Tuesday, December 27, 2016

[PaleoMammalogy • 2016] A New Skull of the Fossil Porpoise Numataphocoena yamashitai (Cetacea: Phocoenidae) from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata Town, Hokkaido, Japan, and Its Phylogenetic Position


Numataphocoena yamashitai 
 Ichishima & Kimura, 2000 

Art work by T. Shinmura  Palaeo-Electronica.org

ABSTRACT

An early Pliocene porpoise, Numataphocoena yamashitai from Hokkaido, Japan, is known from the holotype, a fairly well-preserved skeleton with an incomplete skull and a referred earbone. A new skull referred to Numataphocoena yamashitai found from almost the same locality as the holotype is interesting because it expands knowledge of skull morphology and improves the diagnosis of this taxon. Numataphocoena yamashitai differs from other phocoenids in having the characteristic feature in the maxilla associated with the posterior dorsal infraorbital foramen, narrower and sharper anterior part of the internal acoustic meatus, and a robust anterior process of the periotic. A new cladistic analysis places Numataphocoena yamashitai adjacent to Haborophocoena toyoshimai and Haborophocoena minutus, among a clade of early branching phocoenids, all of which are chronologically and geographically close to each other. The new skull is probably a younger individual because it is about 80% the size of that of the holotype and it shows closed but unfused sutures. Our description of this specimen helps to understand the intraspecies variation of the extinct species Numataphocoena yamashitai.

Key words: skull; Phocoenidae; phylogeny; maxillary terrace; ontogeny; intraspecies variation


FIGURE 6. Life restoration of Numataphocoena yamashitai.
Art work by Tatsuya Shinmura (Ashoro Museum of Paleontology). 


CONCLUSION: 
The new referred skull (NFL 2074) of Numataphocoena yamashitai from the upper part of the Horokaoshirarika Formation (early Pliocene), Numata, Hokkaido, Japan, adds diagnostic characters of the species, recognizes variations among the species and reveals the phylogenetic position among the Phocoenidae. Our cladistic analysis places Numataphocoena yamashitai adjacent to Haborophocoena toyoshimai and Haborophocoena minutus, among a clade of early branching phocoenids, all of which are chronologically and geographically close to each other, being all from Hokkaido. Numataphocoena yamashitai differs from other phocoenids in having a maxillary terrace, narrower and sharper anterior part of the internal acoustic meatus, and a robust anterior process of the periotic. NFL 2074, which is about 80% size of the holotype, NFL 7, is probably younger than the physically mature holotype. NFL 2074 does not have a well-developed nuchal crest like the holotype skull, which is probably the result of intraspecific variation.


 Tanaka, Yoshihiro and Ichishima, Hiroto. 2016. A New Skull of the Fossil Porpoise Numataphocoena yamashitai (Cetacea: Phocoenidae) from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata Town, Hokkaido, Japan, and Its Phylogenetic Position. Palaeontologia Electronica. 19.3.49A; 1-28.  



Abstract
A new periotic of Numataphocoena yamashitai from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata, Hokkaido, Japan shows younger ontogenetic features in comparison to the matured holotype (NFL 7). The referred specimen of N. yamashitai is smaller (approximately 90% in size) than the physically and sexually matured holotype periotic. The holotype (NFL 7) and referred specimen (NFL 2617) show several differences owing to ontogenetic changes, such as: the holotype (NFL 7) has a more strongly swollen medial part of the anterior process, blunt anterior keel, rounded anterodorsal angle, deeper and clearer anterior incisure, more rounded pars cochlearis, deeper and narrower hiatus epitympanicus rather than a flat area, longer posterior process with an extra edge posterolaterally. N. yamashitai differs from other fossil phocoenids in having a narrower and sharper anterior part of the internal acoustic meatus and a robust anterior process. Haborophocoena toyoshimai similarly has a relatively narrow internal acoustic meatus and a robust anterior process, compared with other fossil phocoenids. Between N. yamashitai and H. toyoshimai, N. yamashitai has a narrower internal acoustic meatus and a more robust anterior process. These similarities imply a close relationship between Numataphocoena and Haborophocoena. The new specimen provides diagnostic features and insight into ontogenetic variation of N. yamashitai.

 Keywords: fossil, ontogeny, periotic, Phocoenidae


Yoshihiro Tanaka. 2016. A New and Ontogenetically Younger Specimen of Numataphocoena yamashitai from the Upper Part of the Horokaoshirarika Formation (Lower Pliocene), Numata, Hokkaido, Japan. Paleontological Research. 20(2); 105-115. DOI: 10.2517/2015PR026

H. Ichishima and M. Kimura. 2000. A New Fossil Porpoise (Cetacea; Delphinoidea; Phocoenidae) from the early Pliocene Horokaoshirarika Formation, Hokkaido, Japan. Journal of Vertebrate Paleontology. 20(3); 561-576. DOI: 10.1671/0272-4634(2000)020[0561:ANFPCD]2.0.CO;2

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