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Reply to: Jaw roll and jaw yaw in early mammals

Nature volume 582, pages E9–E12 (2020)Cite this article

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The Original Article was published on 17 June 2020

replying to: D. M. Grossnickle Nature (2020)

The principal objection raised by Grossnickle1 to a cladotherian origin of roll-based processing is that the talonid ‘heel’ or ‘shelf’ of the ancestral cladotherian is not homologous to the talonid basin of therians because the inner or lingual cusp that bit into it (which is conventionally known as the paracone) is not homologous to the lingual cusp of therians (the protocone). We contend that the name assigned to the upper cusp is inconsequential, given the underlying structural and functional continuity between the cladotherian paracone and the therian protocone. Every species on the direct line from Cladotheria to Theria had triangular upper molars, the inner vertex of which bit into a shelf or basin attached to the back of the complementary triangle on the lowers3 (Fig. 1a–i). The name paracone, as currently used, is tied to the identity of the cusp that shears along the posterior face of the trigonid to form the primary trigonid embrasure (Fig. 1a–i, in red; shearing surface 1 in ref. 4). In early cladotherians, this was the upper lingual cusp4,5,6 (Fig. 1c–e), which also contacted or closely approached the talonid. Nearer to Theria, in some early tribosphenidans the anterior labial cusp sheared broadly against the back of the trigonid and is therefore given the name paracone, whereas the lingual cusp—topologically continuous with the cladotherian paracone, but now termed the protocone—maintained its relation to the talonid (Fig. 1f). The embrasure in opossums is again formed by the side of the lingual cusp: surface 4 in ref. 5 and phase I surface of ref. 7 (in which it is shown in hot pink) (Fig. 1i). Therefore, in the topological sense and in the important functional sense of interacting with the talonid, the inner cusp of the ancestral cladotherian upper molar was the antecedent of the inner cusp of the therian molar. Indeed, a comparative survey reveals that the inner cusp is the most conserved feature of the upper molar, and that it is always located below the internal (and largest) of the three roots8,9 (Fig. 1a–i). In sum, regardless of the name given to the internal cusp, there has been a fundamental continuity: always an inner cusp above and always a platform below (Fig. 1a–i). The talonid as a platform is homologous across cladotherians10, which calls into question the identification1,11 of the ancestral cladotherian talonid surface with the therian hypoflexid alone. It is true that the primitive talonid favoured shearing, whereas the therian basin allowed grinding. However, it has previously been observed that mediolateral motion from jaw roll would have increased the efficiency of both kinds of processing4, which exist as points on a continuum rather than a dichotomy.

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Fig. 1: Evolution of mammalian molars and adductor muscles.
Fig. 2: Evolution of jaw kinematics in Mammalia.

Data availability

All referenced data are freely available as described in ref. 2.

References

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Acknowledgements

B.-A.S.B. and J.A.M. were supported by Yale University and the Yale Peabody Museum of Natural History. A.R.M. and E.L.B. were supported by Brown University, by an NSF Graduate Research Fellowship awarded to A.R.M. and by NSF grants 1661129 and 1655756 to E.L.B. E.A.H. was supported by the American Museum of Natural History and an NSF Graduate Research Fellowship. C.M. and A.W.C. were supported by the Harvard Museum of Comparative Zoology.

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Author notes

  1. These authors contributed equally: Bhart-Anjan S. Bhullar, Armita R. Manafzadeh

Authors and Affiliations

  1. Department of Earth and Planetary Sciences, Yale University, New Haven, CT, USA

    Bhart-Anjan S. Bhullar & Juri A. Miyamae

  2. Peabody Museum of Natural History, Yale University, New Haven, CT, USA

    Bhart-Anjan S. Bhullar & Juri A. Miyamae

  3. Department of Ecology and Evolutionary Biology, Brown University, Providence, RI, USA

    Armita R. Manafzadeh & Elizabeth L. Brainerd

  4. Division of Paleontology, American Museum of Natural History, New York, NY, USA

    Eva A. Hoffman

  5. Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA

    Catherine Musinsky & Alfred W. Crompton

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Contributions

All authors conceived and discussed the major elements of the response. B.-A.S.B., A.M. and E.A.H. wrote the paper and prepared figures.

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Correspondence to Bhart-Anjan S. Bhullar.

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Supplementary Information

Sources for data in Figs 1 and 2, listed by taxon.

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Bhullar, BA.S., Manafzadeh, A.R., Miyamae, J.A. et al. Reply to: Jaw roll and jaw yaw in early mammals. Nature 582, E9–E12 (2020). https://doi.org/10.1038/s41586-020-2364-z

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