The anatomy of the upper cretaceous snake Najash rionegrina Apesteguía & Zaher, 2006, and the evolution of limblessness in snakes

Najash rionegrina Apesteguía & Zaher, 2006 , a terrestrial fossil snake from the Upper Cretaceous of Argentina, represents the first known snake with a sacrum associated with robust, well‐developed hind limbs. Najash rionegrina documents an important gap in the evolutionary development towards limblessness, because its phylogenetic affinities suggest that it is the sister group of all modern snakes, including the limbed Tethyan snakes Pachyrhachis, Haasiophis, and Eupodophis. The latter three limbed marine fossil snakes are shown to be more derived morphologically, because they lack a sacrum, but have articulated lymphapophyses, and their appendicular skeleton is enclosed by the rib cage, as in modern snakes.     

Skeletal heterochrony is associated with the anatomical specializations of snakes among squamate reptiles

Snakes possess a derived anatomy, characterized by limb reduction and reorganization of the skull and internal organs. To understand the origin of snakes from an ontogenetic point of view, we conducted comprehensive investigations on the timing of skeletal elements, based on published and new data, and reconstructed the evolution of the ossification sequence among squamates. We included for the first time Varanus, a critical taxon in phylogenetic context. There is comprehensive delay in the onset of ossification of most skeletal elements in snakes when compared to reference developmental events through evolution. We hypothesize that progressing deceleration accompanied limb reduction and reorganization of the snake skull. Molecular and morphological studies have suggested close relationship of snakes to either amphisbaenians, scincids, geckos, iguanids, or varanids. Likewise, alternative hypotheses on habitat for stem snakes have been postulated. Our comprehensive heterochrony analyses detected developmental shifts in ossification for each hypothesis of snake origin. Moreover, we show that reconstruction of ancestral developmental sequences is a valuable tool to understand ontogenetic mechanisms associated with major evolutionary changes and test homology hypotheses. The “supratemporal” of snakes could be homolog to squamosal of other squamates, which starts ossification early to become relatively large in snakes.     

Gravity and the evolution of cardiopulmonary morphology in snakes

Physiological investigations of snakes have established the importance of heart position and pulmonary structure in contexts of gravity effects on blood circulation. Here we investigate morphological correlates of cardiopulmonary physiology in contexts related to ecology, behavior and evolution. We analyze data for heart position and length of vascular lung in 154 species of snakes that exhibit a broad range of characteristic behaviors and habitat associations. We construct a composite phylogeny for these species, and we codify gravitational stress according to species habitat and behavior. We use conventional regression and phylogenetically independent contrasts to evaluate whether trait diversity is correlated with gravitational habitat related to evolutionary transitions within the composite tree topology. We demonstrate that snake species living in arboreal habitats, or which express strongly climbing behaviors, possess relatively short blood columns between the heart and the head, as well as relatively short vascular lungs, compared to terrestrial species. Aquatic species, which experience little or no gravity stress in water, show the reverse — significantly longer heart-head distance and longer vascular lungs. These phylogenetic differences complement the results of physiological studies and are reflected in multiple habitat transitions during the evolutionary histories of these snake lineages, providing strong evidence that heart-to-head distance and length of vascular lung are co-adaptive cardiopulmonary features of snakes.     

The comparative chemical morphology of the mammalian cornea

A simple model of mammalian corneal stroma has been tested against biochemical and ultrastructural measurements performed on a number of species. Contents of water, collagen and total sulphated polyanion were constant, as predicted from the model. Alcian blue CEC results showed great variability between species, with a rise in CEC as corneal size and thickness increased. These variations were due to changes in keratan sulphate content, and particularly to its oversulphated terminal domain, which is absent from mouse cornea. The increase in keratan sulphate content with corneal thickness was balanced by an increase in dermatan sulphate in thin corneas, thus maintaining total sulphated GAG levels at a constant "average", in all the mammals investigated. This balance is probably decided by oxygen tension, which will vary with corneal thickness.     

The comparative morphology of pit organs in elasmobranchs

The pit organs of elasmobranchs (sharks, skates and rays) are free neuromasts of the mechanosensory lateral line system. Pit organs, however, appear to have some structural differences from the free neuromasts of bony fishes and amphibians. In this study, the morphology of pit organs was investigated by scanning electron microscopy in six shark and three ray species. In each species, pit organs contained typical lateral line hair cells with apical stereovilli of different lengths arranged in an “organ‐pipe” configuration. Supporting cells also bore numerous apical microvilli taller than those observed in other vertebrate lateral line organs. Pit organs were either covered by overlapping denticles, located in open grooves bordered by denticles, or in grooves without associated denticles. The possible functional implications of these morphological features, including modification of water flow and sensory filtering properties, are discussed. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Behavioural correlates with hemipenis morphology in New World natricine snakes

Copulatory organs (hemipenes) of male snakes vary markedly among species in shape and ornamentation. We suggest that sexual conflict over copulation duration may have shaped the evolution of hemipenis morphology, favouring more elaborate organs in species in which a long duration of copulation is especially beneficial to males, despite the associated costs to females. To test this proposition, we compare mating behaviour between two species of gartersnakes differing in hemipenis morphology. In addition, we review data on copulation duration and hemipenis morphology and relate hemipenis morphology to phylogeny among of New World natricines. As predicted, copulation duration was significantly shorter in the common gartersnake ( Thamnophis sirtalis ), a species with simple subcylindrical hemipenes, than in the plains gartersnake ( Thamnophis radix ), a species with more complex, bilobed organs. Furthermore, female T. radix frequently exhibited vigorous body rolls during copulation, a behaviour associated with copulation termination, whereas female T. sirtalis never exhibited this behaviour. Copulations were of shorter duration when female T. radix (but not T. sirtalis ) more greatly exceeded males in body size, suggesting that females can more easily disengage from small males. Our review of New World natricines provides only weak evidence for an association between copulation duration and hemipenis morphology. Our mapping of hemipenis morphology onto the New World natricine phylogeny suggests that hemipenis morphology is evolutionarily plastic; both simple and bilobed hemipenes occur in all three major natricine clades, as well as in two of three gartersnake subclades and several sister-species pairs.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 110–120.     

Semen collection and evaluation in free‐ranging Brazilian rattlesnakes (Crotalus durissus terrificus)

Two hundred‐ninety species of reptiles are estimated to need urgent action for conservation, with at least 113 threatened species worldwide. The International Union for Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species includes 80 species of snakes, with six native Brazilian species, a number likely to be an underestimation. Some authors believe that assisted reproduction would be an important tool to improve reproduction in captivity of some reptiles. An efficient technique for semen collection and evaluation is an important step in development of protocols for cryopreservation of semen or artificial insemination in snakes, contributing to the conservation of endangered species. Although these techniques are important, some basic semen parameters are described for four of the ～2,900 snake species in the world. The Brazilian rattlesnake (Crotalus durissus terrificus) was chosen as a model for semen collection in snakes because it is found quite often in Sao Paulo State. Semen was collected once from each animal by the same investigator during the mating season of this species in Brazil. After antiseptic cleansing of the skin around the cloaca, the snakes were injected subcutaneously with a dose of 15 mg/kg of 1% solution of lidocaine around the cloaca. Semen was collected with ventral massages after cloacal relaxation and directly from genital papilla inside the cloaca. A total of 28 ejaculates from 39 animals were obtained, representing collection efficiency of 71.80%. Semen volume and concentration in Brazilian rattlesnakes ranged from 3–70 µl and from 0.94–2.23 × 10⁹ spermatozoa/ml, respectively. Zoo Biol 0:1–6, 2007. © 2007 Wiley‐Liss, Inc.     

The morphology of the suboccipital region in snakes,and the anatomical and functional diversity of the myodural bridge

The myodural bridge, that is, skeletal muscle fibers attaching to the cervical dura mater, has been described from a variety of mammals and other amniotes. To test an earlier assumption about the presence of the myodural bridge in snakes, a comparative study was designed using a group of Colubrine snakes. Serial histological sections revealed no evidence of the myodural bridge in any of the snakes examined. Further analyses, including histology, computed tomography (CT), and micro-CT imaging of other distantly related snakes, also turned up no evidence of a myodural bridge. The close apposition of adjacent neural arches in snakes may preclude muscle tendons from passing through the intervertebral joint to reach the spinal dura. It is hypothesized that the myodural bridge functions in the clearance of the cerebrospinal fluid (CSF) by creating episodic CSF pressure pulsations, and that snakes are capable of creating equivalent CSF pressure pulsations through vertebral displacement.     

Convergence in trophic morphology and feeding performance among piscivorous natricine snakes

Piscivory has independently evolved numerous times amongst snakes, and therefore these animals provide a powerful opportunity to test for convergent evolution in a vertebrate feeding system. In this study, we integrate performance trials with comparative methods to test the hypothesis that piscivory drives convergence in trophic morphology and feeding performance among natricine snakes. Within and across species, increasing the relative length of the quadrate bone in the skull is positively and strongly linked to a reduction in the time needed to swallow large fish prey. Thus, our feeding experiments suggest that a longer quadrate bone enables snakes to better conform their head shape to the shape of the prey during swallowing. Ancestral diet reconstructions and phylogenetically corrected multiple regression analyses further reveal that evolutionary increases in piscivory are coupled to the evolution of relatively longer quadrates, and hence improved feeding performance on fish prey in these animals. The exploitation of similar dietary niches drives the evolution of convergent trophic morphologies and feeding performances in natricine snakes.     

Higher-level relationships of snakes inferred from four nuclear and mitochondrial genes

Higher-level snake relationships are inferred from sequence analyses of one nuclear gene (C-mos) and three mitochondrial genes (12S rRNA, 16S rRNA and cytochrome b). Extant snakes belong to two lineages: the fossorial Scolecophidia, which feed on small prey on a frequent basis, and the ecologically diverse Alethinophidia ('typical' snakes), which feed on large prey on an infrequent basis. The vast majority of Alethinophidia, if not all of them, belong to two clades, corresponding to two distinct prey neutralization modes: unimodal constriction for the Henophidia (locomotor and feeding systems coupled) and injection of toxic saliva, in addition (or not) to diverse alternate modes of constriction, for the Caenophidia (locomotor and feeding systems uncoupled). Within Alethinophidia, non-macrostomatan (small gape) Aniliidae (genus Anilius) and macrostomatan (large gape) Tropidophiidae (genera Trachyboa and Tropidophis), both from the Neotropics, are closest relatives. Although our data are insufficient to robustly infer the ancestral mode of life of snakes, we find evidence of plasticity in the basic ecological and trophic modes of snakes. Consequently, the macrostomatan condition should not be treated a priori as a derived character state devoid of homoplasy.     

Some correlates of cranial and cervical morphology with predatory modes in snakes

Dissection of the cervical and basicranial regions in three species of snakes indicates that compared to Crotalus viridis and Lichanura roseofusca, Masticophis flagellum possesses relatively high numbers of compound axial muscle insertions on the atlas-axis and vertebrae numbers 3-5. It is suggested that the condition in Masticophis facilitates its vertical-neck-horizontal-head foraging posture and has allowed axial muscles inserting on the dorsocaudal braincase in this snake to generate vertical and lateral head movements more effectively.     

The comparative morphology of the wings and axillae of selected Heteroptera

The wings of selected species are here treated as functional deformable aerofoils and their morphology described with this in mind. A novel type of non-tubular vein structure in the forewing and hindwing membranes, a 'channel vein', is described. The presence of small, cuticular membrane thickenings in the hindwing is recorded and their possible functions are discussed.
A comparative account of the structure and musculature of the axillae is presented with a view in particular to clarifying some of the associated terminology.
A mechanism for wing folding is described and previously unrecorded adaptations for this purpose seen in Notonecta are described and discussed.     

Horizontal transfer and subsequent explosive expansion of a DNA transposon in sea kraits (Laticauda)

Transposable elements (TEs) are self-replicating genetic sequences and are often described as important ‘drivers of evolution’. This driving force is because TEs promote genomic novelty by enabling rearrangement, and through exaptation as coding and regulatory elements. However, most TE insertions potentially lead to neutral or harmful outcomes, therefore host genomes have evolved machinery to suppress TE expansion. Through horizontal transposon transfer (HTT) TEs can colonize new genomes, and since new hosts may not be able to regulate subsequent replication, these TEs may proliferate rapidly. Here, we describe HTT of the Harbinger-Snek DNA transposon into sea kraits (Laticauda), and its subsequent explosive expansion within Laticauda genomes. This HTT occurred following the divergence of Laticauda from terrestrial Australian elapids approximately 15–25 Mya. This has resulted in numerous insertions into introns and regulatory regions, with some insertions into exons which appear to have altered UTRs or added sequence to coding exons. Harbinger-Snek has rapidly expanded to make up 8–12% of Laticauda spp. genomes; this is the fastest known expansion of TEs in amniotes following HTT. Genomic changes caused by this rapid expansion may have contributed to adaptation to the amphibious-marine habitat.     

The morphology of the intrinsic tongue musculature in snakes (Reptilia,ophidia): Functional and phylogenetic implications

Tongue musculature in 24 genera of snakes was examined histologically. In all snakes, the tongue is composed of a few main groups of muscles. The M. hyoglossus is a paired bundle in the center of the tongue. The posterior regions of the tongue possess musculature that surrounds these bundles and is responsible for protrusion. Anterior tongue regions contain hyoglossal bundles, dorsal longitudinal muscle bundles and vertical and transverse bundles, which are perpendicular to the long axis of the tongue. The interaction of the longitudinal with the vertical and horizontal muscles is responsible for bending during tongue flicking. Despite general similarities, distinct patterns of intrinsic tongue musculature characterize each infraorder of snakes. The Henophidia are primitive; the Scolecophidia and Caenophidia are each distinguished by derived characters. These derived characters support hypotheses that these latter taxa are each monophyletic. Cylindrophis (Anilioidea) is in some characters intermediate between Booidea and Colubroidea. The condition in the Booidea resembles the lizard condition; however, no synapomorphies of tongue musculature confirm a relationship with any specific lizard family. Although the pattern of colubroids appears to be the most biomechanically specialized, as yet no behavioral or performance feature has been identified to distinguish them from other snakes.     

Effects of feeding on luminal pH and morphology of the gastroesophageal junction of snakes

At the gastroesophageal junction, most vertebrates possess a functional lower esophageal sphincter (LES) which may serve to regulate the passage of liquids and food into the stomach and prevent the reflux of gastric contents into the esophagus. Snakes seemingly lack an LES and consume meals large enough to extend anteriorly from the stomach into the esophagus thereby providing the opportunity for the reflux of gastric juices. To explore whether snakes experience or can prevent gastric reflux, we examined post-feeding changes of luminal pH of the distal esophagus and stomach, the fine scale luminal pH profile at the gastroesophageal junction, and the morphology of the gastroesophageal junction for the Burmese python (Python molurus), the African brown house snake (Lamprophis fuliginosus), and the diamondback water snake (Nerodia rhombifer). For each species fasted, there was no distension of the gastroesophageal junction and only modest changes in luminal pH from the distal esophagus into the stomach. Feeding resulted in marked distension and changes in tissue morphology of the gastroesophageal junction. Simultaneously, there was a significant decrease in luminal pH of the distal esophagus for pythons and house snakes, and for all three species a steep gradient in luminal pH decreasing across a 3-cm span from the distal edge of the esophagus into the proximal edge of the stomach. The moderate acidification of the distalmost portion of the esophagus for pythons and house snakes suggests that there is some anterior movement of gastric juices across the gastroesophageal junction. Given that this modest reflux of gastric fluid is localized to the most distal region of the esophagus, snakes are apparently able to prevent and protect against acid reflux in the absence of a functional LES.     

The comparative morphology of epidermal glands in Pentatomoidea (Heteroptera)

The Heteroptera show a diversity of glands associated with the epidermis. They have multiple roles including the production of noxious scents. Here, we examine the cellular arrangement and cytoskeletal components of the scent glands of pentatomoid Heteroptera in three families, Pentatomidae (stink bugs), Tessaratomidae, and Scutelleridae (shield-backed bugs or jewel bugs). The glands are; (1) the dorsal abdominal glands, (2) the tubular glands of the composite metathoracic gland, and (3) the accessory gland component of the composite metathoracic gland. The dorsal abdominal glands are at their largest in nymphs and decrease in size in adults. The metathoracic gland is an adult-specific gland unit with a reservoir and multiple types of gland cells. The accessory gland is composed of many unicellular glands concentrated in a sinuous line across the reservoir wall. The lateral tubular gland is composed of two-cell units. The dorsal abdominal glands of nymphs are composed of three-cell units with a prominent cuticular component derived from the saccule cell sitting between the duct and receiving canal. The cuticular components that channel secretion from the microvilli of the secretory cell to the exterior differ in the three gland types. The significance of the numbers of cells comprising gland units is related to the role of cells in regenerating the cuticular components of the glands at moulting in nymphs.