某些动物能够通过口腔或鼻腔把从喉部产生的超声波发散出去，利用折回的声音来定方向，这种空间定向的方法被称为回声定位（echolocation）。我们所熟知具有回声定位能力的动物有蝙蝠、鲸、海豚等。而在地面生活具有回声定位能力的哺乳动物仅有北美短尾鼩鼱（Blarina）、栗齿鼩鼱（Sorex）以及马达加斯加岛的马岛猬（tanrec），它们不像蝙蝠那样利用回声定位来寻找食物，而更多的用于搜寻栖息地。
雷比达猪齿兽（Hyopsodus lepidus）颅骨化石的三维成像
此次古生物学家和神经学家发现了一块已灭绝的古新世-始新世（Paleocene-Eocene）有蹄类（偶蹄类）哺乳动物颅骨化石，这是科学家首次发现史前动物回声定位能力的化石证据。这块雷比达猪齿兽（Hyopsodus lepidus）颅骨化石的相关研究报道发表在公共科学图书馆（Public Library of Science）网站上。
研究人员介绍道：“根据猪齿兽颅骨化石的颅腔复原模型形态学研究表明：雷比达猪齿兽的听觉能力高度发达，我们推测这可能与它们利用地面回声定位有关。同时它是一种行动敏捷，能快速移动，通常生活在洞穴里夜行性动物，这可能也与它们地面回声定位能力的演化有关。像回声定位如此特化功能的出现也反映了古代猪齿兽应具有非常特殊的大脑皮质结构。并且，这种史前猪齿兽较大的大脑容量以及大脑各部分的相对比例都与较进化的有蹄类动物非常相似，使得雷比达猪齿兽的大脑演化在较低级的有蹄类动物中尤为显著。”(生物谷 Bioon.com)
doi:10.1371/journal.pone.0030000
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Digital Cranial Endocast of Hyopsodus (Mammalia, “Condylarthra”): A Case of Paleogene Terrestrial Echolocation?
Maeva J. Orliac, Christine Argot, Emmanuel Gilissen
We here describe the endocranial cast of the Eocene archaic ungulate Hyopsodus lepidus AMNH 143783 (Bridgerian, North America) reconstructed from X-ray computed microtomography data. This represents the first complete cranial endocast known for Hyopsodontinae. The Hyopsodus endocast is compared to other known “condylarthran” endocasts, i. e. those of Pleuraspidotherium (Pleuraspidotheriidae), Arctocyon (Arctocyonidae), Meniscotherium (Meniscotheriidae), Phenacodus (Phenacodontidae), as well as to basal perissodactyls (Hyracotherium) and artiodactyls (Cebochoerus, Homacodon). Hyopsodus presents one of the highest encephalization quotients of archaic ungulates and shows an “advanced version” of the basal ungulate brain pattern, with a mosaic of archaic characters such as large olfactory bulbs, weak ventral expansion of the neopallium, and absence of neopallium fissuration, as well as more specialized ones such as the relative reduction of the cerebellum compared to cerebrum or the enlargement of the inferior colliculus. As in other archaic ungulates, Hyopsodus midbrain exposure is important, but it exhibits a dorsally protruding largely developed inferior colliculus, a feature unique among “Condylarthra”. A potential correlation between the development of the inferior colliculus in Hyopsodus and the use of terrestrial echolocation as observed in extant tenrecs and shrews is discussed. The detailed analysis of the overall morphology of the postcranial skeleton of Hyopsodus indicates a nimble, fast moving animal that likely lived in burrows. This would be compatible with terrestrial echolocation used by the animal to investigate subterranean habitat and/or to minimize predation during nocturnal exploration of the environment.
