RESUMEN
Phylogenetic trees establish a historical context for the study of organismal form and function. Most phylogenetic trees are estimated using a model of evolution. For molecular data, modeling evolution is often based on biochemical observations about changes between character states. For example, there are four nucleotides, and we can make assumptions about the probability of transitions between them. By contrast, for morphological characters, we may not know a priori how many characters states there are per character, as both extant sampling and the fossil record may be highly incomplete, which leads to an observer bias. For a given character, the state space may be larger than what has been observed in the sample of taxa collected by the researcher. In this case, how many evolutionary rates are needed to even describe transitions between morphological character states may not be clear, potentially leading to model misspecification. To explore the impact of this model misspecification, we simulated character data with varying numbers of character states per character. We then used the data to estimate phylogenetic trees using models of evolution with the correct number of character states and an incorrect number of character states. The results of this study indicate that this observer bias may lead to phylogenetic error, particularly in the branch lengths of trees. If the state space is wrongly assumed to be too large, then we underestimate the branch lengths, and the opposite occurs when the state space is wrongly assumed to be too small.
RESUMEN
Riojasuchus tenuisceps was a pseudosuchian archosaur from the Late Triassic period in Argentina. Like other ornithosuchids, it had unusual morphology such as a unique "crocodile-reversed" ankle joint, a lesser trochanter as in dinosaurs and a few other archosaurs, robust vertebrae, and somewhat shortened, gracile forelimbs. Such traits have fuelled controversies about its locomotor function-were its limbs erect or "semi-erect"? Was it quadrupedal or bipedal, or a mixture thereof? These controversies seem to persist because analyses have been qualitative (functional morphology) or correlative (morphometrics) rather than explicitly, quantitatively testing mechanistic hypotheses about locomotor function. Here, we develop a 3D whole-body model of R. tenuisceps with the musculoskeletal apparatus of the hindlimbs represented in detail using a new muscle reconstruction. We use this model to quantify the body dimensions and hindlimb muscle leverages of this enigmatic taxon, and to estimate joint ranges of motion and qualitative joint functions. Our model supports prior arguments that R. tenuisceps used an erect posture, parasagittal gait and plantigrade pes. However, some of our inferences illuminate the rather contradictory nature of evidence from the musculoskeletal system of R. tenuisceps-different features support (or are ambiguous regarding) quadrupedalism or bipedalism. Deeper analyses of our biomechanical model could move toward a consensus regarding ornithosuchid locomotion. Answering these questions would not only help understand the palaeobiology and bizarre morphology of this clade, but also more broadly if (or how) locomotor abilities played a role in the survival versus extinction of various archosaur lineages during the end-Triassic mass extinction event.
RESUMEN
Dromaeosaurids (Theropoda: Dromaeosauridae), a group of dynamic, swift predators, have a sparse fossil record, particularly at the end of the Cretaceous Period. The recently described Dineobellator notohesperus, consisting of a partial skeleton from the Upper Cretaceous (Maastrichtian) of New Mexico, is the only diagnostic dromaeosaurid to be recovered from the latest Cretaceous of the southwestern United States. Reinterpreted and newly described material include several caudal vertebrae, portions of the right radius and pubis, and an additional ungual, tentatively inferred to be from manual digit III. Unique features, particularly those of the humerus, unguals, and caudal vertebrae, distinguish D. notohesperus from other known dromaeosaurids. This material indicates different physical attributes among dromaeosaurids, such as use of the forearms, strength in the hands and feet, and mobility of the tail. Several bones in the holotype exhibit abnormal growth and are inferred to be pathologic features resulting from an injury or disease. Similar lengths of the humerus imply Dineobellator and Deinonychus were of similar size, at least regarding length and/or height, although the more gracile nature of the humerus implies Dineobellator was a more lightly built predator. A new phylogenetic analysis recovers D. notohesperus as a dromaeosaurid outside other previously known and named clades. Theropod composition of the Naashoibito Member theropod fauna is like those found in the more northern Late Cretaceous North American ecosystems. Differences in tooth morphologies among recovered theropod teeth from the Naashoibito Member also implies D. notohesperus was not the only dromaeosaurid present in its environment.