest on maxilla; mol., molariform teeth; Mpt, metapterygoid; Mx, maxilla; Op, opercle; Pl, palatine; Pmx, premaxilla; Pop, preopercle; Q, quadrate; Sop, subopercle. Lin et al. (2022).

Exposed on the nodule are an almost complete lower part (splanchnocranium) of a left skull, plus a fragmentary right premaxilla and dentary, and the anterior part of the body, largely covered by scale fragments. Fragments of the coracoid and cleithrum are visible just behind the opercle. 

Based upon examination of the teeth and maxilla, the remains can clearly be assigned to the Family Sparidae (Seabream). It can further be excluded from the genus Dentex by the presence of molariform teeth (which Dentex lacks). The specimen has villiform teeth along the proximal ring of its jaws, which suggests that it belongs to the genera PagrusChrysophrys, or Evynnis. Furthermore, the specimen has large anterior caniform teeth, which would appear to rule out membership of the genus Evynnis

The genera Pagrus and Chrysophrys each contain a single species; the Red Seabream, Pagrus major and the Australasian Snapper, Chrysophrys auratus. These species can only be differentiated morphologically by the presence of a bump on the supraoccipital bone (forehead) of Chrysophrys auratus. It has been suggested that these are in fact different populations of the same species, but recent genetic analysis has revealed that they are separate species, with their most recent common ancestor probably living in the Pliocene or Early Pleistocene. The two species are now often considered to be members of the same species.

Today, Pagrus major is found in the western Pacific, around the coasts of Japan, South China, Taiwan and the Philippines, whereas Chrysophrys auratus is found further east, around the coasts of Australia, New Zealand, and the islands of the South Pacific. However, it does not automatically follow that either species would have had the same distribution in the Pleistocene, when the distribution of climate bands was quite different.

The specimen from the Szekou Formation lacks any sign of a supraoccipital bump, which leads Lin et al. to conclude that it is a specimen of Pagrus major, and that therefore this species was present in the waters around Taiwan in the Late Pleistocene.

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