Endothermy, i.e. the endogenous production of metabolic heat, has evolved multiple times among vertebrates, and several strategies of heat production have been studied extensively by physiologists over the course of the twentieth century. The independent acquisition of endothermy by mammals and birds has been the subject of many hypotheses regarding their origin and associated evolutionary constraints. Many groups of vertebrates, however, are thought to possess other mechanisms of heat production, and alternative ways to regulate thermogenesis that are not always considered in the palaeontological literature. Here, we perform a review of the mechanisms involved in heat production, with a focus on cellular and molecular mechanisms, in a phylogenetic context encompassing the entire vertebrate diversity. We show that endothermy in mammals and birds is not as well defined as commonly assumed by evolutionary biologists and consists of a vast array of physiological strategies, many of which are currently unknown. We also describe strategies found in other vertebrates, which may not always be considered endothermy, but nonetheless correspond to a process of active thermogenesis. We conclude that endothermy is a highly plastic character in vertebrates and provides a guideline on terminology and occurrences of the different types of heat production in vertebrate evolution. This article is part of the theme issue ‘Vertebrate palaeophysiology’.

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Vertebrate bone is composed of three main cell types: osteoblasts, osteoclasts and osteocytes, the latter being by far the most numerous. Osteocytes are thought to play a fundamental role in bone physiology and homeostasis, however they are entirely absent in most extant species of teleosts, a group that comprises the vast majority of bony 'fishes', and approximately half of vertebrates. Understanding how this acellular (anosteocytic) bone appeared and was maintained in such an important vertebrate group has important implications for our understanding of the function and evolution of osteocytes. Nevertheless, although it is clear that cellular bone is ancestral for teleosts, it has not been clear in which specific subgroup the osteocytes were lost. This review aims to clarify the phylogenetic distribution of cellular and acellular bone in teleosts, to identify its precise origin, reversals to cellularity, and their implications. We surveyed the bone type for more than 600 fossil and extant ray-finned fish species and optimised the results on recent large-scale molecular phylogenetic trees, estimating ancestral states. We find that acellular bone is a probable synapomorphy of Euteleostei, a group uniting approximately two-thirds of teleost species. We also confirm homoplasy in these traits: acellular bone occurs in some non-euteleosts (although rarely), and cellular bone was reacquired several times independently within euteleosts, in salmons and relatives, tunas and the opah (Lampris sp.). The occurrence of peculiar ecological (e.g. anadromous migration) and physiological (e.g. red-muscle endothermy) strategies in these lineages might explain the reacquisition of osteocytes. Our review supports that the main contribution of osteocytes in teleost bone is to mineral homeostasis (via osteocytic osteolysis) and not to strain detection or bone remodelling, helping to clarify their role in bone physiology.

Lampridiformes is a peculiar clade of pelagic marine acanthomorph (spiny‐rayed) teleosts. Its phylogenetic position remains ambiguous, and varies depending on the type of data (morphological or molecular) used to infer interrelationships. Because the extreme morphological specializations of lampridiforms may have overwritten the ancestral features of the group with a bearing on its relationships, the inclusion of fossils that exhibit primitive character state combinations for the group as a whole is vital in establishing its phylogenetic position. Therefore, we present an osteological data set of extant (ten taxa) and fossil (14 taxa) acanthomorphs, including early Late Cretaceous taxa for which a close relationship with extant Lampridiformes has been suggested: †Aipichthyoidea, †Pharmacichthyidae, and †Pycnosteroididae. We find that all three taxa plus Lampridiformes form a clade that we call Lampridomorpha. Under this hypothesis, †Aipichthyoidea is paraphyletic. The inclusion of fossils in the analysis changes the topology, highlighting their critical importance in phylogenetic studies of morphological characters. When fossils are included, Lampridomorpha is sister to Euacanthomorpha (all other extant acanthomorphs), concurring with most previous anatomical studies, but conflicting with most molecular results. Lampridomorpha as a whole was a major component of the earliest acanthomorph faunas, notably in the Cenomanian. © 2014 The Linnean Society of London

Osteocytes, cells embedded within the bone mineral matrix, inform on key aspects of vertebrate biology. In particular, a relationship between volumes of the osteocytes and bone growth and/or genome size has been proposed for several tetrapod lineages. However, the variation in osteocyte volume across different scales is poorly characterized and mostly relies on incomplete, two‐dimensional information. In this study, we characterize the variation of osteocyte volumes in ray‐finned fishes (Actinopterygii), a clade including more than half of modern vertebrate species in which osteocyte biology is poorly known. We use X‐ray synchrotron micro‐computed tomography (SRµCT) to achieve a three‐dimensional visualization of osteocyte lacunae and direct measurement of their size (volumes). Our specimen sample is designed to characterize variation in osteocyte lacuna morphology at three scales: within a bone, among the bones of one individual and among species. At the intra‐bone scale, we find that osteocyte lacunae vary noticeably in size between zones of organized and woven bone (being up to six times larger in woven bone), and across cyclical bone deposition. This is probably explained by differences in bone deposition rate, with larger osteocyte lacunae contained in bone that deposits faster. Osteocyte lacuna volumes vary 3.5‐fold among the bones of an individual, and this cannot readily be explained by variation in bone growth rate or other currently observable factors. Finally, we find that genome size provides the best explanation of variation in osteocyte lacuna volume among species: actinopterygian taxa with larger genomes (polyploid taxa in particular) have larger osteocyte lacunae (with a ninefold variation in median osteocyte volume being measured). Our findings corroborate previous two‐dimensional studies in tetrapods that also observed similar patterns of intra‐individual variation and found a correlation with genome size. This opens new perspectives for further studies on bone evolution, physiology and palaeogenomics in actinopterygians, and vertebrates as a whole.

Endothermy, production and retention of heat by the body, appeared convergently in mammals, birds and four spiny-rayed teleost fish lineages. Of these, red-muscle endothermy over most or all of the body has only appeared in two groups: tunas and the opah (). Hitherto, tunas have been the only spiny-rayed fishes known to have bones containing embedded osteocyte cells; others have acellular bone. We examined bone histology in for the first time, demonstrating the presence of cellular bone very similar to that of tunas. This contrasts with the acellular condition of its ectothermic close relatives. The distribution of this character suggests that it co-evolved with red-muscle endothermy, hinting at a common physiological mechanism that would link bone histology to endothermy in these distantly related teleost lineages.

The fossil record of marine ray-finned fishes (Actinopterygii) from the time interval surrounding the Cretaceous–Paleogene (K–Pg) extinction is scarce at a global scale, hampering our understanding of the impact, patterns and processes of extinction and recovery in the marine realm, and its role in the evolution of modern marine ichthyofaunas. Recent fieldwork in the K–Pg interval of the Pindos Unit in Eurytania, continental Greece, shed new light on forgotten fossil assemblages and allowed for the collection of a diverse, but fragmentary sample of actinopterygians from both late Maastrichtian and Paleocene rocks. Late Maastrichtian assemblages are dominated by Aulopiformes (†Ichthyotringidae, †Enchodontidae), while †Dercetidae (also Aulopiformes), elopomorphs and additional, unidentified teleosts form minor components. Paleocene fossils include a clupeid, a stomiiform and some unidentified teleost remains. This study expands the poor record of body fossils from this critical time interval, especially for smaller sized taxa, while providing a rare, paleogeographically constrained, qualitative glimpse of open-water Tethyan ecosystems from both before and after the extinction event. Faunal similarities between the Maastrichtian of Eurytania and older Late Cretaceous faunas reveal a higher taxonomic continuum in offshore actinopterygian faunas and ecosystems spanning the entire Late Cretaceous of the Tethys. At the same time, the scarcity of Paleocene findings offers tentative clues for a depauperate state of Tethyan ichthyofaunas in the aftermath of the K–Pg Extinction.

Teleost fishes comprise one-half of all vertebrate species and possess a duplicated genome. This whole-genome duplication (WGD) occurred on the teleost stem lineage in an ancient common ancestor of all living teleosts and is hypothesized as a trigger of their exceptional evolutionary radiation. Genomic and phylogenetic data indicate that WGD occurred in the Mesozoic after the divergence of teleosts from their closest living relatives but before the origin of the extant teleost groups. However, these approaches cannot pinpoint WGD among the many extinct groups that populate this 50- to 100-million-y lineage, preventing tests of the evolutionary effects of WGD. We infer patterns of genome size evolution in fossil stem-group teleosts using high-resolution synchrotron X-ray tomography to measure the bone cell volumes, which correlate with genome size in living species. Our findings indicate that WGD occurred very early on the teleost stem lineage and that all extinct stem-group teleosts known so far possessed duplicated genomes. WGD therefore predates both the origin of proposed key innovations of the teleost skeleton and the onset of substantial morphological diversification in the clade. Moreover, the early occurrence of WGD allowed considerable time for postduplication reorganization prior to the origin of the teleost crown group. This suggests at most an indirect link between WGD and evolutionary success, with broad implications for the relationship between genomic architecture and large-scale evolutionary patterns in the vertebrate Tree of Life.