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Although the brain fills nearly the entire cranial cavity in birds, it can occupy a small portion of it in crocodilians. The lack of data regarding the volumetric correspondence between the brain and the cranial cavity hampers thorough assessments of the degree of encephalization in non‐neornithean dinosaurs and other extinct archosaurs and, consequently, informed inferences regarding their cognitive capacities. Existing data suggest that, across extant archosaurs, the degree of endocranial doming and the volume of intracranial nonneural components are inversely related. We build upon this information to develop an equation relating these two anatomical features in non‐neornithean dinosaurs and other extinct archosaurs. We rely on measurements of the endocast doming and brain‐to‐endocranial cavity (BEC) index in extant relatives of non‐neornithean dinosaurs, namely, the crurotarsans Caiman crocodilus, Crocodylus niloticus, and Crocodylus porosus; the paleognaths Struthio camelus and Apteryx mantelli; and the fowl Macrocephalon maleo, Gallus gallus, Meleagris gallopavo, Phasianus colchicus, and Anas platyrhynchos. Applying the equation to representative endocasts from major clades of dinosaurs, we found that BEC varies from about 0.6 in ceratopsians and thyreophorans to around 0.7 in ornithopods, pachycephalosaurians, sauropods, and theropods. We, therefore, warn against the use of a catch‐all value, like 0.5, and instead encourage refinement in the adoption of BEC across archosaurs.Illustration of the endocranial doming, which can provide a proxy for the brain‐to‐endocranial cavity index, using the endocast of an adult of the iguanodont dinosaur Proa valdearinnoensis as an example. [ABSTRACT FROM AUTHOR]

Proa valdearinnoensis is a relatively large‐headed and stocky iguanodontian dinosaur from the latest Early Cretaceous of Spain. Its braincase is known from three specimens. Similar to that of other dinosaurs, it shows a mosaic ossification pattern in which most of the bones seem to have fused together indistinguishably while a few (frontoparietal, basioccipital) might have remained loosely attached. The endocasts of the three specimens are described based on CT data and digital reconstructions. They show unmistakable morphological similarities with the endocast of closely related taxa, such as Sirindhorna khoratensis (which is close in age but from Thailand). This supports a high conservatism of the endocranial cavity. The issue of volumetric correspondence between endocranial cavity and brain in dinosaurs is analyzed. Although a brain‐to‐endocranial cavity (BEC) index of 0.50 has been traditionally used, we employ instead 0.73. This is indeed the mid‐value between the situation in adults of Alligator mississippiensis and Gallus gallus, which are members of the extant bracketing taxa of dinosaurs (Crocodilia and Aves). We thence gauge the level of encephalization of P. valdearinnoensis through the calculation of the encephalization quotient (EQ), which remains valuable as a metric for assessing the degree of cognitive function in extinct taxa, especially those with fully ossified braincases like dinosaurs and other archosaurs. The EQ obtained for P. valdearinnoensis (3.611) suggests that this species was significantly more encephalized than most if not all extant nonavian, nonmammalian amniotes. Our work adds to the growing body of data concerning theoretical cognitive capabilities in dinosaurs and supports the idea that an increasing encephalization was fostered not only in theropods but also in parallel in the shorter‐lived lineage of ornithopods. P. valdearinnoensis was ill‐equipped to respond to theropod dinosaurs and possibly lived in groups as a strategy to mitigate the risk of being predated upon. We hypothesize that group‐living and protracted caring of juveniles in this and possibly many other iguanodontian ornithopods favored a degree of encephalization that was outstanding by reptile standards. [ABSTRACT FROM AUTHOR]

Abstract: Three possible hypotheses could explain the polarity of the histological features of basal archosauriform and archosauromorph reptiles: either, the fibrolamellar complex is basal; or, the lamellar-zonal complex is basal or finally, the condition varied, and each complex evolved more than once in these early groups. The answer to this question would have broad implications for our understanding of the physiological, ecological, and behavioral features of the first archosaurs. To this end, we sampled the bone histology of various archosauriforms and basal archosaurs from the Triassic and Lower Jurassic: erythrosuchids, proterochampsids, euparkeriids, and basal ornithischian dinosaurs, including forms close to the origin of archosaurs but poorly assessed phylogenetically. The new data suggest that the possibility of reaching and maintaining very high growth rates through ontogeny could have been a basal characteristic of archosauriforms. This was partly retained (at least during early ontogeny) in most lineages of Triassic pseudosuchians, which nevertheless generally relied on lower growth rates to reach large body sizes. This trend to slower growth seems to have been further emphasized among Crocodylomorpha, which may thus have secondarily reverted toward more generalized reptilian growth strategies. Accordingly, their “typical ectothermic reptilian condition” may be a derived condition within archosauriforms, homoplastic to the generalized physiological condition of basal amniotes. On the other hand, ornithosuchians apparently retained and even enhanced the high growth rates of many basal archosauriforms during most of their ontogenetic trajectories. The Triassic may have been a time of “experimentation” in growth strategies for several archosauriform lineages, only one of which (ornithodirans) eventually stayed with the higher investment strategy successfully. Our data again raise the problem of a possible “phylogenetic signal” being carried by bone histology. Bone histology is highly correlated to “functional” characters as size and growth rates which are intensely involved in species-specific “life-history traits”, are under intense scrutiny by selective pressures and may accordingly evolve very rapidly. This rapid evolutionary rate would in turn produce patterns of species-specific variations that could “erase” higher-order taxonomic signals in bone tissue. In other words, this fast turnover would introduce autapomorphies (and homoplasies) at the level of apical (terminal) taxa that could blur the wider “phylogenetic signal”. Thus, the search for generalized apomorphic (or plesiomorphic) conditions of bone histological character-states at supraspecific levels may often be deceptive. Nevertheless, bone tissue phenotypes can reflect a phylogenetic signal at supraspecific levels if homologous elements are used, and if ontogenetic trajectories and size-dependent differences are taken into consideration. [Copyright &y& Elsevier]