1. Harrison's rule states that parasite body size and the body size of their hosts tend to be positively correlated. After it was proposed a century ago, a number of studies have investigated this trend, but the support level has varied greatly between parasite/host associations. Moreover, while the rule has been tested at the individual species level, we still lack knowledge on whether Harrison's rule holds at the scale of parasite and host communities. 2. Here, we mapped flea (parasites) and rodent (hosts) body sizes across Mongolia and asked whether Harrison's rule holds for parasite/host assemblages (i.e. whether a parasite's average body size in a locality is positively correlated with its host's average body size). In addition, we attempted to disentangle complex relationships between flea size, host size and environmental factors by testing alternative hypotheses for the determinants of fleas' body size variation. 3. We gathered occurrence data for fleas and rodents from 2,370 sites across Mongolia, constructed incidence matrices for both taxa and calculated the average body sizes of fleas and their hosts over half-degree cells. Then, we applied a path analysis, accounting for spatial autocorrelation, trying to disentangle the drivers of the correlation between parasite and host body sizes. 4. We found a strong positive correlation between average flea and host size across assemblages. Surprisingly though, we found that environmental factors simultaneously affected the body sizes of both fleas and hosts in the same direction, leading to a most likely deceptive correlation between parasite and host size across assemblages. 5. We suggest that environmental factors may, to a great extent, reflect the environmental conditions inside the hosts' burrows where fleas develop and attain their adult body size, thus influencing their larval growth. Similarly, rodent body size is strongly influenced by air temperature, in the direction predicted by Bergmann's rule. If our findings are valid in other host-parasite associations, this may explain the dissenting results of both support and lack thereof for Harrison's rule.

Understanding the ontogeny of a taxon is a crucial step to properly elucidate its taxonomy and evolution. However, aside from histological data, osteological criteria for assessing maturity are considered lineage specific or controversial. The sequence of neurocentral suture closure of the axial skeleton of extant crocodilians, which occurs in a posterior–anterior sequence, is a non‐destructive method used to determine maturity in extinct reptiles. However, its use in extinct archosaurs not closely related to crocodilians is debatable, as the ancestral condition of Archosauria is unknown and variation occurs in timing and sequence orientation within the clade. We have assessed the pattern of neurocentral suture closure of Hyperodapedontinae rhynchosaurs, an early archosauromorph clade distantly related to archosaurs. Unlike extant crocodilians, they exhibit an anterior–posterior sequence neurocentral suture closure. Relative size and other ontogenetic markers suggest that neurocentral closure in the Hyperodapedontinae correlates with aging, although closed sutures were rare in the sample. A high number of open or partially open sutures in mature individuals indicate that they remained open during most of their life. Our study indicates that: (1) the delayed neurocentral closure may be a paedomorphic heterochronic process in Hyperodapedontinae, as it contrasts with the fully closed neurocentral sutures of early diverging non‐hyperodapedontine rhynchosaurs; (2) the assumption that open neurocentral sutures indicates immaturity in extinct reptiles is not always correct; and (3) the delayed closure may have originated independently in several archosauromorph lineages, but the ancestral condition of Archosauria probably follows the crocodilian closure pattern.

No abstract

In the field of evolutionary biology, researchers have long been interested in comprehending the patterns of diversification and elucidating the mechanisms responsible for the impressive variety of lifeforms observed within taxonomic groups. Based on the fossil records of Felidae, and using a Bayesian framework, we assess here how speciation and extinction rates vary across the last 25 Myr, and how it can potentially be associated with changes in two major ecomorphological traits of Felidae, body mass and carnassial teeth, over such time span. We found two coupled independent increases in both traits along Felidae evolution, one in Machairodontinae, which gradually increases over the last 15 Myr, and the second in Panthera, which rapidly increases within 5 million years. Furthermore, gradual reductions in body mass and carnassial teeth were also observed for some genera such as Leopardus, Lynx, and Felis. Diversification rates showed a gradual reduction throughout the Miocene and Pliocene, with significant rate shifts occurring around 9 and 2 Myr. We suggest that felids were influenced by major environmental changes over the last 17 Myr that probably shaped both traits studied here in order to hunt larger herbivores, but also to explore new niches that became available.

There is an intense debate in the literature about how life-history traits of species interact and influence each other over evolutionary time because, in most biological systems, the targets of selection are sets of traits. However, most of what we know about the tempo and mode of trait evolution is based on studies that focus on single traits. Here, we applied phylogenetic path analysis to uncover the most probable causal relationships linking seven relevant traits of the Canidae family, focusing on social behaviour and its influence on the reproductive traits of canids. Our findings suggest that biogeographic events throughout the planet’s history are important in understanding how the social behaviour of Canidae was shaped throughout the past 13 Myr, which is more likely to be influencing changes in the length of gestation. In addition, we suggest that cooperative behaviour can be a key element in understanding the evolution of the length of gestation in canids.