The evolution of gigantism in active marine predators

A novel hypothesis to better understand the evolution of gigantism in active marine predators and the diversity of body sizes, feeding strategies and thermophysiologies of extinct and living aquatic vertebrates is proposed. Recent works suggest that some aspects of animal energetics can act as constraining factors for body size. Given that mass-specific metabolic rate decreases with body mass, the body size of active predators should be limited by the high metabolic demand of this feeding strategy. In this context, we propose that shifts towards higher metabolic levels can enable the same activity and feeding strategy to be maintained at bigger body sizes, offering a satisfactory explanation for the evolution of gigantism in active predators, including a vast quantity of fossil taxa. Therefore, assessing the metabolic ceilings of living aquatic vertebrates and the thermoregulatory strategies of certain key extinct groups is now crucial to define the energetic limits of predation and provide quantitative support for this model.     

Trophic niche ontogeny and palaeoecology of early Toarcian Stenopterygius (Reptilia: Ichthyosauria)

Reconstructing ecological niche shifts during ontogeny in extinct animals with no living analogues is difficult without exceptional fossil collections. Here we demonstrate how a previously identified ontogenetic shift in the size and shape of the dentition in the early Toarcian ichthyosaur Stenopterygius quadriscissus accurately predicts a particular dietary shift. The smallest S. quadriscissus fed on small, burst‐swimming fishes, with a steady shift towards faster moving fish and cephalopods with increasing body size. Larger adult specimens appear to have been completely reliant on cephalopods, with fish completely absent from gut contents shortly after onset of sexual maturity. This is consistent with a previously proposed ontogenetic niche shift based on tooth shape and body size, corroborating the idea that dental ontogeny may be a useful predictor of dietary shifts in marine reptiles. Applying the theoretical framework used here to other extinct species will improve the resolution of palaeoecological reconstructions, where appropriate sample sizes exist.     

Three terrestrial Pleistocene coucals (Centropus: Cuculidae) from southern Australia: biogeographical and ecological significance

Coucals are large, predatory, primarily ground‐dwelling cuckoos of the genus Centropus, with 26 extant species ranging from Africa to Australia. Their evolutionary and biogeographical history are poorly understood and their fossil record almost non‐existent. Only one species (Centropus phasianinus) currently inhabits Australia, but there is now fossil evidence for at least three Pleistocene species. One of these (Centropus colossus) was described from south‐eastern Australia in 1985. Here we describe additional elements of this species from the same site, and remains of two further extinct species from the Thylacoleo Caves of the Nullarbor Plain, south‐central Australia. The skeletal morphology and large size of the three extinct species indicates that they had reduced capacity for flight and were probably primarily ground‐dwelling. The extinct species include the two largest‐known cuckoos, weighing upwards of 1 kg each. They demonstrate that gigantism in this lineage has been more marked in a continental context than on islands, contrary to the impression gained from extant species. The evolutionary relationships of the Australian fossil coucals are uncertain, but our phylogenetic analysis indicates a possible close relationship between one of the Nullarbor species and extant Centropus violaceus from the Bismarck Archipelago. The presence of three coucals in southern Australia markedly extends the geographical range of the genus from tropical Australia into southern temperate regions. This demonstrates the remarkable and consistent ability of coucals to colonize continents despite their very limited flying ability.     

Models projecting the fate of fish populations under climate change need to be based on valid physiological mechanisms

Some recent modelling papers projecting smaller fish sizes and catches in a warmer future are based on erroneous assumptions regarding (i) the scaling of gills with body mass and (ii) the energetic cost of ‘maintenance’. Assumption (i) posits that insurmountable geometric constraints prevent respiratory surface areas from growing as fast as body volume. It is argued that these constraints explain allometric scaling of energy metabolism, whereby larger fishes have relatively lower mass‐specific metabolic rates. Assumption (ii) concludes that when fishes reach a certain size, basal oxygen demands will not be met, because of assumption (i). We here demonstrate unequivocally, by applying accepted physiological principles with reference to the existing literature, that these assumptions are not valid. Gills are folded surfaces, where the scaling of surface area to volume is not constrained by spherical geometry. The gill surface area can, in fact, increase linearly in proportion to gill volume and body mass. We cite the large body of evidence demonstrating that respiratory surface areas in fishes reflect metabolic needs, not vice versa, which explains the large interspecific variation in scaling of gill surface areas. Finally, we point out that future studies basing their predictions on models should incorporate factors for scaling of metabolic rate and for temperature effects on metabolism, which agree with measured values, and should account for interspecific variation in scaling and temperature effects. It is possible that some fishes will become smaller in the future, but to make reliable predictions the underlying mechanisms need to be identified and sought elsewhere than in geometric constraints on gill surface area. Furthermore, to ensure that useful information is conveyed to the public and policymakers about the possible effects of climate change, it is necessary to improve communication and congruity between fish physiologists and fisheries scientists.     

Parallel evolutionary trajectories underlie the origin of giant suspension-feeding whales and bony fishes

Giant suspension feeders such as mysticete whales, basking and whale sharks, and the extinct (indicated by ‘†’) †pachycormiform teleosts are conspicuous members of modern and fossil marine vertebrate faunas. Whether convergent anatomical features common to these clades arose along similar evolutionary pathways has remained unclear because of a lack of information surrounding the origins of all groups of large-bodied suspension feeders apart from baleen whales. New investigation reveals that the enigmatic ray-finned fish †Ohmdenia, from the Lower Jurassic (Toarcian, 183.0–175.6 Ma) Posidonia Shale Lagerstätte, represents the immediate sister group of edentulous †pachycormiforms, the longest lived radiation of large vertebrate suspension feeders. †Ohmdenia bisects the long morphological branch leading to suspension-feeding †pachycormiforms, providing information on the sequence of anatomical transformations preceding this major ecological shift that can be compared to changes associated with the origin of modern mysticetes. Similarities include initial modifications to jaw geometry associated with the reduction of dentition, followed by the loss of teeth. The evolution of largest body sizes within both radiations occurs only after the apparent onset of microphagy. Comparing the fit of contrasting evolutionary models to functionally relevant morphological measurements for whales and †pachycormiform fishes reveals strong support for a common adaptive peak shared by suspension-feeding members of both clades.     

Evolutionary pathways toward gigantism in sharks and rays

Through elasmobranch (sharks and rays) evolutionary history, gigantism evolved multiple times in phylogenetically distant species, some of which are now extinct. Interestingly, the world's largest elasmobranchs display two specializations found never to overlap: filter feeding and mesothermy. The contrasting lifestyles of elasmobranch giants provide an ideal case study to elucidate the evolutionary pathways leading to gigantism in the oceans. Here, we applied a phylogenetic approach to a global dataset of 459 taxa to study the evolution of elasmobranch gigantism. We found that filter feeders and mesotherms deviate from general relationships between trophic level and body size, and exhibit significantly larger sizes than ectothermic‐macropredators. We confirm that filter feeding arose multiple times during the Paleogene, and suggest the possibility of a single origin of mesothermy in the Cretaceous. Together, our results elucidate two main evolutionary pathways that enable gigantism: mesothermic and filter feeding. These pathways were followed by ancestrally large clades and facilitated extreme sizes through specializations for enhancing prey intake. Although a negligible percentage of ectothermic‐macropredators reach gigantic sizes, these species lack such specializations and are correspondingly constrained to the lower limits of gigantism. Importantly, the very adaptive strategies that enabled the evolution of the largest sharks can also confer high extinction susceptibility.     

Amphibious adaptations in a newly recognized amphibious fish: Terrestrial locomotion and the influences of body size and temperature

Amphibious animals are adapted for both aquatic and terrestrial habitats. The conflicting requirements for dual habitats are perhaps most pronounced in the air‐breathing fishes, which represent an intermediate stage between the totally aquatic habitat and terrestrial colonization. A key requirement for amphibious fishes is terrestrial locomotion. The different densities and compositions of air and water impose constraints for efficient terrestrial locomotion that differ from those required for aquatic locomotion. I investigated terrestrial locomotion in a small South African fish, Galaxias ‘nebula’, by exposing 60 individual fish to air in specially designed raceways and quantifying movement type and occurrence as a function of availability of water, fish size and environmental temperature. Nebula showed a sustained undulating form of terrestrial locomotion characteristic of amphibious fishes and also a transient ballistic locomotion (jumps) typical of fully aquatic species. Terrestrial movement was influenced by fish size, with medium‐sized fish undertaking more jumps towards water, and fewer jumps away from water, than their smaller or larger conspecifics. In contrast, axial undulation was mainly influenced by temperature. However, there was no consistent pattern in temperature effects presumably because temperature is just one of a suit of environmental factors that may affect terrestrial locomotion. Nebula's amphibious adaptations allow it to cope with the unpredictability inherent in its natural environment.     

Convergence in reduced body size,head size,and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behavior, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis, we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from 6 years of study, we found that relative to mainland population the island populations had smaller body size (i.e., island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, although with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.     

Air‐filled postcranial bones in theropod dinosaurs: physiological implications and the ‘reptile’–bird transition

Pneumatic (air‐filled) postcranial bones are unique to birds among extant tetrapods. Unambiguous skeletal correlates of postcranial pneumaticity first appeared in the Late Triassic (approximately 210 million years ago), when they evolved independently in several groups of bird‐line archosaurs (ornithodirans). These include the theropod dinosaurs (of which birds are extant representatives), the pterosaurs, and sauropodomorph dinosaurs. Postulated functions of skeletal pneumatisation include weight reduction in large‐bodied or flying taxa, and density reduction resulting in energetic savings during foraging and locomotion. However, the influence of these hypotheses on the early evolution of pneumaticity has not been studied in detail previously. We review recent work on the significance of pneumaticity for understanding the biology of extinct ornithodirans, and present detailed new data on the proportion of the skeleton that was pneumatised in 131 non‐avian theropods and Archaeopteryx. This includes all taxa known from significant postcranial remains. Pneumaticity of the cervical and anterior dorsal vertebrae occurred early in theropod evolution. This ‘common pattern’ was conserved on the line leading to birds, and is likely present in Archaeopteryx. Increases in skeletal pneumaticity occurred independently in as many as 12 lineages, highlighting a remarkably high number of parallel acquisitions of a bird‐like feature among non‐avian theropods. Using a quantitative comparative framework, we show that evolutionary increases in skeletal pneumaticity are significantly concentrated in lineages with large body size, suggesting that mass reduction in response to gravitational constraints at large body sizes influenced the early evolution of pneumaticity. However, the body size threshold for extensive pneumatisation is lower in theropod lineages more closely related to birds (maniraptorans). Thus, relaxation of the relationship between body size and pneumatisation preceded the origin of birds and cannot be explained as an adaptation for flight. We hypothesise that skeletal density modulation in small, non‐volant, maniraptorans resulted in energetic savings as part of a multi‐system response to increased metabolic demands. Acquisition of extensive postcranial pneumaticity in small‐bodied maniraptorans may indicate avian‐like high‐performance endothermy.     

Directional selection in the evolution of elongated upper canines in clouded leopards and sabre‐toothed cats

Extremely developed or specialized traits such as the elongated upper canines of extinct sabre‐toothed cats are often not analogous to those of any extant species, which limits our understanding of their evolutionary cause. However, an extant species may have undergone directional selection for a similar extreme phenotype. Among living felids, the clouded leopard, Neofelis nebulosa, has exceptionally long upper canines for its body size. We hypothesized that directional selection generated the elongated upper canines of clouded leopards in a manner similar to the process in extinct sabre‐toothed cats. To test this, we developed an approach that compared the effect of directional selection among lineages in a phylogeny using a simulation of trait evolution and approximate Bayesian computation. This approach was applied to analyse the evolution of upper canine length in the Felidae phylogeny. Our analyses consistently showed directional selection favouring longer upper canines in the clouded leopard lineage and a lineage leading to the sabre‐toothed cat with the longest upper canines, Smilodon. Most of our analyses detected an effect of directional selection for longer upper canines in the lineage leading to another sabre‐toothed cat, Homotherium, although this selection may have occurred exclusively in the primitive species. In all the analyses, the clouded leopard and Smilodon lineages showed comparable directional selection. This implies that clouded leopards share a selection advantage with sabre‐toothed cats in having elongated upper canines.     

Maternal investment increases with altitude in a frog on the Tibetan Plateau

Reproducing females can allocate energy between the production of eggs or offspring of different size or number, both of which can strongly influence fitness. The physical capacity to store developing offspring imposes constraints on maximum clutch volume, but individual females and populations can trade off whether more or fewer eggs or offspring are produced, and their relative sizes. Harsh environments are likely to select for larger egg or offspring size, and many vertebrate populations compensate for this reproductive investment through an increase in female body size. We report a different trade‐off in a frog endemic to the Tibetan Plateau, Rana kukunoris. Females living at higher altitudes (n = 11 populations, 2000–3500 m) produce larger eggs, but without a concomitant increase in female body size or clutch size. The reduced diel and seasonal activity at high altitudes may impose constraints on the maximum body size of adult frogs, by limiting the opportunity for energy accumulation. Simultaneously, producing larger eggs likely helps to increase the rate of embryonic development, causing tadpoles to hatch earlier. The gelatinous matrix surrounding eggs, more of which is produced by large females, may help buffer developing embryos from temperature fluctuations or offer protection from ultraviolet radiation. High‐altitude frogs on the Tibetan Plateau employ a reproductive strategy that favours large egg size independent of body size, which is unusual in amphibians. The harsh and unpredictable environmental conditions at high altitudes can thus impose strong and opposing selection pressures on adult and embryonic life stages, both of which can simultaneously influence fitness.     

Terrestrial capture of prey by the reedfish,a model species for stem tetrapods

Due to morphological resemblance, polypterid fishes are used as extant analogues of Late Devonian lobe‐finned sarcopterygians to identify the features that allowed the evolution of a terrestrial lifestyle in early tetrapods. Previous studies using polypterids showed how terrestrial locomotion capacity can develop, and how air ventilation for breathing was possible in extinct tetrapodomorphs. Interestingly, one polypterid species, the reedfish Erpetoichthys calabaricus, has been noted being capable of capturing prey on land. We now identified the mechanism of terrestrial prey‐capture in reedfish. We showed that this species uses a lifted trunk and downward inclined head to capture ground‐based prey, remarkably similar to the mechanism described earlier for eel‐catfish. Reedfish similarly use the ground support and flexibility of their elongated body to realize the trunk elevation and dorsoventral flexion of the anterior trunk region, without a role for the pectoral fins. However, curving of the body to lift the trunk may not have been an option for the Devonian tetrapodomorphs as they are significantly less elongated than reedfish and eel‐catfish. This would imply that, in contrast to the eel‐like extant species, evolution of the capacity to capture prey on land in early tetrapods may be linked to the evolution of the pectoral system to lift the anterior part of the body.     

Slaying dragons: limited evidence for unusual body size evolution on islands

Aim Island taxa often attain forms outside the range achieved by mainland relatives. Body size evolution of vertebrates on islands has therefore received much attention, with two seemingly conflicting patterns thought to prevail: (1) islands harbour animals of extreme size, and (2) islands promote evolution towards medium body size (‘the island rule’). We test both hypotheses using body size distributions of mammal, lizard and bird species. Location World‐wide. Methods We assembled body size and insularity datasets for the world’s lizards, birds and mammals. We compared the frequencies with which the largest or smallest member of a group is insular with the frequencies expected if insularity is randomly assigned within groups. We tested whether size extremes on islands considered across mammalian phylogeny depart from a null expectation under a Brownian motion model. We tested the island rule by comparing insular and mainland members of (1) a taxonomic level and (2) mammalian sister species, to determine if large insular animals tend to evolve smaller body sizes while small ones evolve larger sizes. Results The smallest species in a taxon (order, family or genus) are insular no more often than would be expected by chance in all groups. The largest species within lizard families and bird genera (but no other taxonomic levels) are insular more often than expected. The incidence of extreme sizes in insular mammals never departs from the null, except among extant genera, where gigantism is marginally less common than expected under a Brownian motion null. Mammals follow the island rule at the genus level and when comparing sister species and clades. This appears to be driven mainly by insular dwarfing in large‐bodied lineages. A similar pattern in birds is apparent for species within orders. However, lizards follow the converse pattern. Main conclusions The popular misconception that islands have more than their fair share of size extremes may stem from a greater tendency to notice gigantism and dwarfism when they occur on islands. There is compelling evidence for insular dwarfing in large mammals, but not in other taxa, and little evidence for the second component of the island rule – gigantism in small‐bodied taxa.     

Detection of Contracaecum multipapillatum (Nematoda: Anisakidae) in the indigenous killifish Aphanius hormuzensis (Teleostei; Aphaniidae) and its histopathological effects: A review of Iranian Aphanius species parasites

The present study reports the occurrence of Contracaecum multipapillatum (Nematoda: Anisakidae) in an indigenous small killifish, Aphanius hormuzensis Teimori, Esmaeili, Hamidan, Reichenbacher, 2018 from Southern Iran and shows its histopathology. A total of 110 A. hormuzensis specimens were collected from Shur (Naband) River, Hormuzgan basin in Southern Iran and examined for their possible parasitic infections. Third‐stage larva of C. multipapillatum was extracted for the first time from the body cavity of 19 fish specimens (one male and 18 female) and identified by molecular and morphological methods. In comparison with non‐infected fishes, the melanomacrophage centers were detected in the tissue sections from liver, kidney and spleen of all the parasite infected fishes. To date, 16 parasites belong to nine families have been recorded from six Aphanius species (out of 15 known species) in Iran. Among them, eight and four parasites have been identified from A. vladykovi, and A. hormuzensis respectively. Since Aphanius species are living in different environments, therefore, they seem to be good hosts for the different types of parasites, and more new parasites are expected to be found in these fishes.     

Locomotion in Extinct Giant Kangaroos: Were Sthenurines Hop-Less Monsters?

Sthenurine kangaroos (Marsupialia, Diprotodontia, Macropodoidea) were an extinct subfamily within the family Macropodidae (kangaroos and rat-kangaroos). These “short-faced browsers” first appeared in the middle Miocene, and radiated in the Plio-Pleistocene into a diversity of mostly large-bodied forms, more robust than extant forms in their build. The largest (Procoptodon goliah) had an estimated body mass of 240 kg, almost three times the size of the largest living kangaroos, and there is speculation whether a kangaroo of this size would be biomechanically capable of hopping locomotion. Previously described aspects of sthenurine anatomy (specialized forelimbs, rigid lumbar spine) would limit their ability to perform the characteristic kangaroo pentapedal walking (using the tail as a fifth limb), an essential gait at slower speeds as slow hopping is energetically unfeasible. Analysis of limb bone measurements of sthenurines in comparison with extant macropodoids shows a number of anatomical differences, especially in the large species. The scaling of long bone robusticity indicates that sthenurines are following the “normal” allometric trend for macropodoids, while the large extant kangaroos are relatively gracile. Other morphological differences are indicative of adaptations for a novel type of locomotor behavior in sthenurines: they lacked many specialized features for rapid hopping, and they also had anatomy indicative of supporting their body with an upright trunk (e.g., dorsally tipped ischiae), and of supporting their weight on one leg at a time (e.g., larger hips and knees, stabilized ankle joint). We propose that sthenurines adopted a bipedal striding gait (a gait occasionally observed in extant tree-kangaroos): in the smaller and earlier forms, this gait may have been employed as an alternative to pentapedal locomotion at slower speeds, while in the larger Pleistocene forms this gait may have enabled them to evolve to body sizes where hopping was no longer a feasible form of more rapid locomotion.     

Endoskeletal structure in Cheirolepis (Osteichthyes,Actinopterygii), An early ray‐finned fish

As the sister lineage of all other actinopterygians, the Middle to Late Devonian (Eifelian–Frasnian) Cheirolepis occupies a pivotal position in vertebrate phylogeny. Although the dermal skeleton of this taxon has been exhaustively described, very little of its endoskeleton is known, leaving questions of neurocranial and fin evolution in early ray‐finned fishes unresolved. The model for early actinopterygian anatomy has instead been based largely on the Late Devonian (Frasnian) Mimipiscis, preserved in stunning detail from the Gogo Formation of Australia. Here, we present re‐examinations of existing museum specimens through the use of high‐resolution laboratory‐ and synchrotron‐based computed tomography scanning, revealing new details of the neuro‐cranium, hyomandibula and pectoral fin endoskeleton for the Eifelian Cheirolepis trailli. These new data highlight traits considered uncharacteristic of early actinopterygians, including an uninvested dorsal aorta and imperforate propterygium, and corroborate the early divergence of Cheirolepis within actinopterygian phylogeny. These traits represent conspicuous differences between the endoskeletal structure of Cheirolepis and Mimipiscis. Additionally, we describe new aspects of the parasphenoid, vomer and scales, most notably that the scales display peg‐and‐socket articulation and a distinct neck. Collectively, these new data help clarify primitive conditions within ray‐finned fishes, which in turn have important implications for understanding features likely present in the last common ancestor of living osteichthyans.     

Fossil lizard from central Europe resolves the origin of large body size and herbivory in giant Canary Island lacertids

The endemic Canary Island lizard clade Gallotia, which includes the largest members of Europe's dominant reptile group, Lacertidae, is one of the classic examples of insular gigantism. For the first time we use fossil data to test the evolutionary reasons for the association between gigantism and herbivory. We describe an almost completely preserved skeleton of J anosikia ulmensis comb. nov. from the early Miocene of Ulm, Germany (MN 2a, ～ 22 Mya). We show that this species and Oligocene Pseudeumeces cadurcensis (Filhol, 1877) are in fact crown lacertids, and the first known pre‐Quaternary record of the total clade of Gallotia. Pseudeumeces confirms the early origin of crown Lacertidae in the Palaeogene of Europe. More importantly, these fossil taxa show that large body size was already achieved on the European mainland by the early Miocene. Furthermore, Pseudeumeces and Janosikia were faunivorous, thus demonstrating that insularity, not large body size, was crucial to the evolution of herbivory in this lineage. Body size change in Gallotia was more complex than previously thought, encompassing size increase [e.g. in the extinct Gallotia goliath (Mertens, 1942)], but more commonly involving miniaturization. The physical environment may play a crucial role in modulating the evolution of body size in this natural laboratory.     

Biology of the sauropod dinosaurs: the evolution of gigantism

The herbivorous sauropod dinosaurs of the Jurassic and Cretaceous periods were the largest terrestrial animals ever, surpassing the largest herbivorous mammals by an order of magnitude in body mass. Several evolutionary lineages among Sauropoda produced giants with body masses in excess of 50 metric tonnes by conservative estimates. With body mass increase driven by the selective advantages of large body size, animal lineages will increase in body size until they reach the limit determined by the interplay of bauplan, biology, and resource availability. There is no evidence, however, that resource availability and global physicochemical parameters were different enough in the Mesozoic to have led to sauropod gigantism. We review the biology of sauropod dinosaurs in detail and posit that sauropod gigantism was made possible by a specific combination of plesiomorphic characters (phylogenetic heritage) and evolutionary innovations at different levels which triggered a remarkable evolutionary cascade. Of these key innovations, the most important probably was the very long neck, the most conspicuous feature of the sauropod bauplan. Compared to other herbivores, the long neck allowed more efficient food uptake than in other large herbivores by covering a much larger feeding envelope and making food accessible that was out of the reach of other herbivores. Sauropods thus must have been able to take up more energy from their environment than other herbivores. The long neck, in turn, could only evolve because of the small head and the extensive pneumatization of the sauropod axial skeleton, lightening the neck. The small head was possible because food was ingested without mastication. Both mastication and a gastric mill would have limited food uptake rate. Scaling relationships between gastrointestinal tract size and basal metabolic rate (BMR) suggest that sauropods compensated for the lack of particle reduction with long retention times, even at high uptake rates. The extensive pneumatization of the axial skeleton resulted from the evolution of an avian‐style respiratory system, presumably at the base of Saurischia. An avian‐style respiratory system would also have lowered the cost of breathing, reduced specific gravity, and may have been important in removing excess body heat. Another crucial innovation inherited from basal dinosaurs was a high BMR. This is required for fueling the high growth rate necessary for a multi‐tonne animal to survive to reproductive maturity. The retention of the plesiomorphic oviparous mode of reproduction appears to have been critical as well, allowing much faster population recovery than in megaherbivore mammals. Sauropods produced numerous but small offspring each season while land mammals show a negative correlation of reproductive output to body size. This permitted lower population densities in sauropods than in megaherbivore mammals but larger individuals. Our work on sauropod dinosaurs thus informs us about evolutionary limits to body size in other groups of herbivorous terrestrial tetrapods. Ectothermic reptiles are strongly limited by their low BMR, remaining small. Mammals are limited by their extensive mastication and their vivipary, while ornithsichian dinosaurs were only limited by their extensive mastication, having greater average body sizes than mammals.     

EVOLUTION OF GIGANTISM IN NINE‐SPINED STICKLEBACKS

The relaxation of predation and interspecific competition are hypothesized to allow evolution toward “optimal” body size in island environments, resulting in the gigantism of small organisms. We tested this hypothesis by studying a small teleost (nine‐spined stickleback, Pungitius pungitius) from four marine and five lake (diverse fish community) and nine pond (impoverished fish community) populations. In line with theory, pond fish tended to be larger than their marine or lake conspecifics, sometimes reaching giant sizes. In two geographically independent cases when predatory fish had been introduced into ponds, fish were smaller than those in nearby ponds lacking predators. Pond fish were also smaller when found in sympatry with three‐spined stickleback (Gasterosteus aculeatus) than those in ponds lacking competitors. Size‐at‐age analyses demonstrated that larger size in ponds was achieved by both increased growth rates and extended longevity of pond fish. Results from a common garden experiment indicate that the growth differences had a genetic basis: pond fish developed two to three times higher body mass than marine fish during 36 weeks of growth under similar conditions. Hence, reduced risk of predation and interspecific competition appear to be chief forces driving insular body size evolution toward gigantism.     

Observations of terrestrial locomotion in wild Polypterus senegalus from Lake Albert,Uganda

Polypterids, the most basal actinopterygians, are a group of fish long-considered living fossils and holding a key position for understanding fish and tetrapod evolution. Knowledge of the natural history of Polypterus is limited, their having been studied in little detail since the early 1900s. The locomotory habits of wild Polypterus senegalus from Lake Albert, Uganda, were investigated in 2014. High-speed videography demonstrated the capability of large Polypterus to move overland successfully. Contrary to previous evidence, field observations found that terrestrial locomotion in Polypterus is not inherently restricted by body size. Evidence that Polypterus exhibit this behaviour as part of their natural life history can be found in the existence of environmental challenges and the presence of adaptations for amphibious life.