メガトゥース・シャークが歯を痛めた原因とは?(What Caused This Megatooth Shark’s Massive Toothache?)

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2022-05-12 ノースカロライナ州立大学(NCState)

世界最大の先史時代のサメに歯科矯正医が必要だったのか、それとも単に昼食がまずかっただけなのか?
ノースカロライナ州立大学とノースカロライナ自然科学博物館の研究者たちは、オトドゥス・メガロドンの変形した歯を調査し、その根本原因を探った。この研究により、古生物学者は、古代のサメの歯の損傷に関連する発達過程や摂食行動についての見識を深めることができるかもしれません。

<関連情報>

ラムニフォームシャークとカルカロニフォームシャークの歯牙病理と脊椎動物における八重歯病理の分類と相同性についてのコメント Dental pathologies in lamniform and carcharhiniform sharks with comments on the classification and homology of double tooth pathologies in vertebrates

Harrison S. Miller, Haviv M. Avrahami, Lindsay E. Zanno
PeerJ  Published:May 11, 2022
DOI:10.7717/peerj.12775

Abstract

Double tooth pathologies are important indicators of trauma, disease, diet, and feeding biomechanics, and are widely documented in mammals. However, diagnosis of double tooth pathologies in extinct non-mammalian vertebrates is complicated by several compounding factors including: a lack of shared terminology reflecting shared etiology, inconsistencies in definitions and key features within and outside of mammals (e.g., gemination, fusion, twinning, concrescence); differences in tooth morphology, heterodonty, regeneration, and implantation between mammals and non-mammalian vertebrates; and the unmet need for diagnostic criteria that can be applied to isolated teeth, which are common in the fossil record. Here we report on double tooth pathologies in the lamniform and carcharhiniform Cenozoic sharks Otodus megalodon (NCSM 33639) and Carcharhinus leucas (NCSM 33640, 33641). All three teeth bear a singular bifid crown with mirrored halves and abnormal internal microstructure—a single, bifurcating pulp cavity in C. leucas and a more than tripling of vessels in O. megalodon (from two to seven main ascending canals). We identify these abnormalities as likely examples of gemination due to their symmetry, which rules out fusion of tooth buds in one tooth file in different developmental stages in polyphyodont taxa; however, we note that incomplete forms of mesiodistal tooth fusion can be morphologically indistinguishable from gemination, and thus fusion cannot be rejected. We further compile and recategorize, when possible, the diversity of tooth pathologies in sharks. The identification of double tooth pathologies in O. megalodon and C. leucas has paleobiological implications. Such pathologies in sharks are largely hypothesized to stem from trauma to developing tooth buds. Carcharhinus leucas is known to feed on prey documented to cause feeding-related oral traumas (e.g., rays, sawfish, spiny fish, and sea urchins). However, O. megalodon, is considered to have largely fed on marine mammals, and perhaps turtles and/or fish, raising the possibility that the dietary diversity of this species is, as of yet, underappreciated. The genetic underpinnings of tooth morphogenesis and regeneration is highly conserved throughout vertebrate evolution, suggesting a homologous framework can be established. However, more research is needed to link developmental, paleobiological, and/or paleoenvironmental factors to gemination/fusion in polyphyodont taxa. We argue that the definitions and diagnostic criteria for dental pathologies in vertebrates require standardization in order to advance macroevolutionary studies of feeding trauma in deep time.

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