The First Mammals

The first mammals appeared in the Late Triassic, but the first fossils are incomplete. Adelobasileus and Sinocon-odon appear to be the most basal mammals,but the first reasonably well-represented mammals are the mor-ganucodontids from the Early Jurassic (Kermack et al., 1973,1981; Jenkins and Parrington, 1976). These early mammals were tiny, with 20-30-mm skulls and total body lengths of less than 150 mm. They probably looked generally like shrews.

10.2.1 The most basal mammals

The oldest known mammal, Adelobasileus from the lower part of the Upper Triassic of Texas, USA (Lucas and Luo, 1993), is based on a specimen representing the braincase region (Figure 10.6(a)). This is enough,

Adelobasileus

Fig. 10.6 Skulls of basal mammals, (a) Adelobasileus,(b, c) Sinoconodon and (d-h) Morganucodon in (a,b, d) lateral, (c, e) dorsal and (f) ventral views; (g) lower jaw in medial view; (h) lateral wall of the braincase (unossified areas shown with regular hatching). [Figure (a) modified from Lucas and Luo, 1993; (b, c) modified from Crompton and Luo, 1993; (d-h) after Kermack etal., 1981.]

Fig. 10.6 Skulls of basal mammals, (a) Adelobasileus,(b, c) Sinoconodon and (d-h) Morganucodon in (a,b, d) lateral, (c, e) dorsal and (f) ventral views; (g) lower jaw in medial view; (h) lateral wall of the braincase (unossified areas shown with regular hatching). [Figure (a) modified from Lucas and Luo, 1993; (b, c) modified from Crompton and Luo, 1993; (d-h) after Kermack etal., 1981.]

however, to show that it is very probably a mammal. The braincase shows a number of features characteristic of early mammals. The reptilian epipterygoid, typically a thin column of bone, has become a broad sheet, termed the alisphenoid in mammals. In front of this, a new element, the orbitosphenoid, contacts the frontal and palatine in front and forms a solid internal wall in the back portion of the eye socket. Towards the rear, the brain is expanded and it is enclosed almost completely in bone. Basal amniotes have the brain enclosed at the side only by the prootic, opisthotic and epipterygoid bones (see p. 108). In Adelobasileus, the prootic sends a large sheet of bone forwards, the anterior lamina of the periotic (or petrosal), which meets the parietal above and alisphenoid in front. The trigeminal nerve, cranial nerve number V, sends two major branches to the snout, temporal and lower jaw regions through the anterior lamina of the petrosal. There are numerous other mammalian characters in this remarkable early specimen (see Box 10.3).

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