MESOZOIC 



JURASSIC 



TRIASSIC 



144 



213 



248 



grasping digits suggest that some multis 

 were adept at climbing trees. Long-limbed 

 animals like Zalambdalestes were capable 

 runners and leapers and might have 

 dashed about like rabbits or jumping mice. 

 Yet what we know of the anatomy of 

 Mesozoic mammals suggests they had a 

 narrower adaptive range than their modem 

 counterparts. Our Mesozoic antecedents 

 are all small; certain triconodonts are com- 

 parable to the tiniest living shrews, and 

 even the largest of the multis only reach 

 the size of opossums. (Size itself puts lim- 

 itations on adaptive virtuosity. An animal 

 had to be sizable to eat the fishes and large 

 lizards that survived beyond the end of the 

 Cretaceous. In addition, larger mammals 

 are capable of behaviors such as long-dis- 

 tance migration.) Mesozoic mammals 



were constrained not only by small size 

 but also by a rather standardized and prim- 

 itive sensory system. This observation is 

 based on the study of endocasts, casts of 

 the brain formed by the infilling of sandy 

 matrix in fossil skulls. Endocasts of multis 

 and other Mesozoic creatures show a rela- 

 tively small cortical area with few, if any, 

 folds, or sulci, suggesting limited intelli- 

 gence. (In contrast, think of the intricate 

 folding of the human brain, which greatly 

 increases the cortical surface.) 



By and large, Mesozoic mammals are 

 all noses and ears. Their olfactory lobes, or 

 smelling centers, are well developed in 

 contrast to their optic regions, or vision 

 centers. Lobes near the back of the brain 

 that represent hearing centers are also well 

 developed. Most of these mammals would 



seem to have had a keen sense of smell 

 and acute, high-frequency hearing, but 

 rather poor vision, like living shrews and 

 hedgehogs. Presumably they were most 

 active at night, a time when the senses of 

 hearing and smell, as opposed to vision, 

 are critical. 



Our team will continue to crawl com- 

 pulsively along the Tugrug slopes in order 

 to piece together a more complete picture 

 of the evolution and natural history of 

 Mesozoic mammals. We are elated that an 

 assortment of skeletons that can fit com- 

 fortably in a shoe box has helped illumi- 

 nate the first two-thirds of mammal evolu- 

 tion. And this summer we hope to 

 experience once again the elation of 

 trundling down the cliffs of Tugrug with a 

 pocketful of fossil skulls. 



EYE SOCKETS 

 NEAR SNOUT 

 aETHYTHERES) 



SYNAPSID OPENING 

 (SYNAPSIDS) 



WATERTIGHT EGG 

 (AMNIOTES) 



HOOFS 

 (UNGULATES) 



ANIMAL 

 CLASSIFICATION 



Kingdom 

 Phylum 

 Class 

 Order 

 Family 

 Genus 

 Species 



43 



