Growing sabers: Mandibular shape and biomechanical performance trajectories during the ontogeny of Smilodon fatalis.

The evolution of organisms can be studied through the lens of developmental systems, as the timing of development of morphological features is an important aspect to consider when studying a phenotype. Such data can be challenging to obtain in fossil amniotes owing to the scarcity of their fossil record. However, the numerous remains of Rancho La Brea allow a detailed study of the postnatal changes in an extinct sabertoothed felid: Smilodon fatalis. Despite numerous previous studies on the ontogeny of Smilodon, an important question remained open: how did the cubs of Smilodon acquire and process food? By applying 3D geometric morphometrics and finite element analyses to 49 mandibles at various developmental stages (22 of S. fatalis, 23 of Panthera leo, and 4 of early diverging felids), we assess the changes in mandibular shape and performance during growth. Both lions and sabertooths exhibit a shift in mandibular shape, aligning with eruption of the lower carnassial. This marks the end of weaning in lions and suggests a prolonged weaning period in S. fatalis owing to its delayed eruption sequence. We also highlight distinct ontogenetic trajectories, with S. fatalis undergoing more postnatal mandibular shape changes. Finally, although S. fatalis appears more efficient than P. leo at performing an anchor bite, this efficiency is acquired through ontogeny and at a quite late age. The delayed shape change compared with P. leo and the low biting efficiency during the growth in Smilodon could indicate an extended duration of the parental care compared with P. leo.

SPASOS 1.1: a program for the inference of ancestral shape ontogenies.

We recently published a method to infer ancestral landmark-based shape ontogenies that takes into account the possible existence of changes in developmental timing. Here we describe SPASOS, a software to perform that analysis. SPASOS is an open-source Windows program written in C. Input data include landmark coordinates for each specimen -with the corresponding information about developmental timing- and a phylogenetic tree showing the relationships among the species sampled. As output, the program produces image files for an easy visualization of the results and data files useful for post-processing. The program incorporates an interpolating function, based on weighting moving averages, which allows analysis of data with scarce information along the ontogenetic trajectory. An empirical evaluation of this function showed its suitability to fill in incomplete ontogenetic trajectories. Finally, we present the results of a reanalysis in SPASOS of a published dataset, where changes in developmental timing were originally inferred by considering PCA scores as shape variables. Both approaches retrieved the same four largest changes in developmental timing, but differed in the ancestral shapes inferred.

PASOS: a method for the phylogenetic analysis of shape ontogenies.

We present a novel phylogenetic approach to infer ancestral ontogenies of shape characters described as landmark configurations. The method is rooted in previously published theoretical developments to analyse landmark data in a phylogenetic context with parsimony as the optimality criterion, in this case using the minimization of differences in landmark position to define not only ancestral shapes but also the changes in developmental timing between ancestor-descendant shape ontogenies. Evolutionary changes along the tree represent changes in relative developmental timing between ontogenetic trajectories (possible heterochronic events) and changes in shape within each stage. The method requires the user to determine the shape of the specimens between two standard events, for instance birth and onset of sexual maturity. Once the ontogenetic trajectory is discretized into a series of consecutive stages, the method enables the user to identify changes in developmental timing associated with changes in the offset and/or onset of the shape ontogenetic trajectories. The method is implemented in a C language program called SPASOS. The analysis of two empirical examples (anurans and felids) using this novel method yielded results in agreement with previous hypotheses about shape evolution in these groups based on non-phylogenetic analyses.

On the lack of a universal pattern associated with mammalian domestication: differences in skull growth trajectories across phylogeny.

As shown in a taxonomically broad study, domestication modifies postnatal growth. Skull shape across 1128 individuals was characterized by 14 linear measurements, comparing 13 pairs of wild versus domesticated forms. Among wild forms, the boar, the rabbit and the wolf have the highest proportion of allometric growth, explaining in part the great morphological diversity of the domesticated forms of these species. Wild forms exhibit more isometric growth than their domesticated counterparts. Multivariate comparisons show that dogs and llamas exhibit the greatest amount of differences in trajectories with their wild counterparts. The least amount is recorded in the pig-boar, and camel and horse pairs. Bivariate analyses reveal that most domesticated forms have growth trajectories different from their respective wild counterparts with regard to the slopes. In pigs and camels slopes are shared and intercepts are different. There is a trajectory extension in most domesticated herbivores and the contrary pattern in carnivorous forms. However, there is no single, universal and global pattern of paedomorphosis or any other kind of heterochrony behind the morphological diversification that accompanies domestication.