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. 2016 May 9:6:25604.
doi: 10.1038/srep25604.

Palaeoneurological clues to the evolution of defining mammalian soft tissue traits

J Benoit  1   2 , P R Manger  2 , B S Rubidge  1   3
Affiliations

Palaeoneurological clues to the evolution of defining mammalian soft tissue traits

J Benoit et al. Sci Rep. .

Abstract

A rich fossil record chronicles the distant origins of mammals, but the evolution of defining soft tissue characters of extant mammals, such as mammary glands and hairs is difficult to interpret because soft tissue does not readily fossilize. As many soft tissue features are derived from dermic structures, their evolution is linked to that of the nervous syutem, and palaeoneurology offers opportunities to find bony correlates of these soft tissue features. Here, a CT scan study of 29 fossil skulls shows that non-mammaliaform Prozostrodontia display a retracted, fully ossified, and non-ramified infraorbital canal for the infraorbital nerve, unlike more basal therapsids. The presence of a true infraorbital canal in Prozostrodontia suggests that a motile rhinarium and maxillary vibrissae were present. Also the complete ossification of the parietal fontanelle (resulting in the loss of the parietal foramen) and the development of the cerebellum in Probainognathia may be pleiotropically linked to the appearance of mammary glands and having body hair coverage since these traits are all controlled by the same homeogene, Msx2, in mice. These suggest that defining soft tissue characters of mammals were already present in their forerunners some 240 to 246 mya.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Lateral view, with the skull transparent, of the maxillary canal (in green) and maxillary antrum (in purple) in (A) the non-avian sauropsid Varanus, (B) the early cynodont Thrinaxodon, (D) the cynognathian Trirachodon, (E) the early probainognathian Ecteninion, (F) the probainognathian Tritylodon, (G) the therocephalian Olivierosuchus, (H) an indeterminated gorgonopsian, and (I) the biarmosuchian Herpetoskylax; and (C) the canal for the ophthalmic nerve in the early cynodont Thrinaxodon in dorsal view with the skull transparent. Rostral is to the left. Scale bar = 10 mm. The condition in Homo (top) illustrates the nomenclatural terms in Mammalia (after 20). Dotted line indicates the caudal extremity of the infraorbital nerve, which is marked by its bifurcation from the rostral alveolar nerve. The position of this dotted line is homologous to the infraorbital foramen in Tritylodon and mammaliaforms. Videos of the CT images of the specimens are available in the Supplementary Information. Abbreviations: Alv.Can., alveolar canal; Caud.Al., caudal alveolar rami; Ext.Nas., external nasal rami of the infraorbital nerve; For., foramina for the maxillary branch of the trigeminal nerve; Int.Nas., internal nasal rami of the infraorbital nerve; Intraorb.For., intraorbital foramen for the infraorbital nerve; I.O.For, infraorbital foramen; I.O.N., infraorbital nerve; Med.Al., medium alveolar nerve; Mx.Ant., maxillary antrum; Mx.Can., maxillary canal; Oph.Fr., frontal ramus of the ophtalmic nerve; Oph.Nas., nasal ramus of the ophtalmic nerve; Rost.Al., rostral alveolar nerve; Sph.P.For., sphenopalatine foramen; Sup.Lab., supralabial ramus of the infraorbital nerve.
Figure 2
Figure 2. The evolution of the bony structures associated with the infraorbital nerve in Therapsida.
Digital reconstruction of the maxillary canal (in green) and maxillary antrum (in purple) based on CT scan images (see Material and Methods). Scale bars = 10 mm. Phylogeny after references Rubidge and Sidor and Liu and Olsen. Lateral views of the skulls with bones transparent. Rostral is to the left. Videos of the CT images of the specimens are available in the Supplementary Information. Abbreviations : Cyn, Cynodontia clade; Prb, Probainognathia clade; Prz, Prozonstrodontia clade; Thr, Therapsida clade.
Figure 3
Figure 3. The skull roof of juvenile trirachodontid (A), BPI/1/4534 and juvenile Pachygenelus (B), BPI/1/5691 with the parietal bone (top, in yellow) and the cranial endocast reconstructed (below, in purple) based on CT scan images (see material and methods).
Abbreviations: Cer, cerebellum; Hem: cerebral hemisphere; Par, parietal bone; Par f, parietal foramen.
See this image and copyright information in PMC

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