[D] Austrocheirus isasii [sG] [T]
Describer
Ezcurra, Agnolin & Novas 2010
Time
Cretaceous Late Maastrichtian
Classification
Saurischia Theropoda Abelisauridae [Abelisauroidae]
Diet
Carnivore
Fossilsite
Pari Aike Formation, Santa Cruz Province, Patagonia, Argentina
Info
Abstract: We describe the new basal abelisauroid dinosaur Austrocheirus isasii gen. et sp. nov. from the Late Cretaceous Pari Aike Formation of southwestern Patagonia, Argentina. The preserved remains include manual bones, a distal tibia, and some pedal and axial elements. Austrocheirus is differentiated from other basal theropods by the presence of metacarpal III with a dorsoventrally compressed shaft and posteriorly displaced collateral tendon fossae located at the same level of the proximal end of distal condyles, and pedal phalanges with a conspicuous longitudinal crest delimitating the dorsal margin of the distal collateral tendon fossae. A cladistic analysis recovered the new species as more derived than Ceratosaurus and Berberosaurus, but within a polytomy at the base of Abelisauroidea, an assignment supported by two abelisauroid synapomorphies: distal end of tibia with a planar vertical scar for the reception of the ascending process of the astragalus that occupies most of its anterior surface and is medially bounded by the longitudinally oriented facet; and scar for the reception of the ascending process with a median vertical ridge, which imbeds into a crescentic vertical groove on the posterior surface of the ascending process of the astragalus forming an interlocking tibiotarsal articulation. Furthermore, Austrocheirus represents the first known medium-sized Late Cretaceous abelisauroid bearing nonatrophied hands. The evidence provided here suggests that the strong reduction of the forelimb recorded in derived abelisaurids is not directly correlated with their increased body-size, but it seems to be an evolutionary event exclusive to this lineage within Ceratosauria.
Holotype: MPM-PV 10003, specimen comprising several incomplete mid-caudal vertebrae, left metacarpal III, proximal left manual phalanx of digit III, distal end of left tibia, fragmentary left metatarsal III, and some fragments of pedal phalanges.
Etymology: The generic name is derived from the Greek words austros (south) and cheirus (hand, manus); in allusion to the non-atrophied manus of this taxon which constitutes the first example of a medium-sized Cretaceous abelisauroid with this condition. The specific name is in honor of Mr. Marcelo Isasi, for his outstanding work in the last 25 years as a technician in palaeontology and discoverer of the holotype specimen of Austrocheirus.
Locality and horizon: Hoyada Arroyo Seco locality (49°57’17’’ S, 72°05’31’’ W), close to the western coast of the Leona River and four kilometers south of the De los Hornos Hill, southwest Santa Cruz Province, Patagonia, Argentina. Pari Aike Formation, Late Cretaceous (early Maastrichtian: Kraemer & Riccardi 1997; Novas et al. 2004).
Diagnosis: Medium-sized theropod (7–8 meters in total length) diagnosed by the following combination of characters (autapomorphies indicated by an asterisk) (1) metacarpal III with a dorsoventrally tapering shaft towards its proximal end* and (2) posteriorly displaced collateral tendon fossae located at the same level of the proximal end of distal condyles; (3) tibia with a slightly convex posterior surface at the distal end; (4) metatarsal III with a very well-developed ventral scar for the insertion of the M. gastrocnemius; (5) pedal phalanges with a conspicuous longitudinal crest delimitating the dorsal margin of the distal collateral tendon fossae*.
Note on the association of the holotype specimen. The bones of MPM-PV 10003 were found closely associated within a two square metres area and match one another perfectly in size. MPM-PV 10003 was found in a green to grey mudstone. No other tetrapod remains were found mixed with it, and the next closest specimen was a fragmentary sauropod found around 300 metres away. Additionally, as discussed below, most of the remains, especially the metacarpal and distal tibia, possess ceratosaurian apomorphies. Thus, all the bones of the holotype of Austrocheirus must correspond to a single individual.
Ezcurra, Agnolin & Novas 2010
Time
Cretaceous Late Maastrichtian
Classification
Saurischia Theropoda Abelisauridae [Abelisauroidae]
Diet
Carnivore
Fossilsite
Pari Aike Formation, Santa Cruz Province, Patagonia, Argentina
Info
Abstract: We describe the new basal abelisauroid dinosaur Austrocheirus isasii gen. et sp. nov. from the Late Cretaceous Pari Aike Formation of southwestern Patagonia, Argentina. The preserved remains include manual bones, a distal tibia, and some pedal and axial elements. Austrocheirus is differentiated from other basal theropods by the presence of metacarpal III with a dorsoventrally compressed shaft and posteriorly displaced collateral tendon fossae located at the same level of the proximal end of distal condyles, and pedal phalanges with a conspicuous longitudinal crest delimitating the dorsal margin of the distal collateral tendon fossae. A cladistic analysis recovered the new species as more derived than Ceratosaurus and Berberosaurus, but within a polytomy at the base of Abelisauroidea, an assignment supported by two abelisauroid synapomorphies: distal end of tibia with a planar vertical scar for the reception of the ascending process of the astragalus that occupies most of its anterior surface and is medially bounded by the longitudinally oriented facet; and scar for the reception of the ascending process with a median vertical ridge, which imbeds into a crescentic vertical groove on the posterior surface of the ascending process of the astragalus forming an interlocking tibiotarsal articulation. Furthermore, Austrocheirus represents the first known medium-sized Late Cretaceous abelisauroid bearing nonatrophied hands. The evidence provided here suggests that the strong reduction of the forelimb recorded in derived abelisaurids is not directly correlated with their increased body-size, but it seems to be an evolutionary event exclusive to this lineage within Ceratosauria.
Holotype: MPM-PV 10003, specimen comprising several incomplete mid-caudal vertebrae, left metacarpal III, proximal left manual phalanx of digit III, distal end of left tibia, fragmentary left metatarsal III, and some fragments of pedal phalanges.
Etymology: The generic name is derived from the Greek words austros (south) and cheirus (hand, manus); in allusion to the non-atrophied manus of this taxon which constitutes the first example of a medium-sized Cretaceous abelisauroid with this condition. The specific name is in honor of Mr. Marcelo Isasi, for his outstanding work in the last 25 years as a technician in palaeontology and discoverer of the holotype specimen of Austrocheirus.
Locality and horizon: Hoyada Arroyo Seco locality (49°57’17’’ S, 72°05’31’’ W), close to the western coast of the Leona River and four kilometers south of the De los Hornos Hill, southwest Santa Cruz Province, Patagonia, Argentina. Pari Aike Formation, Late Cretaceous (early Maastrichtian: Kraemer & Riccardi 1997; Novas et al. 2004).
Diagnosis: Medium-sized theropod (7–8 meters in total length) diagnosed by the following combination of characters (autapomorphies indicated by an asterisk) (1) metacarpal III with a dorsoventrally tapering shaft towards its proximal end* and (2) posteriorly displaced collateral tendon fossae located at the same level of the proximal end of distal condyles; (3) tibia with a slightly convex posterior surface at the distal end; (4) metatarsal III with a very well-developed ventral scar for the insertion of the M. gastrocnemius; (5) pedal phalanges with a conspicuous longitudinal crest delimitating the dorsal margin of the distal collateral tendon fossae*.
Note on the association of the holotype specimen. The bones of MPM-PV 10003 were found closely associated within a two square metres area and match one another perfectly in size. MPM-PV 10003 was found in a green to grey mudstone. No other tetrapod remains were found mixed with it, and the next closest specimen was a fragmentary sauropod found around 300 metres away. Additionally, as discussed below, most of the remains, especially the metacarpal and distal tibia, possess ceratosaurian apomorphies. Thus, all the bones of the holotype of Austrocheirus must correspond to a single individual.