Diego Pol

A basal dromaeosaurid and size evolution preceding avian flight

Fossil evidence for changes in dinosaurs near the lineage leading to birds and the origin of flight has been sparse. A dinosaur from Mongolia represents the basal divergence within Dromaeosauridae. The taxons small body size and phylogenetic position imply that extreme miniaturization was ancestral for Paraves (the clade including Avialae, Troodontidae, and Dromaeosauridae), phylogenetically earlier than where flight evolution is strongly inferred. In contrast to the sustained small body sizes among avialans throughout the Cretaceous Period, the two dinosaurian lineages most closely related to birds, dromaeosaurids and troodontids, underwent four independent events of gigantism, and in some lineages size increased by nearly three orders of magnitude. Thus, change in theropod body size leading to flights origin was not unidirectional.

A pug-nosed crocodyliform from the Late Cretaceous of Madagascar

Although the image of crocodyliforms as ‘unchanged living fossils’ is naive, several morphological features of the group are thought to have varied only within narrow limits during the course of evolution. These include an elongate snout with an array of conical teeth, a dorsoventrally flattened skull and a posteriorly positioned jaw articulation, which provides a powerful bite force. Here we report an exquisitely preserved specimen of a new taxon from the Late Cretaceous of Madagascar that deviates profoundly from this Bauplan, possessing an extremely blunt snout, a tall, rounded skull, an anteriorly shifted jaw joint and clove-shaped, multicusped teeth reminiscent of those of some ornithischian dinosaurs. This last feature implies that the diet of the new taxon may have been predominantly if not exclusively herbivorous. A close relationship with notosuchid crocodyliforms, particularly Uruguaysuchus (Late Cretaceous, Uruguay) is suggested by several shared derived features; this supports a biogeographical hypothesis that Madagascar and South America were linked during the Late Cretaceous .

NEW REMAINS OF SPHAGESAURUS HUENEI (CROCODYLOMORPHA: MESOEUCROCODYLIA) FROM THE LATE CRETACEOUS OF BRAZIL

Abstract Recent discovery of an almost complete skull of Sphagesaurus huenei, a previously poorly known form from the Late Cretaceous of southern Brazil, shows unexpected morphology for a crocodyliform, including an extreme heterodonty with small lower incisors, large upper caniniforms, and reversed triangle-like postcanines; edentulous anterior region of premaxilla; and maxilla with posterior wall over the anterior margin of the suborbital fenestra. The postcanine teeth have extensive wear facets, implying an alternate unilateral occlusion and suggesting the presence of both lateral and fore-aft movement of the jaw. These characteristics appear unique within the crocodyliform lineage, showing that feeding diversity was wider than thought in this clade. The phylogenetic relationships of Sphagesaurus huenei are evaluated through a parsimony analysis. Sphagesaurus lies within a monophyletic group formed by Cretaceous and Tertiary taxa traditionally referred as notosuchians and sebecosuchians. Interestingly, several mammal-like tooth morphologies were present in this clade of terrestrial crocodyliforms.

Biases in Maximum Likelihood and Parsimony: A Simulation Approach to a 10-Taxon Case

Biases present in maximum likelihood and parsimony are investigated through a simulation study in a 10‐taxon case in which several long branches coexist with short branches in the modeled topology. The performance of these methods is explored while increasing the length of the long branches with different amounts of data. Also, simulations with different taxonomic sampling schemes are examined through this study. The presence of a strong bias in parsimony is corroborated: the well‐known long‐branch attraction. Likelihood performance is found to be sensitive to the mere presence extreme of branch length disparity, retrieving topologies compatible with long‐branch attraction and long‐branch repulsion, irrespective of the correctness of the model used.

Skull anatomy of Dakosaurus andiniensis (Thalattosuchia: Crocodylomorpha) and the phylogenetic position of Thalattosuchia

Synopsis New information on Dakosaurus andiniensis from the Latest Jurassic and Early Cretaceous is reported here. One of the specimens described herein consists of an almost complete skull and lower jaw found in the uppermost levels of the Vaca Muerta Formation (Tithonian) of Neuquén Province, Argentina. The new material allows a more complete understanding and diagnosis of this form, previously known only from the fragmentary type specimen. The new remains show that D. andiniensis had an unusual morphology for a marine crocodyliform,namely a remarkably short, high snout with ziphodont dentition. This new information allows testing of the phylogenetic relationships of this taxon, which is depicted as deeply nested within Metriorhynchidae, a clade of marine crocodyliforms with derived adaptations to the marine environment (e.g. paddle‐like forelimbs, hypertrophied nasal salt glands). In particular, D. andiniensis is inferred to be the sister taxon of D. maximus from the Jurassic of Europe. This relationship repeats the phylogenetic pattern seen in other Jurassic marine crocodyliforms from South America and Europe (e.g. Geosaurus), demonstrating the close faunal relationship between these two distant marine basins. The phylogenetic analysis reported here results in a most parsimonious hypothesis that depicts Thalattosuchia nested within Neosuchia and the strength and character evidence supporting this position is presented. In addition, several characters traditionally postulated as dependent upon the longirostrine morphology are critically examined under the light of new evidence. Most of them show a character state distribution that is not strictly compatible with that of the longirostrine condition and, therefore, are interpreted as independent units of phylogenetic evidence.

Unstable taxa in cladistic analysis: identification and the assessment of relevant characters

A common problem in phylogenetic analysis is the presence of unstable taxa that are depicted in multiple positions in optimal topologies. These uncertainties are reflected in strict consensus trees with polytomies that hamper the interpretation of the phylogenetic results. We propose a protocol for detecting unstable branches (either terminal taxa or clades) and identifying particular characters related to their instability in cladistic analysis. This procedure is based on an iterative evaluation of the agreement of triplets among the optimal topologies (i.e. most‐parsimonious trees, MPTs) and examination of character optimizations on these trees. Different types of characters underlying the unstable behaviour of taxa are detected: those with conflicting scorings that support alternative positions of problematic taxa and those with missing data in the unstable taxa that could reduce their instability if they are scored. The entire process is automated through a TNT script that provides a list of characters related to the instability of each unstable taxon. The outcome of this procedure can be used as a guide for further research efforts focused on the revision or addition of (morphological or molecular) phylogenetic data for elucidating the affinities of unstable taxa.

New Araripesuchus Remains from the Early Late Cretaceous (Cenomanian-Turonian) of Patagonia

Abstract Two new crocodyliform specimens found in a recently discovered locality from the Late Cretaceous of Patagonia (Argentina) are described herein. One of them comprises an almost complete skull found in articulation with the lower jaws, while the other consists of the anterior region of the lower jaws and fragmentary remains of the palate. These two specimens differ in the morphology of their lower jaws (e.g., height of mandibular symphysis, pattern of ornamentation on ventral surface of mandibular ramus, concavity of medial surface of splenials, shape of splenial-dentary suture on ventral surface of mandibular symphysis) and probably belong to different taxa. The more complete specimen is considered to be a new taxon, Araripesuchus buitreraensis, diagnosed by the combination of the following characters (autapomorphic characters are indicated with an asterisk): long and acute anterior process of frontals extending anteriorly between the nasals; frontals extending into supratemporal fenestra; narrow parietal dorsal surface between supratemporal fossa; anterior palpebral remarkably broad; large siphoneal foramen in otic recess; T-shaped choanal septum that completely divides the internal nares, having its anterior end as broad as the midregion of the septum*; pterygoid flanges pneumatic and poorly expanded at its lateral end*; transversely elongated depression on ventral surface of pterygoid flanges close to the posterior margin of suborbital fenestra*; longitudinal groove on flat lateral surface of dentaries below toothrow. The second, more fragmentary specimen might represent a different new taxon, although more material is needed in order to make a justified taxonomic decision. The phylogenetic relationships of both specimens are analyzed through a comprehensive cladistic analysis including 50 crocodylomorph taxa. All the most parsimonious hypotheses depict both specimens as closely related to the previously known South American species of Araripesuchus (A. gomesii and A. patagonicus). This group is depicted as the most basal clade of notosuchians, the most diverse group of Cretaceous mesoeucrocodylians from Gondwana.

A bizarre Cretaceous theropod dinosaur from Patagonia and the evolution of Gondwanan dromaeosaurids

Fossils of a predatory dinosaur provide novel information about the evolution of unenlagiines, a poorly known group of dromaeosaurid theropods from Gondwana. The new dinosaur is the largest dromaeosaurid yet discovered in the Southern Hemisphere and depicts bizarre cranial and postcranial features. Its long and low snout bears numerous, small-sized conical teeth, a condition resembling spinosaurid theropods. Its short forearms depart from the characteristically long-armed condition of all dromaeosaurids and their close avian relatives. The new discovery amplifies the range of morphological disparity among unenlagiines, demonstrating that by the end of the Cretaceous this clade included large, short-armed forms alongside crow-sized, long-armed, possibly flying representatives. The new dinosaur is the youngest record of dromaeosaurids from Gondwana and represents a previously unrecognized lineage of large predators in Late Cretaceous dinosaur faunas mainly dominated by abelisaurid theropods.

New evidence on deinonychosaurian dinosaurs from the Late Cretaceous of Patagonia

Most of what is known about the evolution of deinonychosaurs (that is, the group of theropods most closely related to birds) is based on discoveries from North America and Asia. Except for Unenlagia comahuensis and some fragmentary remains from northern Africa, no other evidence was available on deinonychosaurian diversity in Gondwana. Here we report a new, Late Cretaceous member of the clade, Neuquenraptor argentinus gen. et sp. nov., representing uncontroversial evidence of a deinonychosaurian theropod in the Southern Hemisphere. The new discovery demonstrates that Cretaceous theropod faunas from the southern continents shared greater similarity with those of the northern landmasses than previously thought. Available evidence suggests that deinonychosaurians were probably distributed worldwide at least by the beginning of the Cretaceous period. The phylogenetic position of the new deinonychosaur, as well as other Patagonian coelurosaurian theropods, is compatible with a vicariance model of diversification for some groups of Gondwanan and Laurasian dinosaurs.

A Complete Skull of an Early Cretaceous Sauropod and the Evolution of Advanced Titanosaurians

Advanced titanosaurian sauropods, such as nemegtosaurids and saltasaurids, were diverse and one of the most important groups of herbivores in the terrestrial biotas of the Late Cretaceous. However, little is known about their rise and diversification prior to the Late Cretaceous. Furthermore, the evolution of their highly-modified skull anatomy has been largely hindered by the scarcity of well-preserved cranial remains. A new sauropod dinosaur from the Early Cretaceous of Brazil represents the earliest advanced titanosaurian known to date, demonstrating that the initial diversification of advanced titanosaurians was well under way at least 30 million years before their known radiation in the latest Cretaceous. The new taxon also preserves the most complete skull among titanosaurians, further revealing that their low and elongated diplodocid-like skull morphology appeared much earlier than previously thought.