s 

 aasiraoranaire 



by Michael Archer 



The first duck-billed platypus to set 

 webbed foot in Europe arrived in 1798 at 

 the Literary and Philosophical Society of 

 Newcastle on Tyne, England — pickled in 

 a wooden cask. It had been sent by the 

 governor of New South Wales, His Excel- 

 lency Mr. John Hunter, who had watched a 

 "native" spear this "animal of the mole 

 kind" in the Hawkesbury River. Unfortu- 

 nately, the courier who carried the cask 

 into the Society's rooms on her head was 

 nearly suffocated when the bottom of the 

 crate caved in, and the cask and its con- 

 tents dropped over her head. An English 

 historian, commenting later about the 

 event and the wretched woman, mused 

 that "apart from her physical nausea one 

 can picmre her mental horror on seeing a 

 strange creature, half bird, half beast, lying 

 at her feet." 



The unfortunate accident, however, had 

 an instructive aspect; like the defunct cask, 

 the platypus exhibited to the gentlemen of 

 the Society a most unexpected opening. 

 The animal's cloaca not only voids refuse 

 from the intestinal tract and bladder but 

 also ushers into the world the most re- 

 markable production of the platypus — 

 eggs. In contrast, placental mammals have 

 up to three external openings, two used for 

 excretion and one (in females) dedicated 

 to reproduction. The members of the order 

 Monotremata — platypuses and echidnas, 

 or spiny anteaters — are the only living 

 mammals that reproduce by laying eggs. 

 This distinction, in combination with such 

 seemingly archaic features as the unusual 

 structure of the shoulder girdle, has led to 

 a common and not unfair view that 

 monotremes are the most "primitive" 

 order of living mammals. But add to these 

 so-called primitive features the electric 

 sensors in the bill that can detect the mus- 

 cular activity of fleeing prey, and you have 

 a very odd blend of archaic and super- 

 specialized structures. 



Arguments about the evolutionary rela- 

 tionships of monotremes have run the 

 gamut from the bizarre (cousins of turtles) 

 to the implausible (degenerate marsupials) 



48 Natural History 4/94 



to the possible (direct descendants of 

 Mesozoic eupantotheres) and the tantaliz- 

 ing (surviving mammallike reptiles). The 

 best bets at the moment are the last two: 

 monotremes may be either long-lasting 

 descendants of eupantotheres (tiny mam- 

 mals common in Europe and in the Amer- 

 icas during Mesozoic times) or mam- 

 mallike reptiles that have independently 

 acquired mammalian hallmarks, such as 

 three middle ear bones. 



Part of the difficulty in working out the 

 relationships of monotremes has been 

 their lack of well-formed teeth. Platypuses 

 gum their food to death, their grossly de- 

 generate teeth being lost early in life. 

 Echidnas, having lost all trace of teeth, 

 tongue-slurp worms, ants, and termites. 

 Thus, comparisons with extinct mammals, 

 which are often known only from fossil 

 teeth, are difficult to make. After a brave 

 attempt to make sense of the structure of 

 the platypuses' vestigial teeth, mammalo- 

 gist George Gaylord Simpson concluded 

 in 1929 that whatever monotremes were, 

 they were something "quite distinct" from 

 all other groups of mammals. 



Little further light was shed on the ori- 

 gins of monotremes until 1971, when two 

 discoveries were made in South Australia. 

 In the Tirari Desert, Mike Woodbume, of 

 the University of California at Riverside, 

 and I found a fully formed fossil tooth of 

 an early Miocene platypus (later named 

 Obdurodon insignis — "significant lasting 

 tooth"). That same year, Dick Tedford, of 

 the American Museum of Natural History, 

 and his colleagues unearthed another 

 platypus tooth in a fossil deposit near Lake 

 Frome. But it was not until 1984, when our 

 research group at the University of New 

 South Wales found a whole skull and most 

 of the teeth of a fossil platypus some fif- 

 teen million years old at Riversleigh, 

 Queensland, that we at last had the first 

 complete and well-formed dentition of an 

 adult monotreme. 



Hot on the heels of this discovery came 

 another. Fossil fish expert Alex Ritchie, of 

 the Australian Museum in Sydney, was 



mulling over a collection of opalized early 

 Cretaceous fossils (about 120 million 

 years old) gathered from Lightning Ridge, 

 New South Wales, by the Caiman broth- 

 ers, two amateur collectors. Among the 

 brilliantly flashing specimens, he spotted a 

 little jaw fragment sporting three gemlike 

 teeth. Suspecting that it belonged to a 

 mammal but not sure what kind, he sug- 

 gested that I have a look. In their basic 

 structure, the three molars in this jaw were 

 so similar to the teeth we were examining 

 from Riversleigh that we had no doubt that 

 this, Australia's earUest known Mesozoic 

 manunal, was a monotreme. We named 

 the creature Steropodon galmani, "Gal- 

 man's lightning tooth." Not only was this 

 the oldest mammal so far found in Aus- 

 tralia but its discovery sextupled the 

 known age of monoti"emes. 



The surprises continued. In 1991 

 Rosendo Pascual, of the Museo de La 

 Plata in Argentina, wrote to Mike Augee, 

 who was organizing a symposium on the 

 biology of monotremes, telling of a 

 strange tooth he and his team had col- 

 lected from Patagonia, in southern Ar- 

 gentina, at a site that was sixty-one to 

 sixty-three million years old. Although 

 much older, it resembled the teeth that had 

 been described from the Tirari Desert. 

 After Rosendo sent a photograph, the Aus- 

 tiralian Geographic Society, the Royal Zo- 

 ological Society of New South Wales, the 

 Riversleigh Society, and the University of 

 New South Wales quickly offered to fly 

 him and his tantalizing tooth to Sydney. 

 When he arrived and we set his find along- 

 side die teeth from Riversleigh, the only 

 comments were gasps. The two forms 

 were abnost identical despite a separation 

 of nearly forty million years and three 

 continents. 



The Patagonian platypus, which could 

 be called nothing else in view of its stiik- 

 ing similarity to Australian fossil platy- 

 puses, was named Monotrematum sud- 

 americanum, "the South American 

 monotreme." The next year, supported by 

 the Australian Geographic Society and 



In 1984, the fifteen-million-year- 

 old skull and teeth of a platypus 

 came to light in the rich fossil beds 



of Riversleigh in Queensland, 



Australia. In this reconstruction, 



the ancient platypus Obdurodon 



dicksoni basks on a mossy rock in 



its lush rainforest home. 



Painting by Jeanette Muirhead 



