Taxonomic overview of Neogene crocodylians in Myanmar

Although Neogene crocodylians were well documented from Indo-Pakistan, few fossils were known from Southeast Asia, precluding the understanding of their evolutionary and biogeographic history. Here, we describe crocodylians from the Neogene Irrawaddy Formation of central Myanmar and evaluate their taxonomic status. Tebingan, SE of Magway (lower Upper Miocene) yields Gavialis and Crocodylus that differ from the previously known species of each genus, and the Gwebin area, SW of Bagan (Upper Pliocene) produces Crocodylus cf. palaeindicus. Taking into account the materials without provenance data, Neogene crocodylians from Myanmar include at least three gavialids and two Crocodylus that are characterized by different craniomandibular and postcranial features. The body length estimates for the gavialines from Tebingan and an unknown locality in central Myanmar are 7.5 m and 8.6 m, respectively, which exceed the maximum size limit of extant Gavialis gangeticus. Together with the previously reported large taxa, gavialids repeatedly evolved large body sizes in the Neogene of Asia. Gavialis from the Miocene of Myanmar is one of the oldest records of the genus, and its unraised orbital rim suggests that the “telescoped” eyes derived later during the genus evolution. Crocodylus cf. palaeindicus from the Pliocene of Myanmar indicates the species range was extended from western India to Myanmar during the Neogene. The absence of Crocodylus siamensis in the Neogene of India and central Myanmar implies the species originated east of central Myanmar.

尽管新近纪的鳄鱼目动物在印度-巴基斯坦地区有较为详细的记录，但东南亚地区所知的化石数量极少，这阻碍了对它们演化及生物地理历史的研究。在本研究中，我们描述了来自缅甸中部新近纪伊洛瓦底组的鳄鱼目动物，并对其分类地位进行了评估。位于马圭东南方向的特宾地区（下上新世）发现了与已知各属种不同的Gavialis和Crocodylus，而位于巴干西南方向的Gwebin地区（上更新世）则产出了Crocodylus cf. palaeindicus。考虑到缺乏产地数据的材料，缅甸新近纪的鳄鱼目动物至少包括三种鳄鱼目和两种Crocodylus，它们以不同的颅颌部和后颅部特征为特征。特宾地区和缅甸中部一个未知地点的鳄鱼目体型估计长度分别为7.5米和8.6米，超过了现存Gavialis gangeticus的最大体型限制。与之前报道的大型物种一起，鳄鱼目在亚洲的新近纪中反复演化出大型体型。缅甸上新世的Gavialis是本属最古老的记录之一，其未抬高的眶缘表明“望远镜状”的眼睛是在属的演化过程中后来才出现的。缅甸上新世的Crocodylus cf. palaeindicus表明该物种在新近纪期间从印度西部扩展到了缅甸。印度和缅甸中部的Neogene期缺乏Crocodylus siamensis的存在暗示该物种起源于缅甸中部以东。