<Li> Its teeth look adapted for eating fish: the first two molars had cusps in a straight row, which made them more suitable for gripping and slicing than for grinding; and these molars are curved backwards, to help in grasping slippery prey . </Li> <P> The family tree above shows Hadrocodium as an "aunt" of crown mammals . This mammaliaform, dated about 195M years ago in the very early Jurassic, exhibits some important features: </P> <Ul> <Li> The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design . </Li> <Li> In therapsids and early mammaliaforms the eardrum may have stretched over a trough at the rear of the lower jaw . But Hadrocodium had no such trough, which suggests its ear was part of the cranium, as it is in crown - group mammals--and hence that the former articular and quadrate had migrated to the middle ear and become the malleus and incus . On the other hand, the dentary has a "bay" at the rear that mammals lack . This suggests that Hadrocodium's dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that Hadrocodium or a very close ancestor may have been the first to have a fully mammalian middle ear . </Li> <Li> Therapsids and earlier mammaliaforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain . This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it . Hadrocodium's braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward . In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain--in other words it now became possible for mammaliaforms both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating . </Li> </Ul> <Li> The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design . </Li>

When do mammals appear in the fossil record