ABSTRACT

During the land-to-water transition in the Eocene epoch, the cetacean skeleton underwent modifications to accommodate life in the seas. These changes are well-documented in the fossil record. The forelimb transformed from a weight-bearing limb with mobile joints to a flipper with an immobile carpus. We used micro-CT imaging to assess evolutionary changes in carpal size, orientation, and articulation within Eocene cetacean taxa associated with the transition from a terrestrial to amphibious niche. We compared Ambulocetus natans, a well-preserved amphibious archaeocete, with other archaeocetes, and with Eocene terrestrial artiodactyls, the sister group to Cetacea. A cylindrical carpus in terrestrial taxa evolved into a mediolaterally flattened, cambered carpus in the semi-aquatic and fully aquatic cetaceans. Specifically, the pisiform bone shifted from a ventral orientation in terrestrial taxa to a lateral orientation, in plane with the carpus, within semi-aquatic and fully aquatic taxa. Flattening of the carpus, including lateral rotation of the pisiform, likely relates to functional shifts from weight-bearing terrestrial locomotion to aquatic locomotion. This laterally projecting pisiform morphology is retained in all extant cetaceans. Our results suggest this shift, along with other modifications to the carpus, predominantly occurred during the middle Eocene and facilitated an obligatorily aquatic lifestyle in late Eocene cetaceans.

摘要

在始新世时期由陆地向水域过渡期间，鲸类骨骼进行了改造，以适应海洋生物的需求。这些变化在化石记录中有充分记载。前肢从具有活动关节的承重肢体变为具有固定腕骨的鳍状肢。我们使用微型CT成像来评估始新世鲸类目群内腕骨大小，方向和关节运动的进化变化，这些变化与从陆地利基向两栖利基的过渡有关。我们比较了安非球菌，保存完好的两栖古生物，其他古细菌和始新世陆生偶蹄动物，是鲸类的姐妹群。陆生类群中的圆柱形腕鱼在半水生和完全水生的鲸类动物中演变成中外侧扁平的，弯曲的腕鱼。具体来说，在半水生和全水生类群中，梨状骨从陆生类群的腹侧方向转变为与腕骨成平面的横向方向。腕的扁平化，包括梨形的横向旋转，很可能与从承重的陆地运动到水生运动的功能转变有关。在所有现存的鲸类动物中都保留了这种侧向突出的梨形形态。我们的结果表明，这种转变以及对腕骨的其他修改，