A Pocketful o' 

 -ossils 



by Michael J. Novacek 



Tugrugeen Shireh, a line of cliffs near 

 an alkaline lake in the Gobi Desert of 

 Mongolia, is not marked on any road map. 

 Indeed, there are virtually no maps for this 

 poorly charted region of the world. But 

 over the past four summers, "Tugrug" has 

 become a paleontological mecca for our 

 joint team from the American Museum of 

 Natural History in New York and the 

 Mongolian Academy of Sciences. Not 

 only have we found exquisitely preserved 

 theropod dinosaurs, such as the agile 

 flesh-eater Velocirapton and the dinosaur- 

 like bird Mononykus (see "New Limb on 

 the Avian Family Tree," Natural History, 

 September 1993), but we have also uncov- 

 ered a wealth of tiny fossil skulls and 

 skeletons, remains of mammals that lived 

 in the shadows of the dinosaurs. 



These mammal bones are preserved in 

 Brazil-nut-sized concretions of hard, dark 

 sandstone and iron-bearing minerals. 

 These concretions continually erode from 

 the soft, white sandstone that makes up the 

 bulk of the Tugrug cliffs as they are bat- 

 tered by high winds and seasonal rain- 

 storms, but they still provide a durable 

 coating that protects the more fragile fossil 

 bone underneath. Such conditions practi- 

 cally guarantee our discovery of more 

 mammals every season, even on slopes we 

 have crawled across many times before. 



These fossils represent mammal com- 

 munities that lived about eighty million 

 years ago, near the end of the Mesozoic 

 era, the Age of Dinosaurs. Although the 

 following period, the Tertiary, is consid- 

 ered the Age of Mammals, the iirst two- 

 thirds of the entire history of mammals 

 was played out in the Mesozoic. Unlike 

 most Mesozoic localities, which yield 



only isolated teeth or bits of jaws with 

 teeth, Tugrug and other Gobi sites have 

 given us fine skulls and entire skeletons. 

 The fossils have provided critical clues to 

 the evolutionary steps linking Mesozoic 

 with modem mammals, as well as with 

 their primitive vertebrate relatives. The 

 more complete fossils have also revealed 

 secrets of locomotion, feeding, sensory 

 systems, and possible life styles of these 

 ancient creatures. 



The earliest mammals were the tricon- 

 odonts, shrewlike creatures that appeared 

 some 200 million years ago, during the 

 Triassic period. They were tiny; an adult 

 could snooze comfortably curled up in a 

 teaspoon. Most likely, triconodonts laid 

 eggs, as do the living duck-billed platypus 

 and echidna. During the succeeding Juras- 

 sic and Cretaceous periods, the tricon- 

 odonts were joined by other mammalian 

 lineages. Although many of these Meso- 

 zoic "experiments" waned and died out 

 before or at the time of dinosaur extinc- 

 tion, sixty-five million years ago, some 

 survived and diversified into the modem 

 mammals — animals as different as kanga- 

 roos, koalas, primates, bats, whales, ele- 

 phants, and aardvarks. 



Thus, mammals from Mesozoic sites 

 reveal a biological empire in transition, 

 with archaic creamres destined to go ex- 

 tinct before the Age of Mammals had even 

 begun, living nose to nose (or fang to 

 claw) with the precursors of modem mam- 

 mals. Tugrug preserves a pastiche of 

 mammal species. These cliffs do not con- 

 tain the generally older triconodonts, but 

 they have yielded abundant remains of a 

 group known as the multituberculates. 

 With their long, gnawing incisors; blade- 



Some eighty million years ago, in the arid regions of central Asia, a 



family o/Protoceratops sleeps while rat-sized mammals known as 



Deltatheridium/orage by night. Deltatheridium, which may have been a 



marsupial, or pouched mammal, may also have used its acute sense of 



hearing and smell to detect live prey such as insects or tiny lizards. 



Painting by Ely Kish 



41 



