We describe seven associated skeletal remains of Ischyrhiza mira, a Late Cretaceous sclerorhynchid sawfish, from the Campanian‒lower Maastrichtian of Tennessee and Alabama, U.S.A., to decipher its paleobiology. Ischyrhiza mira had about 16 or 17 functional spines and about the same number of replacement spines on each side of the rostrum in which tall erect spines occupied the anterior one-half to two-third of the rostrum followed posteriorly by smaller spines. Whereas small hat-shaped dermal denticles were distributed on the rostrum, large thorn-like dermal denticles were present on the dorsal side of the body characteristic of sluggish, benthic batoids. We concur with the interpretation that specimens previously identified as rostral spines of Peyeria are actually enlarged thorn-like dermal denticles of a sclerorhynchid. We suggest that the ratio between the rostrum length and total body length of sclerorhynchids was generally about 1:3.27. Our vertebra-based ontogenetic analysis of I. mira gives an age estimate of 12.4 years for a 190-cm-long individual, the size at birth of about 0.5 m, and the maximum possible length for the species of no more than 3 m. Compared to extant pristid sawfishes, I. mira probably became sexually mature much earlier with a slightly faster rate of rostrum development.

The emergence and subsequent evolution of pectoral fins is a key point in vertebrate evolution, as pectoral fins are dominant control surfaces for locomotion in extant fishes.^1,2,3 However, major gaps remain in our understanding of the diversity and evolution of pectoral fins among cartilaginous fishes (Chondrichthyes), a group with an evolutionary history spanning over 400 million years with current selachians (modern sharks) appearing about 200 million years ago.^4,5,6 Modern sharks are a charismatic group of vertebrates often thought to be predators roaming the open ocean and coastal areas, but most extant species occupy the seafloor.⁴ Here we use an integrative approach to understand what facilitated the expansion to the pelagic realm and what morphological changes accompanied this shift. On the basis of comparative analyses in the framework of a time-calibrated molecular phylogeny,⁷ we show that modern sharks expanded to the pelagic realm no later than the Early Cretaceous (Barremian). The pattern of pectoral fin aspect ratios across selachians is congruent with adaptive evolution, and we identify an increase of the subclade disparity of aspect ratio at a time when sea surface temperatures were at their highest.⁸ The expansion to open ocean habitats likely involved extended bouts of sustained fast swimming, which led to the selection for efficient movement via higher aspect ratio pectoral fins. Swimming performance was likely enhanced in pelagic sharks during this time due to the elevated temperatures in the sea, highlighting that shark evolution has been greatly impacted by climate change.

The megatooth shark, Otodus megalodon, is an iconic Neogene lamniform shark known only from its teeth and vertebrae. Its thermophysiology is previously inferred to have been regionally endothermic, like the extant lamnids that are active predatory lamniforms. By considering the entire Lamnidae as the ecological and physiological analogue to O. megalodon, a recent study proposed inferred body dimensions of O. megalodon based on morphometric analyses on body forms of extant lamnids. Here, we reassessed the recent study by testing whether a two-dimensional approach used in the study can actually tease out the difference between ectothermic and endothermic fishes in the first place. Morphometric comparisons of the whole body and different body parts (e.g. head, different fins, and precaudal body with and without fins) among the 15 extant species of Lamniformes were conducted using principal component analyses and simple clustering methods. Our study strongly indicates that, two-dimensionally, there is no relationship between thermophysiology and body form in lamniforms. The reality is that there are presently no scientific means to support or refute the accuracy of any of the previously published body forms of O. megalodon. We also clarify that regional endothermy likely evolved in multiple clades independently through lamniform phylogeny.

While sexual size dimorphism (SSD) is abundant in nature, there is huge variation in both the intensity and direction of SSD. SSD results from a combination of sexual selection for large male size, fecundity selection for large female size and ecological selection for either. In most vertebrates, it is variation in the intensity of male–male competition that primarily underlies variation in SSD. In this study, we test four hypotheses regarding the adaptive value of SSD in sharks—considering the potential for each of fecundity, sexual, ecological selection and reproductive mode as the primary driver of variation in SSD between species. We also estimate past macroevolutionary shifts in SSD direction/intensity through shark phylogeny. We were unable to find evidence of significant SSD in early sharks and hypothesise that SSD is a derived state in this clade, that has evolved independently of SSD observed in other vertebrates. Moreover, there is no significant relationship between SSD and fecundity, testes mass or oceanic depth in sharks. However, there is evidence to support previous speculation that reproductive mode is an important determinant of interspecific variation in SSD in sharks. This is significant as in most vertebrates sexual selection is thought to be the primary driver of SSD trends, with evidence for the role of fecundity selection in other clades being inconsistent at best. While the phylogenetic distribution of SSD among sharks is superficially similar to that observed in other vertebrate clades, the relative importance of selective pressures underlying its evolution appears to differ.

Major shifts in habitat often occur during life history and can have significant impacts on the morphology and function of an animal; however, little is known about how such ecological changes influence the locomotor system of large aquatic vertebrates. Scalloped hammerheads (Sphyrna lewini) are large sharks found in warm temperate and tropical waters. Smaller scalloped hammerheads are generally found in near-shore habitats, but as they grow larger, individuals spend time in deep-water, pelagic habitats. We measured a number of morphological traits of scalloped hammerheads, ranging from 32 to 130 cm, to determine whether there are allometric changes in morphology in association with this shift in habitat. We found that head morphology, caudal fin area and lateral span scaled with negative allometry, whereas the lengths of their pectoral, dorsal and caudal fins, and their pectoral and caudal fin aspect ratios, scaled with positive allometry. Furthermore, the largest shark in our dataset exhibited an optimal body fineness ratio for locomotor efficiency. This suggests that the changes in ecology have profound influences on the functional morphology of scalloped hammerheads. We discuss how these drastic morphological changes relate to potential changes in scalloped hammerhead swimming function and performance.