An island continent with a highly endemic biota sharing certain elements with other southern continents, Australia has long fascinated students of evolution and biogeogra-phy. Until recently, though, no mammalian fossils older than Miocene were known from this important landmass down under. Discoveries in the Early Cretaceous of Australia (table 2.12) are now beginning to shed light on the antiquity of mammals there and are raising fundamental new questions on the origin and relationships of some major mammal groups.
The first Early Cretaceous mammal to be reported from Australia is from Lightning Ridge, New South Wales (figure 2.13). The occurrence is in the Wallangulla Sandstone Member of the Griman Creek Formation in a whitish claystone interbedded with clayball conglomer-
table 2.12. Early Cretaceous Mammals of Australia (see figure 2.13). Localities: 1, Lightning Ridge, New South Wales (Griman Creek Formation, Albian); 2, Dinosaur Cove (Eumeralla Formation, Aptian-Albian); 3, Flat Rocks, Victoria (Wonthaggi Formation, Aptian)
?Mammalia incertae sedis (1) Mammalia incertae sedis (1) Australosphenida Ausktribosphenidae
Ausktribosphenos nyktos (3) Bishops whitmorei (3) Gen. et sp. indet. (3) Kollikodontidae
Kollikodon ritchei (1) Steropodontidae
Steropodon galmani (1) ?Steropodontidae
Teinolophos trusleri (3) Family incertae sedis Gen. et sp. indet. (2)
ates, representing a shallow estuarine environment (Archer et al., 1985; Flannery et al., 1995). Palynomorphs and other evidence indicate a middle Albian age for the unit. A diverse vertebrate fauna, including dinosaurs (see, e.g., Weishampel, 1990), is known from Lightning Ridge. Many of the fossils have been transported. Notably, the fossils are pseudomorphs: the bone has been replaced by opal, which is mined from the unit. The fossils are highly prized by collectors. Steropodon galmani, known by a dentary with the last three molars, was first described as a monotreme possibly belonging to the platypus family (Archer et al., 1985). It was later placed in its own family, Steropodontidae, and considered to represent a sister taxon to the two living families of monotremes, Ornithorhynchidae and Tachy-glossidae (Flannery et al., 1995). Monotremes are generally considered to represent a clade that diverged extremely early in mammalian evolution. However, the teeth of Steropodon are remarkably advanced, suggesting a closer relationship to therians (sensu lato) than previously thought (Archer et al., 1985; Kielan-Jaworowska, Crompton, and Jenkins, 1987; Luo, Cifelli, and Kielan-Jaworowska, 2001). A second mammal from Lightning Ridge, Kollikodon ritchei, was originally based on a dentary with three molars. A well-preserved maxilla with teeth was subsequently found and is figured herein as figure 6.5D2 (courtesy of Anne Musser). Flannery et al. (1995) argued that the morphology of the dentary indicates that Kolli-kodon might be a monotreme, but we assign it to Monotremata only tentatively (see chapter 6). Both Steropodon and Kollikodon are rather large by the standard of Meso-
zoic mammals, possibly reflecting cold conditions in the Early Cretaceous of Australia (Rich et al., 1989). Mono-tremes clearly achieved some degree of diversity in the Australian Mesozoic. Ornithorhynchidae are now known from the Tertiary of South America, suggesting possible dispersal to that continent no later than early Paleocene (Pascual et al., 1992a).
Two other specimens from Lightning Ridge deserve passing mention. One, an edentulous maxilla, belongs to a mammal but is not further identifiable. The other is a complex tooth of debatable affinities. If mammalian, the specimen most resembles the pattern seen in upper molars of Dryolestidae (Clemens et al., 2003). Dryolestids survived through much of the Late Cretaceous in South America (see later); hence their presence in the Early Cretaceous of Australia might be expected. If mammalian (dryolestid or not), this specimen is notable for its large size.
Early Cretaceous mammals have been recovered from two other sites in Australia, both on the coast of Victoria (figure 2.13). Dinosaur Cove, about 200 km west of Melbourne, is in the Eumeralla Formation. Two undescribed specimens, a monotreme humerus and a fragment of a mammal tooth, have been reported from this site (Rich et al., 1999).
By far the most significant and eye-opening mammalian fossils from the Cretaceous of Australia hail from the Flat Rocks site, about 120 km southeast of Melbourne. The site is under active investigation by Thomas H. Rich, Patricia Vickers-Rich, and associates (see Rich and Vickers-Rich, 2000). Flat Rocks, which lies in the Won-thaggi Formation, consists of fluvial volcaniclastic sandstones and mudstones. It is placed in the Aptian, based on combined palynological and radiometric data. Evidence of cryoturbation stratigraphically below the fossiliferous horizon attests to cold temperatures and seasonally frozen ground (Rich et al., 1999). First to be described from Flat Rocks is Ausktribosphenos nyktos, based on several den-taries. The cheek teeth have an advanced morphology that belies their age, suggesting the possibility of eutherian affinities (Rich et al., 1997, 1999, 2001a). Other evidence, including jaw structure (Kielan-Jaworowska et al., 1998), suggests that Ausktribosphenos may represent an early, divergent clade that also includes monotremes (Luo, Cifelli, and Kielan-Jaworowska, 2001; Luo et al., 2002; Rauhut et al., 2002, see chapter 6). Monotremes were clearly differentiated by the Aptian, as shown by another element of the Flat Rocks fauna: Teinolophus trusleri, a possible relative of Steropodon, mentioned earlier (see Rich et al., 1999; Rich, Vickers-Rich, et al., 2001). The most recent addition to the roster of named mammals from Flat Rocks is Bish
Hobart figure 2.13. Mesozoic mammals localities of Australia (all Early Cretaceous; black diamonds). Localities: 1, Lightning Ridge (Wallangulla Sandstone Member, Griman Creek Formation; Albian, New South Wales); 2, Dinosaur Cove (Eumeralla Formation; ?Aptian-Albian, Victoria); 3, Flat Rocks (Wonthaggi Formation; Aptian, Victoria).
Hobart ops whitmorei. Known by several dentaries and the entire postcanine dentition, Bishops is an ausktribosphenid and, like Ausktribosphenos, its affinities are contended.
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