Toyotamaphimeia

History and naming

The first bones belonging to Toyotamaphimeia were discovered on May 3, 1964, during the construction of a new school building on the grounds of Ôsaka University. A field survey was conducted shortly afterwards, confirming the presence of more fossils, however not yet identifying their crocodilian nature. Following the survey several digs were organized starting on 9 June 1964. The skull was found on September 17 during the second dig. A third excavation was held in December which yielded more material of Toyotamaphimeia as well as fossil shellfish, insects and plant remains. Finally a fourth excavation took place in January 1965. Following analysis of the fossils, the material was assigned to the genus Tomistoma and named Tomistoma machikanense. In 1983, 18 years after the initial discovery, the skull was redescribed and deemed different enough from Tomistoma to erect a new genus, Toyotamaphimeia creating the comb. nov. Toyotamaphimeia machikanensis. In turn, Aoki also changed the species name from machikanense to machikanensis, as the new genus name was feminine. Similar remains were also known from Taiwan and had been classified as Tomistoma taiwanicus and Toyotamaphimeia sp. respectively, the former of which suggested as a species of Toyotamaphimeia already in 1983. A 2023 study concluded that both belonged to a single taxon: the Taiwanese form retained the species name, but was placed in Toyotamaphimeia, creating the new combination T. taiwanicus.

The generic name derives from Toyotama-hime, a goddess of Japanese mythology with the ability to change her appearance to that of a crocodile. The species epithet of T. machikanensis means "from Mountain Machikane" ((:ja:待兼山)), while that of T. taiwanicus derives from Taiwan.

Description

The holotype of Toyotamaphimeia is a nearly complete skeleton consisting of a skull, an entire cervical and dorsal series of vertebrae, various ribs, 33 osteoderms as well as almost half the bones of the limbs, hip region and pectoral girdle. Most of the tail past the 3rd caudal vertebra is missing, making it difficult to determine the exact length of the animal. The first research paper that described the type species tentatively suggested a body length estimate of 8 m based on the assumption that the caudal vertebrae in total would measure 4 m long.

''Toyotamaphimeia'''s skull is triangular in shape and longirostrine. It's fairly large, measuring over 1 m from the tip of the premaxillary to the posterior end of the parietal. Most of that length is taken up by the maxilla and the nasal bones penetrate the premaxilla dorsally, extending deep into the premaxilla to the level of the 3rd maxillary alveoli, but not coming in contact with the nares. The skulltable of the holotype is crushed and damaged just before the orbits. The dentaries are broken off at the anterior end and each preserves 10 alveoli. The absence of any grooves or confluence of alveoli suggests that the specimen is mature, which is consistent with its great size.

Paleobiology

The holotype specimen (MOUF00001) preserves a series of pathologies described by Katsura in 2004. The mandible is broken off at the tip, the tibia and fibula have been fractured and healed and some of the osteoderms present preserve healing bite marks. The fact that these injuries healed is evidence that the animal survived for a while after being injured and Katsura suggests that they may have been the result of intraspecific fights, furthermore hypothesizing that this could mean the Osaka University specimen may have been a male.

Although the holotype of Toyotamaphimeia is the first substantial and best preserved evidence of crocodilians in Japan, there are other remains found across the islands. The northernmost finds were made in the Iwate Prefecture (northern Honshu) while their range extends south to Nagasaki Prefecture (Kyushu Island). At this latitude Toyotamaphimeia would have existed at the thermal limit of crocodilians. The Ibaraki Formation, where the remains of Toyotamaphimeia have been found, is part of the Osaka Group, which consists of lacustrine and fluvial deposits of the Pliocene to Pleistocene. Specifically, the fossils belong to the Kasuri Tuff, which dates to the Chibanian age of the Pleistocene. Molluscs, pollen and plant fossils (species of lotus and water caltrop found in the Kasuri Tuff suggest a moderate climate. Toyotamaphimeia would have most likely coexisted in this area alongside Stegodon orientalis, Cervus kazusensis, Panthera youngi, Bubalus teihardi and Stephanorhinus. The pollen found in the region suggests a vegetation consisting of alders, beeches, pines and Cryptomeria (Japanese redwood).

Phylogeny

Below is a cladogram based morphological studies comparing skeletal features that shows Toyotamaphimeia as a member of Tomistominae, related to the false gharial:

Based on morphological studies of extinct taxa, the tomistomines (including the living false gharial) were long thought to be classified as crocodiles and not closely related to gavialoids. However, recent molecular studies using DNA sequencing have consistently indicated that the false gharial (Tomistoma) (and by inference other related extinct forms in Tomistominae) actually belong to Gavialoidea (and Gavialidae). Following this interpretation, Iijima et al. found Toyotamaphimeia to have been a basal member of Gavialinae, clading together with the Miocene Penghusuchus and the then newly named Hanyusuchus. The resulting group was later supported by a 2023 study following the inclusion of a geologically older species Toyotamaphimeia taiwanicus, although the resulting tree was poorly resolved and contained a large polytomy. In said study, Cho and Tsai argued that Toyotamaphimeia originated in Taiwan and evolved to acquire a large body size with gigantothermic physiology, and that it eventually migrated out of Taiwan and dispersed farther north to Japan. They also stated that tomistomines were variably recovered as either a group of crocodyloid or gavialoid depending on whether or not postcranial characters were included. The presence of an East Asian lineage, however, was found through both methods.

The phylogenetic trees of Iijima et al. (2022) as well as Cho and Tsai (2023) are featured below.

References

References

1. (2023). "Crocodylian princess in Taiwan: Revising the taxonomic status of ''Tomistoma taiwanicus'' from the Pleistocene of Taiwan and its paleobiogeographic implications". Journal of Paleontology.
2. "Valuable Specimen which Osaka University Possesses".
3. (1965). "Discovery of Crocodile Fossil from the Ôsaka Group". National Science Museum Monographs.
4. Aoki, R.. (1983). "A new generic allocation of ''Tomistoma machikanense'', a fossil crocodilian from the Pleistocene of Japan". Copeia.
5. (2006). ""Anatomy of a Japanese tomistomine crocodylian, ''Toyotamaphimeia machikanensis'' (Kamei et Matsumoto, 1965), from the middle Pleistocene of Osaka Prefecture: the reassessment of its phylogenetic status within Crocodylia"". National Science Museum Monographs.
6. (2020). "Vertebrae-Based Body Length Estimation in Crocodylians and Its Implication for Sexual Maturity and the Maximum Sizes". Integrative Organismal Biology.
7. Katsura, Yoshihiro. (2004-06-01). "Paleopathology of ''Toyotamaphimeia machikanensis'' (Diapsida, Crocodylia) from the Middle Pleistocene of Central Japan". Historical Biology.
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9. (2018-05-01). "Toyotamaphimeia cf. machikanensis (Crocodylia, Tomistominae) from the Middle Pleistocene of Osaka, Japan, and crocodylian survivorship through the Pliocene-Pleistocene climatic oscillations". Palaeogeography, Palaeoclimatology, Palaeoecology.
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