Mammaliaformes - Mammaliaformes in Life

Mammaliaformes in Life

Early mammaliaforms were generally shrew-like in appearance and size, and most of their distinguishing characteristics were internal. In particular, the structure of the mammaliaform (and mammal) jaw and arrangement of teeth is nearly unique. Instead of having many teeth that are frequently replaced, mammals have one set of baby teeth and later one set of adult teeth which fit together precisely. This is thought to aid in the grinding of food to make it quicker to digest. Warm-blooded animals require more calories than those that are cold-blooded, so quickening the pace of digestion is a necessity. The drawback to the fixed dentition is that worn teeth cannot be replaced, as was possible for the reptilian ancestors of mammaliaforms. To compensate, mammals developed prismatic enamel, characterized by crystallite discontinuities that helped spread out the force of the bite.

Lactation, along with other characteristically mammalian features, is also thought to characterize the Mammaliaformes, but these traits are difficult to study in the fossil record. While the early mammaliaforms likely had some form of lactation, their mammary glands probably were not associated with distinct mammae with nipples but rather were distributed in patches on the belly side with the young licking milk from the fur. Prior to hatching, the same glands would provide moisture to the leathery eggs, a situation still found in monotremes.

Some early mammaliaforms did have fur. An insulative covering is necessary to keep a homeothermic animal warm if it is very small, less than 5 cm (1.97 in) long; The 3.2 cm (1.35 in) Hadrocodium must have had fur, therefore, but the 10 cm (3.94 in) Morganucodon may not have needed it. The docodont Castorocauda, further removed from crown group mammals than Hadrocodium, had two layers of fur, guard hairs and underfur, as do mammals today.

Like monotremes today, the legs of early mammaliaforms were somewhat sprawling, giving a rather "reptilian" type of gait. In some forms the hind feet likely bore a spur similar to those found in the platypus and echidnas. Such a spur would have been connected to a venom gland for protection or mating competition.

Hadrocodium lacks the multiple bones in its lower jaw seen in reptiles. These are still retained, however, in earlier mammaliaforms.