New remains of an ornithuromorph bird from Bajo de la Carpa Formation (Santonian, Upper Cretaceous), Paso Córdoba, Río Negro

LECUONA, AGUSTINA; BAIANO, MATIA; LEARDI JUAN MARTÍN; POL, DIEGO; SALGADO, LEONARDO

Trelew

Congreso; XXXV Jornadas Argentinas de Paleontología de Vertebrados; 2022

Museo Paleontológico Egidio Feruglio

The remains presented here correspond to an avian left tarsometatarsus (field number CCS-5) recovered from the Bajo de la Carpa Formation (Santonian, Upper Cretaceous), of Paso Córdoba locality, southwest of General Roca, Río Negro, Argentina. This element is 49.2 mm long, with incomplete proximal and distal ends, and 10.4 mm midshaft width. The metatarsals II to IV are fused through its length, with shallow grooves between adjacent metatarsals on their proximal halves, and even shallower along their distal halves. This condition is similar to Patagopteryx deferrariisi, but the opposite is seen in Kaririavis mater and Neuquenornis volans, with metatarsals almost fused proximally and unfused distally. The three metatarsals are similar in width, where the IIIrd is 1.18 times the metatarsal IV. This condition differs from Patagopteryx that has a proportionately narrow metatarsal IV (mtt III/mtt IV width ratio = ca. 1.73) and Kaririavis with a wider metatarsal III (mtt III/mtt IV width ratio = ca. 2). The proximal vascular foramen is present between metatarsals III and IV that opens on the anterior and plantar surfaces, also present in Patagopteryx, but absent in Kaririavis. The presence of a distal vascular foramen between the trochlea of metatarsals III and IV cannot be determined because the trochlea of metatarsal IV has not been preserved. No articular surface for metatarsal I is recognized on the metatarsal II, nor a swelling on the anterior surface of metatarsals II and III for the attachment of the M. tibialis anterior. The plantar surface is somewhat excavated on the proximal half, a deeper excavation is present in Patagopteryx (MACN-N-11) and Neuquenornis (MUCPv-142). A preliminary phylogenetic analysis of Avialae recovers Kaririavis as the basalmost Ornithuromorpha and nests CCS-5 within this clade in a large polytomy. The latter, that also includes Patagopteryx, is due to the scarce remains of CCS-5 and its combination of plesiomorphic (e.g. trochlea of metatarsal II not medially curved) and apomorphic (e.g. metatarsal fusion) characters. Many characters shared with Patagopteryx suggest it could represent a new specimen of this taxon, however a more exhaustive phylogenetic analysis is needed to evaluate if it can be assigned to a new species. In any case, CCS-5 increases the vertebrate remains of Bajo de la Carpa Fm. at Paso Córdoba locality, which yields Velocisaurus, Achillesaurus, Notosuchus, Comahuesuchus, and Dinilysia, approaching to the largest diversity represented by the contemporaneous fossil fauna of Neuquén city localities.