Browsing by Author "Ksepka, Daniel T."
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ItemThe basal penguin (Aves, Sphenisciformes) Perudyptes devriesi and a phylogenetic evaluation of the penguin fossil record. (Bulletin of the American Museum of Natural History, no. 337)(American Museum of Natural History., 2010) Ksepka, Daniel T.; Clarke, Julia A.We present the first detailed description of Perudyptes devriesi, a basal penguin from the middle Eocene (~42 Ma) Paracas Formation of Peru, and a new analysis of all published extinct penguin species as well as controversial fragmentary specimens. The Perudyptes devriesi holotype includes key regions of the skull and significant postcranial material, thus helping to fill a major phylogenetic and stratigraphic (~20 million year) gap between the earliest fossil penguins (Waimanu manneringi and Waimanu tuatahi, ~58-61.6 Ma) and the next oldest partial skeletons. Perudyptes devriesi is diagnosable by five autapomorphies: (1) an anteroventrally directed postorbital process, (2) marked anterior expansion of the parasphenoid rostrum, (3) posterior trochlear ridge of the humerus projecting distal to the middle trochlear ridge and conformed as a large, broadly curved surface, (4) convex articular surface for the antitrochanter of the femur, and (5) extremely weak anterior projection of the lateral condyle of the tibiotarsus. The skull of Perudyptes is characterized by deep temporal fossae and an elongate, narrow beak that differs from other reported stem penguins in its short mandibular symphysis. The wing skeleton of Perudyptes preserves a combination of plesiomorphic features also observed in the basal penguin Waimanu and derived features shared with more crownward penguins. Features of the wing optimized as primitive for Sphenisciformes include retention of a discrete dorsal supracondylar tubercle on the humerus and presence of a modestly projected pisiform process on the carpometacarpus. Derived features present in Perudyptes and all more crownward penguins, but absent in Waimanu, include a more flattened humerus, development of a trochlea for the tendon of m. scapulotriceps at the distal end of the humerus, and bowing of the anterior face of the carpometacarpus. A combined molecular and morphological dataset for Spheniciformes was expanded by adding 25 osteological and soft tissue characters as well as 11 taxa. In agreement with previous results, Perudyptes devriesi is identified as one of the most basal members of Sphenisciformes. This analysis also confirms the placement of the middle/late Miocene (~11-13 Ma) fossil Spheniscus muizoni as a member of the Spheniscus clade and places the late Miocene (~10 Ma) Madrynornis mirandus as sister taxon to extant Eudyptes. These two species, known from relatively complete partial skeletons, are the oldest crown clade penguin fossils and represent well-corroborated temporal calibration points for the Spheniscus-Eudyptula divergence and Megadyptes-Eudyptes divergence, respectively. Our results reaffirm that the Miocene penguin taxon Palaeospheniscus, recently proposed to represent a member of the crown radiation, belongs outside of the crown clade Spheniscidae. The phylogenetic positions of small Eocene Antarctic penguin taxa (Delphinornis, Marambiornis, and Mesetaornis) recently proposed as possible direct ancestors to crown Spheniscidae were further evaluated using alternate coding strategies for incorporating scorings from isolated elements that preserve critical morphologies and are thought to represent these taxa, although they cannot yet be reliably assigned to individual species. Under all scoring regimes, Delphinornis, Marambiornis, and Mesetaornis were recovered as distantly related to Spheniscidae. Using synapomorphies identified in the primary analysis, we evaluated the phylogenetic position of fragmentary specimens, including the holotypes of valid but poorly known species, specimens currently unassignable to the species level, and morphologically distinct specimens that have not yet been named. All pre-Miocene specimens can be excluded from Spheniscidae based on presence of plesiomorphies lost in all crown penguins, consistent with a recent radiation for the penguin crown clade. This study provides additional support for a scenario of penguin evolution characterized by an origin of flightlessness near the K-T boundary, dispersal throughout the Southern Hemisphere during the early Paleogene, and a late Cenozoic origin for the crown clade Spheniscidae. Stratigraphic distribution and phylogenetic relationships of fossil penguins are consistent with distinct radiations during the Eocene, Oligocene, and Miocene. While the Eocene and Oligocene penguin faunas are similar in many respects, the Miocene fauna is characterized by smaller average size and novel cranial morphologies, suggesting that an ecological shift in diet occurred close to the origin of crown Spheniscidae. ItemErketu ellisoni, a long-necked sauropod from Bor Guvé (Dornogov Aimag, Mongolia). American Museum novitates ; no. 3508(New York, NY : American Museum of Natural History, 2006) Ksepka, Daniel T.; Norell, Mark.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlekh Ukhaany Akademi.The first specimen of the new sauropod Erketu ellisoni, from the Lower Cretaceous of the eastern Gobi of Dornogov, Mongolia, is described here. The specimen comprises a well-preserved articulated anterior cervical series, an articulated lower hindlimb, and a sternal plate. This sauropod displays a unique combination of features including low, bifid neural spines, elongate cervical centra, and crescent-shaped sternal plates. Computed tomography imaging reveals the vertebrae were extensively invaded with pneumatic camellae. The holotype individual of Erketu was of modest mass relative to other neosauropods, but had an extremely elongate neck. Phylogenetic analysis indicates Erketu is a member of the Somphospondyli and may belong to a more exclusive clade therein. ItemThe illusory evidence for Asian Brachiosauridae : new material of Erketu ellisoni and a phylogenetic reappraisal of basal Titanosauriformes. (American Museum novitates, no. 3700)(American Museum of Natural History., 2010) Ksepka, Daniel T.; Norell, Mark.Phylogenetic relationships among the diverse Cretaceous sauropods of East Asia have long been controversial. Debate has centered on whether there is any evidence for an endemic clade of Asian species ("Euhelopodidae") and on the placement of these taxa within the context of higher sauropod phylogeny. While most Cretaceous sauropod taxa from Asia are recognized as part of Somphospondyli, recent discoveries have suggested Brachiosauridae may have dispersed into Asia as well. We present new fossils and analyses bearing on these issues. Additional material of the holotype individual of Erketu ellisoni recovered on a subsequent visit to the type locality expands the character data available for this unique sauropod. Associated sauropod dorsal and caudal vertebrae were collected from the same horizon, at a location approximately 2 km from the holotype excavation. The dorsal vertebra exhibits synapomorphies suggesting a representative of Titanosauria co-occurred with Erketu ellisoni. These new specimens, as well as recent discoveries of contemporary Asian sauropod taxa, allow a basis for phylogenetic reappraisal of Erketu and related taxa. Phylogenetic results support a sister group relationship between the Asian Cretaceous sauropods Erketu and Qiaowanlong. Although Qiaowanlong was described as a brachiosaurid, it joins Erketu on the somphospondylian side of the Brachiosauridae-Somphospondyli divergence, erasing the evidence for the dispersal of Brachiosauridae into Asia. ItemA new choristodere from the Cretaceous of Mongolia. American Museum novitates ; no. 3468(New York, NY : American Museum of Natural History, 2005) Ksepka, Daniel T.; Gao, Keqin, 1955-; Norell, Mark.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.The remains of a choristodere recently discovered at Two Volcanoes, a new locality in the Gobi Desert of Mongolia, are described in this paper. Consisting of a fairly complete skull and partial postcranial skeleton, this specimen represents a new species of the genus Tchoiria. The new species differs from Tchoiria namsarai in having a much smaller number of teeth. Several elements preserved in this specimen are unknown in T. namsarai and thus provide new information about the genus. Phylogenetic analysis with the addition of data from the new specimen confirms the basal position of Tchoiria in Simoedosauridae. ItemNew material of Mongolemys elegans Khosatzky and Mlynarski, 1971 (Testudines, Lindholmemydidae), from the late Cretaceous of Mongolia with comments on bone histology and phylogeny. (American Museum novitates, no. 3766)(American Museum of Natural History., 2013-01-25) Cadena, Edwin A.; Ksepka, Daniel T.; Norell, Mark.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.Mongolemys elegans Khosatzky and Mlynarski, 1971, is a freshwater lindholmemydid turtle that is very abundant in late Cretaceous (Maastrichtian) pond deposits from the Gobi Desert of Mongolia. Here, we present new data on the morphology, bone histology, and phylogenetic position of M. elegans based on hatchlings, juveniles, and adults collected by American Museum of Natural History and the Mongolian Academy of Sciences joint field expeditions at the Bugin Tsav locality. Phylogenetic analysis using a morphological dataset supports the placement of M. elegans as a stem testudinoid. Bone histology of M. elegans shows similar patterns of thickness and bone tissue type for the internal and external cortexes as in other freshwater turtles. Microstructural samples of fossil bone from M. elegans show exceptional preservation of osteocyte lacuno-canicular networks, and higher values of osteocyte density at the external cortex in contrast to cancellous bone and the internal cortex. ItemOrnithomimosaur cranial material from Ukhaa Tolgod (Omnogov, Mongolia). American Museum novitates ; no. 3448(New York, NY : American Museum of Natural History, 2004) Ksepka, Daniel T.; Norell, Mark.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.The 1998 American Museum of Natural History-Mongolian Academy of Sciences expedition uncovered partial ornithomimosaur remains from the Xanadu sublocality at Uhkaa Tolgod. The specimen includes the rostral portion of the snout, the anterior portion of the mandible, and vertebral fragments. These remains cannot be assigned with certainty to any known ornithomimosaur genus. Examination of these materials allows new comments on ornithomimosaur palatal anatomy. ItemRedescription and phylogenetic position of the early Miocene penguin Paraptenodytes antarcticus from Patagonia ; American Museum novitates, no. 3525(New York, NY : American Museum of Natural History, 2006) Bertelli, Sara, Ph. D.; Giannini, Norberto P.; Ksepka, Daniel T.Paraptenodytes antarcticus is one of the best-known and most complete fossil penguins. This taxon is so distinctive that it has traditionally been classified in its own subfamily (Sphenisciformes: Paraptenodytinae) separate from all living penguins (Spheniscinae). The well-preserved partial skull of P. antarcticus is one of our richest sources of data on early penguin cranial morphology. We provide an updated description of the skull of P. antarcticus in a comparative context and use this information to explore the phylogenetic relationships of this taxon. Three cladistic analyses using an osteology dataset, a larger morphological dataset (including osteological, soft tissue, behavior, and oological characters) and a combined (morphological + molecular) dataset all recover Paraptenodytes as the sister taxon to a clade including all extant penguins. The placement of Paraptenodytes outside the crown clade of extant penguins reveals the order in which many spheniscid synapomorphies were acquired and lends support to the hypothesis that modern penguins had Subantarctic ancestors.