The enigmatic spherical-horned <i>Tsaidamotherium</i> reveals an unsuccessful grazing adaptation of early giraffoids

Abstract

<p><i>Tsaidamotherium</i> was previously believed to be an ��<i>Ovibos</i>-like�� bovid because of its specialized basicranial morphology, which closely parallels that of living <i>Ovibos</i>. However, the plate-like headgear of <i>Tsaidamotherium</i> is so strange that confidently categorizing it as bovid horncores is challenging. In this paper, we conducted a comparative development study on the morphology of <i>Tsaidamotherium</i>. The headgear of a juvenile <i>Tsaidamotherium</i> exhibits a stepwise fusion state, with a distinct seam between the headgear and the calvaria. This morphology contradicts the typical development of bovid horncore anlagen, where fusion occurs in the early stages and is limited to a small area, making the fusion seam unobservable. Developmental conservatism dictates that primary bovid horncores form in pairs and strictly develop on the frontals. However, <i>Tsaidamotherium</i> possesses only one headgear primarily supported by the parietal. Therefore, the <i>Tsaidamotherium</i> headgear cannot be classified as bovid horncores but aligns better with giraffoid ossicones in terms of both development and morphology. <i>Tsaidamotherium</i>, as a miniaturized grazing giraffoid, likely represents a giraffoid lineage that adapted to open-land grazing, competing with various bovids but ultimately facing defeat. This may serve as another example to understand the early evolution and radiation of giraffoids: basal giraffoids exhibited diversity and expanded into open ecosystems. While most failed as small grazers, some survived as high-level browsers, eventually giving rise to <i>Giraffa</i>, which succeeded with an extremely long neck among giraffoids.</p>