Heterochrony, dental ontogenetic diversity and the circumvention of constraints in marsupial mammals and extinct relatives

In marsupial mammals and their extinct relatives -collectively, metatherians- only the last premolar is replaced, but the timing of dental eruption is variable within the group. Our knowledge of fossils metatherians is limited, but is critical to understanding several aspects of the evolution and morphological diversification of this clade. We analyzed the sequence of eruption of 76 specimens of metatherians, including Sparassodonta, an extinct clade of specialized carnivores from South America. In Sparassodonta (1) the P3/p3 erupt simultaneously, in common with some didelphids (in other didelphids, p3 erupts before P3, whereas in the remaining didelphids, some peramelids, one caenolestid, and Pucadelphys this order is reversed); (2) the upper and lower molars at the same locus erupt more in synchrony than in other carnivorous metatherians in which the lower molars clearly precede the upper equivalents; (3) the upper canine in thylacosmilids and proborhyaenids is hypselodont; (4) specie...有袋类哺乳动物及其已灭绝近亲——统称为后兽类（metatherians）——中，仅最后一枚前臼齿会发生替换，但该类群内的牙齿萌出时序存在变异。目前学界对化石后兽类的认知仍较为有限，但其对于理解该演化支的演化历程与形态多样性的诸多关键方面至关重要。本研究分析了76件后兽类标本的牙齿萌出序列，其中包括袋鬣狗类（Sparassodonta）——一类来自南美洲的已灭绝特化食肉演化支。针对袋鬣狗类，我们发现：（1）其上第三前臼齿（P3）与下第三前臼齿（p3）同步萌出，这一点与部分负鼠科（didelphids）物种一致；而在其他负鼠科物种中，下第三前臼齿（p3）的萌出早于上第三前臼齿（P3），其余负鼠科类群、部分袋狸科（peramelids）、1种袋鼩猬科（caenolestid）物种以及普氏袋犬（Pucadelphys）的萌出顺序则恰好相反；（2）同位置的上下臼齿萌出同步性相较于其他食肉后兽类更强，后者的下臼齿明显早于同位置的上臼齿萌出；（3）袋剑虎类（thylacosmilids）与原剑虎类（proborhyaenids）的上犬齿为高冠齿（hypselodont）；（4）