ABSTRACT

One of the largest members of Sigmodontinae, the extinct Megaoryzomys curioi from Santa Cruz Island in the Galápagos Archipelago (Ecuador), is traditionally treated as a representative of Thomasomyini. We contrasted this hypothesis based on a direct study of craniodental material of M. curioi, including a well-preserved skull assessed through CT-scan examination, within a broader sampling of brachydonts sigmodontines. M. curioi lacks or presents a poorly expressed suspensory process of the squamosal bone, while in Thomasomyini this structure is well developed. In the same way, M. curioi does not exhibit the condition in the dorsal aperture of the ectotympanic displayed by the Thomasomyini, and the morphology of the mandible also contrasts with the widespread condition shown by Thomasomyini. Besides, the procingulum of the first lower molar in M. curioi lacks the typical pattern of Thomasomyini composed of two defined conulids. Overall craniodental morphology in M. curioi favours its allocation to the Oryzomyini. This alternative hypothesis resolves two biogeographical issues connected with the target genus: (1) Galápagos Islands were colonised by members of a single sigmodontine tribe (i.e., Oryzomyini) and (2) within the sigmodontine radiation; members of the Oryzomyini were uniquely capable of reaching oceanic islands.

摘要

Sigmodontinae 最大的成员之一，来自加拉帕戈斯群岛（厄瓜多尔）圣克鲁斯岛的已灭绝Megaoryzomys curioi，传统上被视为 Thomasomyini 的代表。我们基于对M. curioi的颅牙材料的直接研究来对比这一假设，包括通过 CT 扫描检查评估的保存完好的头骨，在更广泛的短齿象 sigmodontines 样本中。M. curioi缺乏或表现出较差的鳞状骨悬突，而在 Thomasomyini 中，这种结构发育良好。同样地，M. curioi没有表现出Thomasomyini所显示的外鼓膜背孔的情况，下颌骨的形态也与Thomasomyini所显示的广泛情况形成对比。此外，M. curioi 中第一下磨牙的前齿缺乏由两个明确的锥体组成的典型的Thomasomyini 模式。M. curioi 的整体颅牙形态倾向于将其分配给 Oryzomyini。这种替代假设解决了与目标属相关的两个生物地理问题：(1) 加拉帕戈斯群岛被一个单一的 sigmodontine 部落（即 Oryzomyini）的成员殖民，以及 (2) 处于 sigmodontine 辐射范围内；Oryzomyini 的成员具有独特的能力到达海洋岛屿。