Central to Rich's placental interpretation are A. nyktos's tribosphenic molars. All marsupials and placentals (termed therian mammals collectively) are characterized by this tooth type, in which hollows and cusps of corresponding upper and lower molars occlude in a mortar-and-pestle fashion to macerate food. According to Rich, details of the talonid basin, a lower molar hollow, in A. nyktos speak against a monotreme interpretation. And considering the absence of certain other features, Rich excludes the possibility that this creature was a marsupial. Rich also points to A. nyktos's inferred dental formula--the number of premolars and molars--as evidence of its placental nature: Placental mammals and, according to Rich's figuring, A. nyktos have five premolars and three molars, described as the 5/3 pattern.
Though intriguing, this mouthful of evidence has not convinced some scholars. "The talonid basin of A. nyktos doesn't look like that of any self-respecting placental mammal," declares San Diego State University paleontologist David Archibald. He points to a series of small cusps found on the tongue side of A. nyktos's talonid that are not seen in placental talonids. Any special similarities between this Australian beast and placental mammals, says Archibald, are probably the result of convergent evolution. Nor does Archibald think that the dental formula is conclusive. He notes that a 5 /3 pattern has also been observed for some monotremes, and suspects that this pattern may be a primitive trait for all therian mammals.
Image: LESLEY KOOL
Both Archibald and Richard L. Cifelli of the University of Oklahoma believe that A. nyktos looks as though it is related to an extinct monotreme called Steropodon (a fossil of which is known from the Early Cretaceous of Australia) or the peramurans, the group that gave rise to marsupials and placentals. Cifelli centers his arguments on A. nyktos's "really bizarre" premolar. "This style of molarization," he asserts, is "strongly suggestive of peramurans." The notion of A. nyktos as an intermediate step between monotremes and peramurans would lend support to the idea that monotremes share a much more recent common ancestor with marsupials and placentals than previously thought.
A more conservative opinion comes from University of Alberta paleontologist Richard C. Fox. Like Rich, he thinks that A. nyktos is a primitive tribosphenic mammal, but disagrees that it can be categorized more specifically than that. "It has some placental-like characters, but not the whole suite of characters that we would use to determine that it was a placental definitely."
So far, Rich's ideas have won tentative support from David W. Krause of the State University of New York at Stony Brook. "In terms of what we know about the evolution of early mammals, Rich's is the most parsimonious interpretation," he says. Because so little is known about Gondwanan mammals, Krause is wary of dismissing Rich's interpretation "just because we don't expect, based on current knowledge of early mammalian evolution on Gondwana, to see a placental mammal in the Early Cretaceous of Australia." He adds that if this fossil had been discovered in the Late Cretaceous of North America, "I don't think anyone would have doubted for an instant that it was anything but the jaw of a slightly weird placental."
Krause's own research has turned up unexpected mammals in Madagascar and India. Previously known only from Argentina, these "gondwanatheres" were announced within weeks of the A. nyktos find. And Krause notes, "Both of these discoveries together underscore to me that we really do need to keep an open mind about the biogeography of early mammals on Gondwana."