Dental morphology of <i>Adalatherium hui</i> (Mammalia, Gondwanatheria) from the LATE Cretaceous of Madagascar

The dentition in the only known specimen of <i>Adalatherium hui</i>, a gondwanatherian mammal from the Late Cretaceous of Madagascar, is markedly different from that of any previously known mammaliaform, extinct or extant. The two preserved upper incisors and single lower incisor of <i>Adalatherium</i> are each very large, open-rooted, and bear a restricted band of enamel. A shallow-rooted, rudimentary upper canine is present. The upper postcanine dentition is composed of a small, simple, two-rooted tooth mesially and four large, multiple-rooted (five roots or more) teeth distally. The latter are quadrangular in outline, each with four major cusps and three perimetric ridges bordering a central valley that opens buccally. There are four lower postcanines, also with four major cusps each. The first postcanine has two roots, but the following ones each have (or likely had) at least four. The two distal lower postcanines bear a prominent mesiobuccal basin. The cusps and other topographic features of the postcanines cannot be unambiguously homologized with those of other known mammaliaforms. Diastemata are present in the upper dentition between the incisors and canines and between the canines and postcanines, and in the lower dentition between the incisors and postcanines. <i>Adalatherium</i>, like several gondwanatherians from the Late Cretaceous and Paleogene of Argentina, retains relatively plesiomorphic, ‘normal’ radial enamel (i.e., single-layered schmelzmuster, non-decussating small prisms, interprismatic matrix anastomosing around prisms). This stands in contrast to the modified radial enamel found in other gondwanatherians from the Late Cretaceous of India and Madagascar, which exhibit prominent interrow sheets of interprismatic matrix.产自马达加斯加晚白垩世的冈瓦纳兽类哺乳动物霍氏阿达莱兽（Adalatherium hui），其唯一已知标本的齿列与此前已发现的所有哺乳形类（mammaliaform），无论灭绝或现生，均存在显著差异。该物种保存的两枚上门齿与单枚下门齿均体型硕大、牙根开放，且仅在有限区域分布釉质；同时存在一枚牙根较浅的退化上门犬齿。 上颌后犬齿列由近中侧一枚小型简单的双根齿，与远中侧四枚大型多根（至少五根）牙齿构成：后者外形呈四边形，每枚均具四个主齿尖与三条周缘脊，围绕着一处向颊侧开放的中央谷。下颌后犬齿共计四枚，同样各带有四个主齿尖；其中第一枚后犬齿为双根结构，其余（或推测原始状态下）至少具备四根牙根。下颌远侧的两枚后犬齿带有显著的近中颊侧凹陷。后犬齿的齿尖及其他形貌特征，无法与其他已知哺乳形类的对应结构进行明确的同源性比对。 上颌齿列在门齿与犬齿之间、犬齿与后犬齿之间均存在齿隙；下颌齿列则在门齿与后犬齿之间存在齿隙。与阿根廷晚白垩世及古近纪的多种冈瓦纳兽类类似，霍氏阿达莱兽保留了相对原始的‘典型’放射状釉质（radial enamel）：即单层釉质构筑（schmelzmuster）、釉柱无交叉排列、柱间质（interprismatic matrix）环绕釉柱并呈吻合网状。这与印度、马达加斯加晚白垩世其他冈瓦纳兽类的特化放射状釉质形成鲜明对比——后者具有发达的柱间质层间片（interrow sheets）。