Wednesday, December 11, 2019

[Paleontology • 2019] Asfaltovenator vialidadiProbable Basal Allosauroid from the early Middle Jurassic Cañadón Asfalto Formation of Argentina Highlights Phylogenetic Uncertainty in Tetanuran Theropod Dinosaurs

Asfaltovenator vialidadi
 Rauhut & Pol, 2019

Illustration: Gabriel Lío

Tetanurae, the most successful clade of theropod dinosaurs, including modern birds, split into three major clades early in their evolutionary history: Megalosauroidea, Coelurosauria, and Allosauroidea. The oldest tetanurans occur in the earliest Middle Jurassic, but the early fossil record of the clade is still poor. Here we report one of the oldest known and most complete pre-Late Jurassic tetanuran, the probable allosauroid Asfaltovenator vialidadi gen. et sp. nov., which has an unusual character combination, uniting features currently considered to be apomorphic of different tetanuran lineages. A phylogenetic analysis resulted in a monophyletic Carnosauria (Allosauroidea + Megalosauroidea), and the inclusion of the new taxon significantly changes topology within carnosaurs. The analysis shows concentrated homoplasy in proximal nodes at the base of Tetanurae, and a temporal peak at the Pliensbachian-Toarcian extinction event, recently identified as a potential driver of tetanuran radiation. These results highlight the complex morphological evolution in the early radiation of tetanuran theropods, in which convergences and parallelisms were extremely common. This pattern seems to be a common feature in rapid radiation events of major clades of vertebrates and might explain the common difficulties to unravel phylogenetic relationships of important lineages at the base of major clades.

Figure 1: Cranial anatomy of Asfaltovenator vialidadi, MPEF PV 3440.
(A) composite reconstruction of the skull and lower jaws, based on disarticulated cranial elements. (B), graphic reconstruction of articulated skull. (C), braincase in occipital view. (D,E) posterior end of left mandible in dorsal view; (D) photo; (E) outline drawing.
 Abbreviations: a, angular; aa, antarticular; ao, antorbital fenestra; aof, antorbital fossa; ar, articular; bsr, basisphenoid recess; bt, basal tubera; cp, cornual process; d, dentary; en, external nares; eor, exoccipital ridge; fm, foramen magnum; g, groove; itf, infratemporal fenestra; j, jugal; jf, jugal foramen; l, lacrimal; lf, lacrimal fenestrae; m, maxilla; mf, maxillary fenestra; n, nasal; nf, nasal foramina; o, orbit; oc, occipital condyle; pap, paroccipital process; pcf, posterior exit of mid-cerebral vein; pm, premaxilla; po, postorbital; pra, prearticular; ptf, posttemporal foramen; q, quadrate; qj, quadratojugal; sa, surangular; snf, supranarial fossa; soc, supraoccipital; sq, squamosal; stf, supratemporal fenestra. Scale bars are 10 cm (A–C) and 5 cm (D,E).

Figure 2: Selected skeletal elements of Asfaltovenator vialidadi, MPEF PV 3440.
(A) left nasal in lateral view. (B), right maxilla in lateral view. (C) anterior end of left maxilla in lateral view. (D) left ectopterygoid in ventral view. (E) jugal process of left ectopterygoid in lateral view. (F) left dentary in lateral view. (G) last two cervical vertebrae and centrum of first dorsal vertebra in left lateral view.
Abbreviations: aof, antorbital fenestra; avp, anteroventral process; en, external nares; epi, epipophysis; fao, antorbital fossa; j, facet for articulation with the jugal; l, facet for articulation with lacrimal; mf, maxillary fenestra; nc, nasal crest; ns, neural spine; pap, parapophysis; pf, pneumatic foramen; pl, pleurocoel; prz, prezygapophysis; r, ridge that forms the ventral border of the antorbital fossa; snf, supranarial fossa; vf, ventral fossa. Scale bars are 5 cm.

Systematic Palaeontology: 
Theropoda Marsh, 1881, 
Tetanurae Gauthier, 1986, 
Allosauroidea (Marsh, 1878), 

Asfaltovenator vialidadi gen. et sp. nov.

Etymology: Generic name for the Cañadón Asfalto Formation and venator, Greek for hunter. The species epithet honours the Administración de Vialidad Provincial of Chubut and the Dirección Nacional de Vialidad, for their aid to paleontological expeditions of the Museo Paleontológico Egidio Feruglio.


Figure 3: Skeletal reconstruction and postcranial anatomy of Asfaltovenator vialidadi, MPEF PV 3440. Centre: body outline with preserved elements indicated. (A) articulated cervical vertebrae three to five. (B) cervical vertebra 7. (C) articulated dorsal vertebrae four to seven (better preserved right side, reversed). (D) right humerus in anterior view. (E) right radius and ulna, medial view. (F) right manus, metacarpus in dorsal and digits in lateral view. (G) articulated proximal ends of right tibia and fibula in lateral and proximal views.
Abbreviations: ag, anterior groove; cc, cnemial crest; cr, cervical rib; dc, distal carpal; di, diapophysis; dpc, deltpectoral crest; ec, ectepicondyle; ent, entepicondyle; epi, epipophysis; fi, fibula; fic, fibular condyle; hy, hyposphene; im, intermedium; it, internal tuberosity; lr, lateral ridge; mc, metacarpal; ns, neural spine; ol, olecranon; pa, parapophysis; pl, pleurocoel; poz, postzygapophysis; ppdl, paradiapophyseal lamina; prz, prezygapophysis; ra, radial. Scale bars are 100 cm (skeletal reconstruction), 5 cm (A–C) and 10 cm (D–G).

Locality and Horizon: Ca. 1.6 km NE of the village of Cerro Cóndor, lacustrine layers of the Cañadón Asfalto Formation, late Toarcian to Bajocian9.

Diagnosis: Large basal tetanuran diagnosed by the following character combination (autapomorphies are marked with*): premaxillary teeth with well-developed distal, but only minute mesial serrations*; postorbital with small cornual dorsal process; exoccipital with pronounced horizontal ridges between paroccipital processes and foramen magnum*; ossified antarticular in the mandible; platycoelous cervical vertebrae; neural spines of cervical vertebrae three and four triangular and backswept*; anterior cervical epipophyses tab-like and elongated; mid-cervical vertebrae with median pit between parapophyses ventrally; ventral keel absent in posterior cervical and poorly developed in anterior dorsal vertebrae; well-developed paradiapohyseal lamina in middle and posterior dorsal vertebrae; dorsals 11 and 12 with small additional anterior centrodiapophyseal lamina*; articulated metacarpus broader than long; manual digit III significantly more slender and shorter than digits II and III.

Oliver W. M. Rauhut and Diego Pol. 2019. Probable Basal Allosauroid from the early Middle Jurassic Cañadón Asfalto Formation of Argentina Highlights Phylogenetic Uncertainty in Tetanuran Theropod Dinosaurs. Scientific Reports. 9: 18826. DOI: 10.1038/s41598-019-53672-7

 Hallan uno de los más antiguos y completos dinosaurios carnívoros del Jurásico