New tools suggest a middle Jurassic origin for mammalian endothermy: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time: Advances in state-of-the-art techniques uncover new insights on the evolutionary patterns of mammalian endothermy through time

Early Permian synapsid impressions illuminate the origin of epidermal scales and aggregation behavior

Fossil evidence of skin structure1,2 and group behavior3,4 in the stem group of mammals, the early synapsids, is sparse and restricted to a few occurrences. We describe here exceptional resting trace fossils, Bromackerichnus requiescens n. igen. n. isp., from the early Permian Tambach Formation at the Bromacker locality, Thuringia, Germany.5 They are the only definite early synapsid-and, more specifically, sphenacodontid-full-body impressions. They include skin impressions of the limbs, trunk, and tail that show epidermal scales and are associated with Dimetropus leisnerianus footprints. Through a morphological comparison with modern taxa,6 we demonstrate for the first time the unequivocal occurrence of epidermal scales in early synapsids. A review of the early amniote and stem amniote trace and body fossil skin record highlights that this constitutes the oldest occurrence of epidermal scales in synapsids, long before the acquisition of hair.7 Moreover, we find the first fossil occurrence of epidermal scales in eight different tetrapod groups during the early Permian. This implies an earlier common origin of epidermal scales. The higher chance of preservation and spreading in the early Permian was probably due to a structural reinforcement of the scales as an adaptation to global warming and aridization8 at the end of the Late Paleozoic Ice Age. Moreover, sphenacodontid resting, swimming, and locomotion traces of multiple individuals of different sizes co-occur on the same bedding planes, in a relatively small area. This is interpreted as the earliest documented aggregation behavior in sphenacodontid synapsids that gathered in and around small ponds.

Pseudosuchian thermometabolism: A review of the past two decades

Pseudosuchia, one of the two main clades of Archosauria, is today only represented by some 20 extant species, the crocodilians, representing only a fraction of its extinct diversity. Extant crocodilians are ectotherms but present morphological and anatomical features usually associated with endothermy. In 2004, it was proposed that pseudosuchians were ancestrally endothermic and the features observed in extant crocodilians are the remains of this lost legacy. This contribution has two parts: the first part covers 20 years of studies on this subject, first exploring the evidence for a loss of endothermy in extant crocodilians, before covering the variety of proxies used to infer the thermophymetabolic regime of extinct pseudosuchians. In the second part, the quantitative results of these previous studies are integrated into a comprehensive ancestral state reconstruction to discuss a potential scenario for the evolution of thermometabolism. Pseudosuchian endothermy would then have been lost close to the node Crocodylomorpha. The end-Triassic mass extinction is proposed to have played the role of a filter, leading to the extinction of endothermic pseudosuchians and the survival of ectothermic ones. This difference in survival in Pseudosuchia is compared to those of dinosaurs, and difference in their metabolism is also considered. Pseudosuchian endothermy might have been of a different level than the dinosaurian one and more studies are expected to clarify this question.

Life history data derived from the dental histological analysis of Giraffa camelopardalis: Implications for the palaeohistology of extinct giraffids

The analysis of incremental marks in the enamel, dentine and cementum of extant and extinct species provides important information about the rate and pattern of tooth growth, which permits inferences about key life history traits. Traditionally, such research has mainly focused on primates, while other mammalian groups have remained relatively unexplored. In some cases, this has led to the misidentification of incremental markings and the miscalculation of dental growth parameters in non-primate taxa, which has highlighted the importance of obtaining more reliable comparative frameworks. Here, we partially fill this gap by providing a detailed analysis of the dental microstructure in the extant giraffe Giraffa camelopardalis. We specifically studied the histology of the different cusps (i.e. protoconid, metaconid, hypoconid, entoconid and hypoconulid) of two first lower molars and two third lower molars with different degree of wear to identify the different incremental markings and to calculate dental growth parameters such as daily secretion rate and enamel formation front angle for each cusp and tooth. Our results show that incremental markings in enamel were more apparent as compared to those in dentine and/or cementum and have permitted a deeper analysis of the former tissue. Enamel laminations, which had a daily periodicity, were the most common incremental lines in all teeth. Supradaily Retzius lines and subdaily cross-striations and laminations were also recognised in dental enamel, revealing multiple secretory pulses of the ameloblasts in the giraffe. Generally, values of enamel growth parameters (i.e. daily secretion rate and enamel formation front angle) obtained for the first lower molar were comparable to those reported for closely related taxa, while those calculated for the third lower molar present a higher degree of variation that may be linked to differences in general somatic rates of growth. Nevertheless, enamel growth parameters were highly variable within each tooth, suggesting caution when making general (palaeo)biological inferences from dental histology. The giraffe dentine and cementum also register incremental lines. In the dentine, most of these features were classified as daily von Ebner's lines and their counting and measurement revealed values of secretion rates that agree with those previously reported in other artiodactyls. The age calculated from the incremental lines in the dental cementum matches that deduced from dental wear, suggesting that the counting of yearly lines in this tissue is a reliable tool to estimate individual age in giraffids. This study further suggests ways to refine future analyses of dentine and cementum and sets the stage for dental palaeohistology of extinct giraffids and closely related ungulates for which life history information is still unknown.

Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint

The acquisition of the load-bearing dentary-squamosal jaw joint was a key step in mammalian evolution1-5. Although this innovation has received decades of study, questions remain over when and how frequently a mammalian-like skull-jaw contact evolved, hindered by a paucity of three-dimensional data spanning the non-mammaliaform cynodont-mammaliaform transition. New discoveries of derived non-mammaliaform probainognathian cynodonts from South America have much to offer to this discussion. Here, to address this issue, we used micro-computed-tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes6-8, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major. We find homoplastic evolution in the jaw joint in the approach to mammaliaforms, with ictidosaurs (Riograndia plus tritheledontids) independently evolving a dentary-squamosal contact approximately 17 million years before this character first appears in mammaliaforms of the Late Triassic period10-12. Brasilodon, contrary to previous descriptions6-8, lacks an incipient dentary condyle and squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate-articular joint. We postulate that the jaw joint underwent marked evolutionary changes in probainognathian cynodonts. Some probainognathian clades independently acquired 'double' craniomandibular contacts, with mammaliaforms attaining a fully independent dentary-squamosal articulation with a conspicuous dentary condyle and squamosal glenoid in the Late Triassic. The dentary-squamosal contact, which is traditionally considered to be a typical mammalian feature, therefore evolved more than once and is more evolutionary labile than previously considered.

The origins of mammal growth patterns during the Jurassic mammalian radiation

We use synchrotron x-ray tomography of annual growth increments in the dental cementum of mammaliaforms (stem and crown fossil mammals) from three faunas across the Jurassic to map the origin of patterns of mammalian growth patterns, which are intrinsically related to mammalian endothermy. Although all fossils studied exhibited slower growth rates, longer life spans, and delayed sexual maturity relative to comparably sized extant mammals, the earliest crown mammals developed significantly faster growth rates in early life that reduced at sexual maturity, compared to stem mammaliaforms. Estimation of basal metabolic rates (BMRs) suggests that some fossil crown mammals had BMRs approaching the lowest rates of extant mammals. We suggest that mammalian growth patterns first evolved during their mid-Jurassic adaptive radiation, although growth remained slower than in extant mammals.

Jurassic fossil juvenile reveals prolonged life history in early mammals

Living mammal groups exhibit rapid juvenile growth with a cessation of growth in adulthood1. Understanding the emergence of this pattern in the earliest mammaliaforms (mammals and their closest extinct relatives) is hindered by a paucity of fossils representing juvenile individuals. We report exceptionally complete juvenile and adult specimens of the Middle Jurassic docodontan Krusatodon, providing anatomical data and insights into the life history of early diverging mammaliaforms. We used synchrotron X-ray micro-computed tomography imaging of cementum growth increments in the teeth2-4 to provide evidence of pace of life in a Mesozoic mammaliaform. The adult was about 7 years and the juvenile 7 to 24 months of age at death and in the process of replacing its deciduous dentition with its final, adult generation. When analysed against a dataset of life history parameters for extant mammals5, the relative sequence of adult tooth eruption was already established in Krusatodon and in the range observed in extant mammals but this development was prolonged, taking place during a longer period as part of a significantly longer maximum lifespan than extant mammals of comparable adult body mass (156 g or less). Our findings suggest that early diverging mammaliaforms did not experience the same life histories as extant small-bodied mammals and the fundamental shift to faster growth over a shorter lifespan may not have taken place in mammaliaforms until during or after the Middle Jurassic.

New evidence from high-resolution computed microtomography of Triassic stem-mammal skulls from South America enhances discussions on turbinates before the origin of Mammaliaformes

The nasal cavity of living mammals is a unique structural complex among tetrapods, acquired along a series of major morphological transformations that occurred mainly during the Mesozoic Era, within the Synapsida clade. Particularly, non-mammaliaform cynodonts document several morphological changes in the skull, during the Triassic Period, that represent the first steps of the mammalian bauplan. We here explore the nasal cavity of five cynodont taxa, namely Thrinaxodon, Chiniquodon, Prozostrodon, Riograndia, and Brasilodon, in order to discuss the main changes within this skull region. We did not identify ossified turbinals in the nasal cavity of these taxa and if present, as non-ossified structures, they would not necessarily be associated with temperature control or the development of endothermy. We do, however, notice a complexification of the cartilage anchoring structures that divide the nasal cavity and separate it from the brain region in these forerunners of mammals.

Parental brain through time: The origin and development of the neural circuit of mammalian parenting

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Nasal turbinates of the dicynodont Kawingasaurus fossilis and the possible impact of the fossorial habitat on the evolution of endothermy

The nasal region of the fossorial anomodont Kawingasaurus fossilis was virtually reconstructed from neutron-computed tomographic data and compared with the terrestrial species Pristerodon mackayi and other nonmammalian synapsids. The tomography of the Kawingasaurus skull reveals a pattern of maxillo-, naso-, fronto- and ethmoturbinal ridges that strongly resemble the mammalian condition. On both sides of the nasal cavity, remains of scrolled maxilloturbinals were preserved that were still partially articulated with maxilloturbinal ridges. Furthermore, possible remains of the lamina semicircularis as well as fronto- or ethmoturbinals were found. In Kawingasaurus, the maxilloturbinal ridges were longer and stronger than in Pristerodon. Except for the nasoturbinal ridges, no other ridges in the olfactory region and no remains of turbinates were recognized. This supports the hypothesis that naso-, fronto-, ethmo- and maxilloturbinals were a plesiomorphic feature of synapsids, but due to their cartilaginous nature in most taxa were, in almost all cases, not preserved. The well-developed maxilloturbinals in Kawingasaurus were probably an adaptation to hypoxia-induced hyperventilation in the fossorial habitat, maintaining the high oxygen demands of Kawingasaurus' large brain. The surface area of the respiratory turbinates in Kawingasaurus falls into the mammalian range, which suggests that they functioned as a countercurrent exchange system for thermoregulation and conditioning of the respiratory airflow. Our results suggest that the environmental conditions of the fossorial habitat led to specific sensory adaptations, accompanied by a pulse in brain evolution and of endothermy in cistecephalids, ~50 million years before the origin of endothermy in the mammalian stem line. This supports the Nocturnal Bottleneck Theory, in that we found evidence for a similar evolutionary scenario in cistecephalids as proposed for early mammals.

Craniodental anatomy in Permian-Jurassic Cynodontia and Mammaliaformes (Synapsida, Therapsida) as a gateway to defining mammalian soft tissue and behavioural traits

Mammals are diagnosed by more than 30 osteological characters (e.g. squamosal-dentary jaw joint, three inner ear ossicles, etc.) that are readily preserved in the fossil record. However, it is the suite of physiological, soft tissue and behavioural characters (e.g. endothermy, hair, lactation, isocortex and parental care), the evolutionary origins of which have eluded scholars for decades, that most prominently distinguishes living mammals from other amniotes. Here, we review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestors, the Permian-Jurassic Cynodontia and Mammaliaformes, can potentially be used to document the origin of some of the most crucial defining features of mammals. We discuss how these soft tissue and behavioural traits are highly integrated, and how their evolution is intermingled with that of craniodental traits, thus enabling the tracing of their previously out-of-reach phylogenetic history. Most of these osteological and dental proxies, such as the maxillary canal, bony labyrinth and dental replacement only recently became more easily accessible-thanks, in large part, to the widespread use of X-ray microtomography scanning in palaeontology-because they are linked to internal cranial characters. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

At the root of the mammalian mind: The sensory organs, brain and behavior of pre-mammalian synapsids

All modern mammals are descendants of the paraphyletic non-mammaliaform Synapsida, colloquially referred to as the "mammal-like reptiles." It has long been assumed that these mammalian ancestors were essentially reptile-like in their morphology, biology, and behavior, i.e., they had a small brain, displayed simple behavior, and their sensory organs were unrefined compared to those of modern mammals. Recent works have, however, revealed that neurological, sensory, and behavioral traits previously considered typically mammalian, such as whiskers, enhanced olfaction, nocturnality, parental care, and complex social interactions evolved before the origin of Mammaliaformes, among the early-diverging "mammal-like reptiles." In contrast, an enlarged brain did not evolve immediately after the origin of mammaliaforms. As such, in terms of paleoneurology, the last "mammal-like reptiles" were not significantly different from the earliest mammaliaforms. The abundant data and literature published in the last 10 years no longer supports the "three pulses" scenario of synapsid brain evolution proposed by Rowe and colleagues in 2011, but supports the new "outside-in" model of Rodrigues and colleagues proposed in 2018, instead. As Mesozoic reptiles were becoming the dominant taxa within terrestrial ecosystems, synapsids gradually adapted to smaller body sizes and nocturnality. This resulted in a sensory revolution in synapsids as olfaction, audition, and somatosensation compensated for the loss of visual cues. This altered sensory input is aligned with changes in the brain, the most significant of which was an increase in relative brain size.

Gradistics: An underappreciated dimension in evolutionary space

The growth of complexity is an unsolved and underappreciated problem. We consider possible causes of this growth, hypotheses testing, molecular mechanisms, complexity measures, cases of simplification, and significance for biomedicine. We focus on a general ability of regulation, which is based on the growing information storage and processing capacities, as the main proxy of complexity. Natural selection is indifferent to complexity. However, complexification can be inferred from the same first principle, on which natural selection is founded. Natural selection depends on potentially unlimited reproduction under limited environmental conditions. Because of the demographic pressure, the simple ecological niches become fulfilled and diversified (due to species splitting and divergence). Diversification increases complexity of biocenoses. After the filling and diversification of simple niches, the more complex niches can arise. This is the 'atomic orbitals' (AO) model. Complexity has many shortcomings but it has an advantage. This advantage is ability to regulatory adaptation, including behavioral, formed in the evolution by means of genetic adaptation. Regulatory adaptation is much faster than genetic one because it is based on the information previously accumulated via genetic adaptation and learning. Regulatory adaptation further increases complexity of biocenoses. This is the 'regulatory advantage' (RA) model. The comparison of both models allows testable predictions. We focus on the animal evolution because of the appearance of higher regulatory level (nervous system), which is absent in other lineages, and relevance to humans (including biomedical aspects).

Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy

Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings^1,2. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most of the fossil evidence is ambiguous^3-17. Here we show that this key evolutionary transition can be investigated using the morphology of the endolymph-filled semicircular ducts of the inner ear, which monitor head rotations and are essential for motor coordination, navigation and spatial awareness^18-22. Increased body temperatures during the ectotherm-endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively affecting semicircular duct biomechanics^23,24, while simultaneously increasing behavioural activity^25,26 probably required improved performance²⁷. Morphological changes to the membranous ducts and enclosing bony canals would have been necessary to maintain optimal functionality during this transition. To track these morphofunctional changes in 56 extinct synapsid species, we developed the thermo-motility index, a proxy based on bony canal morphology. The results suggest that endothermy evolved abruptly during the Late Triassic period in Mammaliamorpha, correlated with a sharp increase in body temperature (5-9 °C) and an expansion of aerobic and anaerobic capacities. Contrary to previous suggestions^3-14, all stem mammaliamorphs were most probably ectotherms. Endothermy, as a crucial physiological characteristic, joins other distinctive mammalian features that arose during this period of climatic instability²⁸.