|Allkaruen koi |
Codorniú, Carabajal, Pol, Unwin & Rauhut, 2016
Pterosaurs are an extinct group of highly modified flying reptiles that thrived during the Mesozoic. This group has unique and remarkable skeletal adaptations to powered flight, including pneumatic bones and an elongate digit IV supporting a wing-membrane. Two major body plans have traditionally been recognized: the primitive, primarily long-tailed paraphyletic “rhamphorhynchoids” (preferably currently recognized as non-pterodactyloids) and the derived short-tailed pterodactyloids. These two groups differ considerably in their general anatomy and also exhibit a remarkably different neuroanatomy and inferred head posture, which has been linked to different lifestyles and behaviours and improved flying capabilities in these reptiles. Pterosaur neuroanatomy, is known from just a few three-dimensionally preserved braincases of non-pterodactyloids (as Rhamphorhynchidae) and pterodactyloids, between which there is a large morphological gap. Here we report on a new Jurassic pterosaur from Argentina, Allkaruen koi gen. et sp. nov., remains of which include a superbly preserved, uncrushed braincase that sheds light on the origins of the highly derived neuroanatomy of pterodactyloids and their close relatives. A µCT ray-generated virtual endocast shows that the new pterosaur exhibits a mosaic of plesiomorphic and derived traits of the inner ear and neuroanatomy that fills an important gap between those of non-monofenestratan breviquartossans (Rhamphorhynchidae) and derived pterodactyloids. These results suggest that, while modularity may play an important role at one anatomical level, at a finer level the evolution of structures within a module may follow a mosaic pattern.
|Figure 1: Selected skeletal elements of Allkaruen koi. |
(A–C), Holotype braincase (MPEF-PV 3613) in left lateral (A), dorsal (B) and posteroventral (C) views. (D, E), Mandible (MPEF-PV 3609) in dorsal (D) and right lateral (E) view. (F, G), Cervical vertebra (MPEF-PV 3615) in ventral (F) and right lateral (G) view.
Abbreviations: ag, alveolar groove; alv, alveoli; atr, anterior tympanic recess; bo, basioccipital; btr, basipterygoid recess; bpt, basipterygoid process; bs, basisphenoid; bt, basal tuber; cmv, caudal middle cerebral vein foramen; co, condyle; ct, crista tuberalis; d, dentary; eo-op, exoccipital/opisthotic; f, frontal; fm, foramen magnum; lts, laterosphenoid; mf, metotic foramen; nc, nuchal crest; np, nasal process; ns, neural spine; oc, occipital condyle; p, parietal; pf, pneumatic foramen; poz, postzygapophysis; pro, prootic; prz, prezygapophysis; ptf, posttemporal fenestra; ptr, posterior tympanic recess; sc, sagittal crest. Roman numerals indicate cranial nerves. Scale bars are 10 mm. DOI: 10.7717/PeerJ.2311
Pterosauria Kaup, 1834
Breviquartossa Unwin, 2003
Allkaruen koi gen. et sp. nov.
Etymology. Genus name from the native Tehuelche word ‘all’ meaning brain, and ‘karuen,’ meaning ancient. Species name from Tehuelche ‘koi’ meaning lake, referring to the lacustrine setting of the type locality.
Holotype. MPEF-PV (Museo Paleontológico Egidio Feruglio) 3613, braincase, a mandible (MPEF-PV 3609) and a cervical vertebrae (MPEF-PV 3615) (Figs. 1–7; Figs. S1–S3).
Referred material. Referred material includes a mid cervical vertebrae (MPEF-PV -3616.
Type locality and horizon. Locality La Lluvia, Cañadón Carrizal, Cerro Cóndor, Chubut, Argentina. Cañadón Asfalto Formation, latest Early-early Middle Jurassic (Cúneo et al., 2013).
The discovery of strongly correlated character state distributions in Darwinopterus led Lü et al. (2010) to suggest that major anatomical regions might have behaved as integrated modules that changed at different times and rates during pterosaur evolution. However, Allkaruen demonstrates that, whereas modular evolution might have operated at an inclusive morphological level (e.g., skull + neck versus the remainder of the postcranium), evolution within at least one of these modules (the neurocranium and braincase) seems to have followed a mosaic pattern.
The late Early-early Middle Jurassic age of Allkaruen (Cúneo et al., 2013) also provides new information on the timing of transformations during the evolution of the derived pterodactyloid skull from that of basal pterosaurs. The derived features of the cranium of Allkaruen indicate that some typical “pterodactyloid” skull features had already evolved by the time of the Early/Middle Jurassic boundary (Fig. 7), before the origin of pterodactyloids and the appearance of their modified postcranial skeleton. Prior to this discovery, a large suite of cranial features was presumed to have appeared somewhat later, during the late Middle to Late Jurassic, the age of the basal monofenestratans, Darwinopterus (Lü et al., 2010) and Wukongopterus (Wang et al., 2009) and the oldest pterodactyloids (Andres, Clark & Xu, 2014).
Unfortunately, the Early-Middle Jurassic is a period with a very poor pterosaur fossil record, in contrast to the relatively diverse assemblage of pterosaurs known from both the Late Triassic and the Late Jurassic–Cretaceous (Barrett et al., 2008; Butler, Benson & Barrett, 2013; Benson et al., 2014). The early evolutionary origin and diversification inferred for derived pterosaurs (Fig. 4), adds further evidence in support of the hypothesis that the origin and diversification of major vertebrate lineages (e.g., dinosaurs (Allain & Läng, 2009; Pol & Rauhut, 2012), crocodyliforms (Pol & Gasparini, 2009), turtles (Sterli, Pol & Laurin, 2013), mammals (Luo et al., 2011)) occurred prior to the Early/Middle Jurassic boundary (Allain & Läng, 2009; Cúneo et al., 2013). This pattern was previously obscured by the worldwide poor fossil record of terrestrial vertebrates during this evolutionarily critical period of time.
Laura Codorniú, Ariana Paulina Carabajal, Diego Pol, David Unwin and Oliver W.M. Rauhut. 2016. A Jurassic Pterosaur from Patagonia and the Origin of the Pterodactyloid Neurocranium. PeerJ. 4:e2311. DOI: 10.7717/PeerJ.2311