The amphibamiform Nanobamus macrorhinus from the early Permian of Texas

Nanobamus macrorhinus is a small amphibamiform temnospondyl from the early Permian Arroyo Formation of Texas. It is most readily characterized by an elongate and partially subdivided naris. This condition is superficially reminiscent of that seen in the coeval trematopids, the group to which N. macrorhinus was originally referred to under an interpretation of the holotype as a larval form. This was discounted by later workers, but the amphibamiform affinities of the specimen were not formalized until recently. The specimen has never been described in the context of its amphibamiform affinities and remains poorly characterized, never having been sampled in a phylogenetic analysis. Here we present a complete, updated osteological description of N. macrorhinus, including an improved characterization of its unique mosaic of plesiomorphic and apomorphic features and clarification of the taxon’s autapomorphies. Our analysis of the taxon’s phylogenetic position within Amphibamiformes shows that N. macrorhinus is recovered as diverging after basal amphibamiforms such the micropholids and before derived amphibamiforms such as the amphibamids. This is supported by the unique mixture of retained plesiomorphies such as non-foreshortened postparietals and an oval choana and apomorphies such as a narrow interorbital region and slender palatal rami of the pterygoid. These results reflect the complexity of terrestrial amphibamiform diversity and provide further insight into the evolutionary history of the lissamphibian stem in terrestrial environments.

Methods

To test the relationships of Nanobamus macrorhinus within Dissorophoidea at large and more specifically within Amphibamiformes, we utilized the matrix of Schoch (2019), which originally included 34 taxa and 108 characters. Nanobamus could be coded for 65 of the total 108 characters (60% completeness); no postcranial characters could be coded. One coding was changed from that of Schoch (2019): for character 39 (external narial opening), we changed Georgenthalia clavinasica from 0 to 1, reflecting the ‘keyhole-shaped’ naris described by Anderson et al. (2008a) that is shared with N. macrorhinus. We performed a maximum parsimony analysis in PAUP 4.0b164 for Macintosh (Swofford, 2002) with multistate characters ordered, a heuristic search, simple stepwise addition, and Dendrysekos helogenes Steen, 1934 as the outgroup. As with Schoch (2019), changing all multistate characters to be unordered did not produce any changes to the strict consensus, so these characters were maintained as ordered in all subsequent permutations and in the supplemental data matrix. Following a reviewer’s suggestion, we also ran the analysis with random addition sequence stepwise addition (10,000 reps), which resulted in the strict consensus tree remaining the same. A bootstrap analysis with 1,000 replicates was also performed. Following Schoch (2019), three permutations with slightly reduced character sampling were performed: (1) the removal of character 17 (postparietal length); (2) the removal of character 30 (a second postparietal length character); and (3) the removal of both characters, with bootstrap analyses performed on each permutation. Schoch’s original analysis examined these permutations because of previous concerns over the independence or redundancy of these characters.

长鼻纳米螈（Nanobamus macrorhinus）是一种小型两栖形类（amphibamiform）离片椎类（temnospondyl），发现于美国德克萨斯州早二叠世的阿罗约组（Arroyo Formation）。其最显著的鉴别特征为修长且部分分隔的外鼻孔。该外观特征与同期的塌肋螈科（trematopids）物种高度相似，而长鼻纳米螈最初正是基于将其正模标本解读为幼体形态的研究结论，被归入该类群。这一解读后续被研究者否定，但该标本的两栖形类亲缘关系直至近年才得到正式确认。此前该标本从未结合其两栖形类亲缘关系开展过系统性描述，相关特征认知仍较为匮乏，且从未被纳入系统发育分析进行特征取样。

本文首次对长鼻纳米螈进行了完整且更新的骨骼解剖学描述，包括对其独特的祖征与衍征镶嵌组合的精细化刻画，并明确了该分类单元的自衍征。通过对其在两栖形亚目（Amphibamiformes）内系统发育位置的分析，我们发现长鼻纳米螈的分化分支位于微螈科（micropholids）等基底两栖形类之后，而在两栖螈科（amphibamids）等衍化两栖形类之前。这一结果得到了其独特特征组合的支持：既保留了未缩短的后顶骨、卵圆形内鼻孔等祖征，也具备狭窄的眶间区域、纤细的翼骨腭突等衍征。本研究结果揭示了陆生两栖形类多样性的复杂性，并为陆生环境下的滑体两栖类（Lissamphibia）干群演化历史提供了新的认知视角。

研究方法
为验证长鼻纳米螈在广义离螈超科（Dissorophoidea）内，尤其是两栖形亚目内的系统发育关系，我们采用了Schoch（2019）的形态矩阵，该矩阵最初包含34个分类单元与108个形态特征。我们可为长鼻纳米螈编码65个特征（占总特征数的60%，完整性为60%），但无法编码任何颅后特征。我们对一处特征编码进行了修正：针对特征39（外鼻孔开口形态），我们将Georgenthalia clavinasica的编码从0改为1，以反映Anderson等人（2008a）描述的、与长鼻纳米螈共有的“钥匙孔形”外鼻孔。

我们在Macintosh平台的PAUP 4.0b164软件（Swofford, 2002）中开展了最大简约法分析，设置多态特征为有序状态，采用启发式搜索与逐步添加法，并以Dendrysekos helogenes Steen, 1934作为外类群。与Schoch（2019）的研究一致，将所有多态特征改为无序状态并未对严格合意树产生任何影响，因此在所有后续置换分析与补充数据矩阵中，我们均保留多态特征为有序状态。根据一位审稿人的建议，我们还采用随机逐步添加法（10000次重复）进行了分析，结果显示严格合意树未发生改变。我们同时开展了包含1000次重复的自举检验分析。

参照Schoch（2019）的方案，我们进行了三组特征采样略有缩减的置换分析：（1）移除特征17（后顶骨长度）；（2）移除特征30（另一处后顶骨长度特征）；（3）同时移除上述两处特征，并对每组置换分析均开展自举检验。Schoch最初开展此类置换分析，是为了解决此前关于这些特征独立性或冗余性的学术争议。