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A new member of fossil balaenid (Mysticeti, Cetacea) from the early Pliocene of Hokkaido, Japan.

ABSTRACT: The family Balaenidae includes two genus and four extant species. Extinct balaenids are known for at least four genus and 10 species. The oldest known record of members of the Balaenidae is known from the early Miocene, but still need more early members of the family to provide better phylogenetic hypotheses. FCCP 1049 from the lower part of the Chippubetsu Formation, Fukagawa Group (3.5-5.2 Ma, Zanclean, early Pliocene) was preliminary described and identified as Balaenula sp. by Furusawa and Kimura in 1982. Later works discussed that FCCP 1049 is different from the genus, and is placed in different clade from Balaenula astensis. The result of our phylogenetic analysis places FCCP 1049 basal to Balaenella brachyrhynus, and is again separated from B. astensis. In this study, FCCP 1049 is re-described and named as Archaeobalaena dosanko gen. et sp. nov. Archaeobalaena dosanko is distinguishable from other balaenids by having a deep promontorial groove of the pars cochlearis of the periotic. Archaeobalaena dosanko can be differentiated from other balaenids, except Morenocetus parvus by having a slender zygomatic process, and posteriorly oriented postorbital process in dorsal view. Archaeobalaena dosanko adds detailed skull, periotic and bulla morphologies for the earlier balaenids.

SUBMITTER: Tanaka Y

PROVIDER: S-EPMC7211833 | biostudies-literature | 2020 Apr

REPOSITORIES: biostudies-literature

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A new member of fossil balaenid (Mysticeti, Cetacea) from the early Pliocene of Hokkaido, Japan.

Tanaka Yoshihiro Y Furusawa Hitoshi H Kimura Masaichi M

Royal Society open science 20200422 4

The family Balaenidae includes two genus and four extant species. Extinct balaenids are known for at least four genus and 10 species. The oldest known record of members of the Balaenidae is known from the early Miocene, but still need more early members of the family to provide better phylogenetic hypotheses. FCCP 1049 from the lower part of the Chippubetsu Formation, Fukagawa Group (3.5-5.2 Ma, Zanclean, early Pliocene) was preliminary described and identified as <i>Balaenula</i> sp. by Furusaw ...[more]

PMID: 32431893

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Project description:BackgroundAnatomical comparisons of the ear region of baleen whales (Mysticeti) are provided through detailed osteological descriptions and high-resolution photographs of the petrotympanic complex (tympanic bulla and petrosal bone) of all extant species of mysticete cetaceans. Salient morphological features are illustrated and identified, including overall shape of the bulla, size of the conical process of the bulla, morphology of the promontorium, and the size and shape of the anterior process of the petrosal. We place our comparative osteological observations into a phylogenetic context in order to initiate an exploration into petrotympanic evolution within Mysticeti.Principal findingsThe morphology of the petrotympanic complex is diagnostic for individual species of baleen whale (e.g., sigmoid and conical processes positioned at midline of bulla in Balaenoptera musculus; confluence of fenestra cochleae and perilymphatic foramen in Eschrichtius robustus), and several mysticete clades are united by derived characteristics. Balaenids and neobalaenids share derived features of the bulla, such as a rhomboid shape and a reduced anterior lobe (swelling) in ventral aspect, and eschrichtiids share derived morphologies of the petrosal with balaenopterids, including loss of a medial promontory groove and dorsomedial elongation of the promontorium. Monophyly of Balaenoidea (Balaenidae and Neobalaenidae) and Balaenopteroidea (Balaenopteridae and Eschrichtiidae) was recovered in phylogenetic analyses utilizing data exclusively from the petrotympanic complex.SignificanceThis study fills a major gap in our knowledge of the complex structures of the mysticete petrotympanic complex, which is an important anatomical region for the interpretation of the evolutionary history of mammals. In addition, we introduce a novel body of phylogenetically informative characters from the ear region of mysticetes. Our detailed anatomical descriptions, illustrations, and comparisons provide valuable data for current and future studies on the phylogenetic relationships, evolution, and auditory physiology of mysticetes and other cetaceans throughout Earth's history.

Project description:The transition from terrestrial ancestry to a fully pelagic life profoundly altered the body systems of cetaceans, with extreme morphological changes in the skull and feeding apparatus. The Oligocene Epoch was a crucial time in the evolution of cetaceans when the ancestors of modern whales and dolphins (Neoceti) underwent major diversification, but details of dental structure and evolution are poorly known for the archaeocete-neocete transition. We report the morphology of teeth and ultrastructure of enamel in archaeocetes, and fossil platanistoids and delphinoids, ranging from late Oligocene (Waitaki Valley, New Zealand) to Pliocene (Caldera, Chile). Teeth were embedded in epoxy resin, sectioned in cross and longitudinal planes, polished, etched, and coated with gold palladium for scanning electron microscopy (SEM) observation. SEM images showed that in archaeocetes, squalodontids and Prosqualodon (taxa with heterodont and nonpolydont/limited polydont teeth), the inner enamel was organized in Hunter-Schreger bands (HSB) with an outer layer of radial enamel. This is a common pattern in most large-bodied mammals and it is regarded as a biomechanical adaptation related to food processing and crack resistance. Fossil Otekaikea sp. and delphinoids, which were polydont and homodont, showed a simpler structure, with inner radial and outer prismless enamel. Radial enamel is regarded as more wear-resistant and has been retained in several mammalian taxa in which opposing tooth surfaces slide over each other. These observations suggest that the transition from a heterodont and nonpolydont/limited polydont dentition in archaeocetes and early odontocetes, to homodont and polydont teeth in crownward odontocetes, was also linked to a marked simplification in the enamel Schmelzmuster. These patterns probably reflect functional shifts in food processing from shear-and-mastication in archaeocetes and early odontocetes, to pierce-and-grasp occlusion in crownward odontocetes, with the implication of less demanding feeding biomechanics as seen in most extant odontocetes.

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Dataset Information

A new member of fossil balaenid (Mysticeti, Cetacea) from the early Pliocene of Hokkaido, Japan.

Publications

A new member of fossil balaenid (Mysticeti, Cetacea) from the early Pliocene of Hokkaido, Japan.

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