Naples Virginia L, McAfee Robert K
Department of Biological Sciences, Northern Illinois University, Illinois, 60115, USA.
Department of Biological & Allied Health Sciences, Ohio Northern University, Ada, Ohio, 45810, USA.
F1000Res. 2014 Apr 4;3:86. doi: 10.12688/f1000research.3282.1. eCollection 2014.
Hapalops, a smaller-sized and early sloth of the Megatheroidea, appeared in the middle Miocene Santa Cruz formation of Argentina. This genus is part of the group from which later, larger megatheroids arose, i.e., Nothrotheriops and Megatherium. Many cranial characters support this idea; however Hapalops is not merely a smaller antecedent of the later forms. Specifically, Hapalops retains short anterior caniniform teeth, and a temporomandibular joint elevated above the cheek tooth row; a combination distinct among sloths. An elevated temporomandibular joint occurs in Bradypus, a tree sloth with anterior chisel-shaped teeth instead of caniniforms, and the tree sloth Choloepus, which is aligned with the megalonychids, has anterior caniniforms. Hapalops has an elongated zygomatic ascending process that is reminiscent of that in Bradypus; however, the Bradypus skull is extremely foreshortened while that of Hapalops is elongated, as in nothrotheres, but not deepened as in megatheres. Previous work identified many sloth cranial character complexes, and functional limitations on skull feature combinations. The unique Hapalops character patterns indicate a selective feeder with a mediolaterally oriented grinding stroke during mastication.
哈帕洛兽是大地懒超科中体型较小的早期树懒，出现在阿根廷中新世中期的圣克鲁斯组。该属是后来出现的体型较大的大地懒类群的一部分，即北方大懒兽属和大地懒属。许多颅骨特征支持这一观点；然而，哈帕洛兽不仅仅是后来这些形态的较小祖先。具体而言，哈帕洛兽保留了短的前犬形齿，以及高于颊齿列的颞下颌关节；这种组合在树懒中是独特的。颞下颌关节抬高的情况出现在三趾树懒身上，它有前凿形齿而非犬形齿，而与大爪地懒类相关的二趾树懒有前犬形齿。哈帕洛兽有一个细长的颧突上升支，这让人联想到三趾树懒的；然而，三趾树懒的头骨极度缩短，而哈帕洛兽的头骨是细长的，类似北方大懒兽，但不像大地懒那样加深。先前的研究确定了许多树懒颅骨特征复合体，以及头骨特征组合的功能限制。哈帕洛兽独特的特征模式表明它是一种选择性进食者，咀嚼时具有向内外方向的研磨动作。
