Data from: The cranial endocast of the Middle Devonian dipnonan Dipterus valenciennesi and a fossilized dipnoan otoconial mass

The well known Middle Devonian (Eifelian–Givetian) lungfish Dipterus valenciennesi from Scotland, UK, has been studied for more than one hundred years though our understanding of the neurocranium and cranial cavity is incomplete. Micro-CT scanning demonstrates that the internal cast of the cranial cavity, the endocast, possesses a mix of primitive and derived characters. The olfactory bulbs are sessile, as in the derived extant Lepidosirenidae. However, Dipterus valenciennesi possesses the primitive condition of a shallow telencephalon unlike Upper Devonian forms such as Rhinodipterus kimberleyensis and extant lungfish taxa. Further information revealed by micro-CT scanning has allowed coding for characters pertaining to the neurocranial cristae, previously unobservable in Dipterus valenciennesi, allowing hypotheses of relationships between Devonian Dipnoi and the position of Dipterus valenciennesi to be reassessed. New analyses do not refine the phylogenetic position of Dipterus valenciennesi but do increase support for most recently established Devonian dipnoan phylogeny. The first record of a three-dimensionally preserved fossilized otoconial mass from the utricular recess in a fossil dipnoan is also described. Comparison of morphometric data of the endosseous and soft tissue manifestations of the labyrinth system in extant dipnoan taxa demonstrates that there is largely good correspondence between the two. When compared to Dipterus valenciennesi, extant taxa exhibit semi-circular canals that are reduced in length. Furthermore, compared to Dipterus, in extant dipnoan taxa the size of the otoconial mass has increased relative to the utricular and sacculolagenar pouches containing them. The functional implications for these observations suggest that the Dipnoi have evolved towards a more sedentary lifestyle and behaviour at least since the Middle Devonian.

产自英国苏格兰的著名中泥盆世（艾费尔期-吉维特期）肺鱼类物种瓦伦西亚双鳍鱼（Dipterus valenciennesi），虽已被研究百余年，但学界对其脑颅与颅腔的认知仍存在不足。显微CT扫描（Micro-CT）显示，其颅腔内模（endocast）兼具原始性状与衍征性状。嗅球呈固着状态，与现生衍征类群南美肺鱼科（Lepidosirenidae）一致；但不同于晚泥盆世的金伯利鼻双鳍鱼（Rhinodipterus kimberleyensis）及现生肺鱼类群，瓦伦西亚双鳍鱼具有端脑较浅的原始特征。此次显微CT扫描还获取了此前在瓦伦西亚双鳍鱼中未被观测到的脑颅嵴相关性状编码信息，借此得以重新评估泥盆纪肺鱼亚纲（Dipnoi）类群间的亲缘关系假说，以及瓦伦西亚双鳍鱼的系统发育位置。新的系统发育分析虽未明确瓦伦西亚双鳍鱼的系统发育位置，但确实提升了近年建立的泥盆纪肺鱼类系统发育框架的支持强度。本文还首次报道了化石肺鱼椭圆囊隐窝内三维保存的耳石块（otoconial mass）。通过对比现生肺鱼类内耳迷路系统的骨化结构与软组织形态的形态计量数据，发现二者整体对应性良好。与瓦伦西亚双鳍鱼相比，现生肺鱼类的半规管（semi-circular canals）长度更短；此外，相较于瓦伦西亚双鳍鱼，现生肺鱼类的耳石块相对容纳其的椭圆囊与球囊-听壶囊（sacculolagenar pouches）的体积有所增大。上述观察结果的功能意义表明，肺鱼类至少自中泥盆世起，便已向更静栖的生活方式与行为模式演化。