Fossil remains of the earliest known monotreme (the rat-sized Teinolophous trusleri) date from about 115 million years ago, when the world was still very much dominated by dinosaurs (Musser 2003). The earliest known animal that is definitely believed to have been a member of the Ornithorhynchidae or platypus family (based on fossilised teeth and some pieces of leg bone) is Monotrematum sudamericanum. It lived in Patagonia (in southern Argentina) about 61-63 million years ago – at a time when Australia, Antarctica and South America were still joined together as part of a single supercontinent known as Gondwana (Pascual et al. 1992). M. sudamericanum is believed to have measured around 70 centimetres in length, which is a little larger than the size of a modern platypus (Pian et al. 2013).
The fossil remains of three other ancient species related to the platypus have been recovered from the Lake Eyre region of central Australia (Obdurodon insignis) and at Riversleigh in northwestern Queensland (Obdurodon dicksoni and Obdurodon tharalkooschild) (Musser 1998, 2003; Pian et al. 2013). These animals lived between about 15 and 25 million years ago and are all believed to have had a bill and associated electroreceptors (Asahara et al. 2016), though bill size and shape (as shown at left for Obdurodon dicksoni) were not necessarily the same as in a modern platypus. In contrast to the modern platypus, species of Obdurodon were also apparently equipped with permanent teeth (Asahara et al. 2016).
The earliest known fossil remains that correspond to the dimensions and shape of the modern platypus’s skeleton date from around 4 million years ago (Musser 1998).
Photo of skull courtesy of T. Rowe (U. of Texas)
LITERATURE CITED
Asahara M, Koizumi M, Macrini TE, Hand SJ and Archer M (2016) Comparative cranial morphology in living and extinct platypuses: feeding behaviour, electroreception and loss of teeth. Science Advances 2, e1601329.
Musser AM (1998) Evolution, biogeography and palaeoecology of the Ornithorhynchidae. Australian Mammalogy 20, 147-162.
Musser AM (2003) Review of the monotreme fossil record and comparison of palaeontological and molecular data. Comparative Biochemistry and Physiology Part A 136, 927–942.
Pascual R, Archer M, Jaureguisar EO, Prado JL, Godthelp H and Hand SJ (1992) The first non-Australian monotreme: an early paleocene South American platypus (Monotremata, Ornithorhynchidae). Pp. 1-14 in Platypus and Echidnas (ML Augee, ed.) Royal Zoological Society of New South Wales: Mosman NSW.
Pian R, Archer M and Hand SJ (2013) A new, giant platypus, Obdurodon tharalkooschild, sp. nov. (Monotremata, Ornithorhynchidae), from the Riversleigh World Heritage Area, Australia. Journal of Vertebrate Paleontology 33, 1255-1259.