Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley, California, United States of America.
Department of Geosciences, Virginia Tech, Blacksburg, Virginia, United States of America.
PLoS One. 2024 May 31;19(5):e0297894. doi: 10.1371/journal.pone.0297894. eCollection 2024.
Placerias hesternus, a Late Triassic dicynodont, is one of the last megafaunal synapsids of the Mesozoic. The species has a tusk-like projection on its maxillary bone, known as the caniniform process. This process has been hypothesized to be sexually dimorphic since the 1950s, however this claim has not been thoroughly investigated quantitatively. Here, we examined maxillae, premaxillae, quadrates, and fibulae from a single population from the Placerias Quarry in the Blue Mesa Member of the Chinle Formation, near St. Johns, Arizona, USA to determine if the caniniform process is dimorphic. We made a total of 25 measurements from the four bones and used a maximum likelihood framework to compare the fit of unimodal versus bimodal distributions for each set of measurements. Our results from complete maxillae reveal that the caniniform process has two distinct morphs, with a shorter and longer form. This interpretation is substantiated both by strong statistical support for bimodal distribution of caniniform lengths, and by clustering analysis that clearly distinguishes two morphs for the maxillae. Clustering analysis also shows support for potential dimorphism in the shape of the quadrate. However, no measurements from elements other than the maxilla have a strong likelihood of bimodal distribution. These results support the long-standing hypothesis that the caniniform in Placerias was dimorphic. Alternative explanations to sexual dimorphism that could account for the dimorphism among these fossils include the presence of juveniles in the sample or time-averaged sampling of a chronospecies, but both have been previously rejected for the Placerias Quarry population. The lack of strong dimorphism in non-maxilla elements and increased variation in caniniform length of the large-caniniform morph suggest that the caniniform is a secondary sexual trait, possibly used in intraspecific competition.
晚三叠世的 Placerias hesternus 是中生代最后一批巨型合弓兽之一。该物种的上颌骨上有一个类似于獠牙的突起，称为犬齿状突起。自 20 世纪 50 年代以来，人们一直假设这个突起具有性别二态性，但这一说法尚未得到彻底的定量研究。在这里，我们检查了来自美国亚利桑那州圣约翰附近蓝礁组的 Placerias Quarry 的单个种群的上颌骨、前上颌骨、方骨和腓骨，以确定犬齿状突起是否具有二态性。我们总共从这四块骨头中进行了 25 项测量，并使用最大似然框架比较了每种测量结果的单峰和双峰分布的拟合程度。我们从完整上颌骨得到的结果表明，犬齿状突起有两种不同的形态，一种较短，另一种较长。这一解释不仅得到了犬齿状突起长度双峰分布的强有力的统计支持，而且聚类分析也清楚地将两种形态区分开来。聚类分析还表明，方骨的形态也可能存在潜在的二态性。然而，除了上颌骨之外的其他元素的测量值都没有双峰分布的强烈可能性。这些结果支持了犬齿状突起在 Placerias 中具有性别二态性的长期假说。除了性二态性之外，还有其他可能导致这些化石存在二态性的解释，包括样本中存在幼年个体或对一个chronospecies 的时间平均采样，但这些解释都已经被先前对 Placerias Quarry 种群的研究否定了。非上颌骨元素中没有强烈的二态性，以及大犬齿状突起的犬齿状突起长度变化较大，这表明犬齿状突起是一种次要的性特征，可能用于种内竞争。
