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.
Highlights
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
Facial and body tactile sensibility is often directly related to pilosity, and given that stem mammaliaforms and a number of non-mammaliaform therapsids were apparently nocturnal and may not have strongly relied on vision to monitor their environment[8,9,10], the evolution of hair must have dramatically influenced the survival of the lineage leading to mammaliaforms, i.e. the Late Paleozoic and early Mesozoic non-mammaliaform therapsids commonly known as the ‘mammal-like reptiles’[11,12]
In accordance with the direct phylogenetic relationship unifying Mammaliaformes and non-mammaliaform therapsids (NMT), and despite similarities with the anatomy of the maxillary canal in extant reptiles (Fig. 1A), the identification of the rami of the maxillary canal are based on the name of the corresponding ramus of the CNV2 in therian mammals
Summary
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. 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, Msx[2], in mice. These suggest that defining soft tissue characters of mammals were already present in their forerunners some 240 to 246 mya. As the maxillary canal is similar in most specimens examined, a detailed description is provided only for early cynodonts, the best documented taxon in our sample, and only diagnostic differences are listed for other taxa
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