Funston Gregory F, Kynigopoulou Zoi, Williamson Thomas E, Brusatte Stephen L
School of GeoSciences, University of Edinburgh, Edinburgh, UK.
Royal Ontario Museum, Toronto, Ontario, Canada.
J Anat. 2025 Sep-Oct;247(3-4):819-841. doi: 10.1111/joa.70010. Epub 2025 Jul 14.
The life histories of Palaeocene mammals are poorly known, but may have been central to their success in diversifying across terrestrial ecosystems after the end-Cretaceous extinction. Among these mammalian groups, the eutherian Taeniodonta are particularly enigmatic, with few modern analogues and no living descendants, despite being one of the only lineages to apparently traverse the Cretaceous-Palaeogene (K-Pg) boundary. Here, we investigate the life history of an early Palaeocene taeniodont, Conoryctes comma, based on a multi-individual, multi-element sample. Nearly all elements sampled exhibit similar osteohistological architecture, with a small internal zone of compacted coarse cancellous bone surrounded by an internal cortex of periosteally derived fibrolamellar bone of variable thickness, and an outer cortex of lamellar bone. The well-vascularized fibrolamellar complex in the limb bones, lacking cyclical growth marks, is indicative of overall rapid growth to near adult body size. Cyclical growth marks are present in the outer cortex after the transition to slow-growing lamellar bone, but not in the inner cortex, suggesting sexual maturity was reached in 1 year. In some elements, an internal non-cyclical growth mark shares histological similarities with weaning marks in living mammals and other contemporary Palaeocene mammals, and occurred at the body size predicted for this transition in therian mammals. The unusual presence of compacted coarse cancellous bone near the midshafts of multiple limb bones may be related to cortical thickening, and is similar to the arrangement described in some fossorial mammals, supporting previous assertions of this lifestyle in Conoryctes. Altogether, these palaeohistological signals suggest a life history in C. comma similar to living eutherians, despite uncertainty about whether it is within crown Placentalia or a close outgroup. Thus, our data are consistent with an early origin of placental-like reproductive strategies in their eutherian ancestors, although this attribute was likely shared more broadly among Mesozoic mammal lineages prior to the end-Cretaceous extinction.
古新世哺乳动物的生活史鲜为人知，但这可能是它们在白垩纪末大灭绝后成功在陆地生态系统中多样化的关键因素。在这些哺乳动物类群中，真兽类的绦齿目尤其神秘，尽管它们是明显跨越白垩纪-古近纪（K-Pg）界线的少数谱系之一，但现代类似物很少，也没有现存后代。在这里，我们基于一个多个体、多元素的样本，研究了一种早期古新世绦齿目动物——逗号锥齿兽（Conoryctes comma）的生活史。几乎所有采样的骨骼元素都表现出相似的骨组织学结构，内部有一个由致密的粗松质骨组成的小区域，周围是一层厚度不一的骨膜衍生纤维层状骨的内部皮质，以及一层板层骨的外部皮质。四肢骨骼中血管丰富的纤维层状复合体缺乏周期性生长标记，表明其整体生长迅速，接近成年体型。在向生长缓慢的板层骨转变后，外部皮质出现了周期性生长标记，但内部皮质没有，这表明其在1岁时达到性成熟。在一些骨骼元素中，内部的非周期性生长标记在组织学上与现存哺乳动物和其他当代古新世哺乳动物的断奶标记相似，并且发生在有袋类哺乳动物这种转变所预测的体型时。多根四肢骨骼中部附近致密粗松质骨的异常存在可能与皮质增厚有关，并且与一些穴居哺乳动物中描述的结构相似，这支持了之前关于逗号锥齿兽这种生活方式的断言。总的来说，这些古组织学信号表明逗号锥齿兽的生活史与现存真兽类相似，尽管不确定它是否属于胎盘类冠群或一个密切的外类群。因此，我们的数据与胎盘类生殖策略在其真兽类祖先中的早期起源一致，尽管这种特征在白垩纪末大灭绝之前可能在中生代哺乳动物谱系中更广泛地存在。
