The mandible and dentition of the Early Cretaceous monotreme <i>Teinolophos trusleri</i>

Abstract

The monotreme Teinolophos trusleri Rich, Vickers-Rich, Constantine, Flannery, Kool &amp; van Klaveren, 1999 from the Early Cretaceous of Australia&#13;\nis redescribed and reinterpreted here in light of additional specimens of that species and compared with the exquisitely preserved Early Cretaceous&#13;\nmammals from Liaoning Province, China. Together, this material indicates that although T. trusleri lacked a rod of postdentary bones contacting&#13;\nthe dentary, as occurs in non-mammalian cynodonts and basal mammaliaforms, it did not share the condition present in all living mammals, including&#13;\nmonotremes, of having the three auditory ossicles, which directly connect the tympanic membrane to the fenestra ovalis, being freely suspended&#13;\nwithin the middle ear cavity. Rather, T. trusleri appears to have had an intermediate condition, present in some Early Cretaceous mammals from&#13;\nLiaoning, in which the postdentary bones cum ear ossicles retained a connection to a persisting Meckel’s cartilage although not to the dentary.&#13;\nTeinolophos thus indicates that the condition of freely suspended auditory ossicles was acquired independently in monotremes and therian mammals.&#13;\nMuch of the anterior region of the lower jaw of Teinolophos is now known, along with an isolated upper ultimate premolar. The previously&#13;\nunknown anterior region of the jaw is elongated and delicate as in extant monotremes, but differs in having at least seven antemolar teeth, which&#13;\nare separated by distinct diastemata. The dental formula of the lower jaw of Teinolophos trusleri as now known is i2 c1 p4 m5. Both the deep&#13;\nlower jaw and the long-rooted upper premolar indicate that Teinolophos, unlike undoubted ornithorhynchids (including the extinct Obdurodon),&#13;\nlacked a bill.