Platysternon Research Articles

Phylogenetic Relationships of Mesozoic Turtles

Presented herein is the first morphological analysis of turtle relationships to examine the monophyly of many turtle groups by using only single species as terminals and by integrating a large number of primitive fossil taxa. The data matrix consists of 136 osteological parsimony informative characters with 169 derived character states for 45 fossil and 22 living species of the clade Testudinata. The results corroborate the monophyly of a large number of previously hypothesized clades, but refute the accepted hypothesis regarding the basal split of living turtles. In particular, the primitive turtles Proterochersis robusta, Kayentachelys aprix, Mongolochelys efremovi, Meiolania platyceps, and Kallokibotion bajazidi are removed from their current position as crown turtles and placed along the phylogenetic stem of this clade. The age of the turtle crown is thereby adjusted from the Late Triassic to the Late Jurassic, which is relevant to testing molecular clock hypotheses. This revised topology has important implications for the evolution of several character complexes, because it implies that the common ancestor of all living turtles must have had a partially braced brain case and a primitive trochlear mechanism. Other noteworthy conclusions include the tentative exclusion of protostegids from Chelonioidea, the placement of Platysternon megacephalum outside of Chelydridae, and the tentative interpretation of Sandownia harrisi as a basal eucryptodire.

Complete mitochondrial genome of Chinese big-headed turtle, Platysternon megacephalum, with a novel gene organization in vertebrate mtDNA

The mitochondrial genome of the Chinese big-headed turtle, Platysternon megacephalum, was obtained using polymerase chain reaction (PCR). The entire mtDNA sequence, the longest mitochondrial genome in turtles reported so far, is 19 161 bp. This mitochondrial genome exhibits a novel gene order, which greatly differs from that of any other vertebrates. It is characterized by four distinctive features: 1) the translocation of a gene cluster including three tRNA genes (tRNA His, tRNA Ser, tRNA Leu(CUN)) and ND5 gene, 2) two tRNA Thr pseudogenes, 3) a duplication of pseudo tRNA Thr/tRNA Pro/D-loop region and 4) 3 non-coding spacers. These unique identities represent a new mitogenomic gene order in vertebrates. The TDRL model was proposed to account for the generation of the gene order in P. megacephalum.

The complete mitochondrial genome of the enigmatic bigheaded turtle (Platysternon): description of unusual genomic features and the reconciliation of phylogenetic hypotheses based on mitochondrial and nuclear DNA

BackgroundThe big-headed turtle (Platysternon megacephalum) from east Asia is the sole living representative of a poorly-studied turtle lineage (Platysternidae). It has no close living relatives, and its phylogenetic position within turtles is one of the outstanding controversies in turtle systematics. Platysternon was traditionally considered to be close to snapping turtles (Chelydridae) based on some studies of its morphology and mitochondrial (mt) DNA, however, other studies of morphology and nuclear (nu) DNA do not support that hypothesis.ResultsWe sequenced the complete mt genome of Platysternon and the nearly complete mt genomes of two other relevant turtles and compared them to turtle mt genomes from the literature to form the largest molecular dataset used to date to address this issue. The resulting phylogeny robustly rejects the placement of Platysternon with Chelydridae, but instead shows that it is a member of the Testudinoidea, a diverse, nearly globally-distributed group that includes pond turtles and tortoises. We also discovered that Platysternon mtDNA has large-scale gene rearrangements and possesses two, nearly identical, control regions, features that distinguish it from all other studied turtles.ConclusionOur study robustly determines the phylogenetic placement of Platysternon and provides a well-resolved outline of major turtle lineages, while demonstrating the significantly greater resolving power of comparing large amounts of mt sequence over that of short fragments. Earlier phylogenies placing Platysternon with chelydrids required a temporal gap in the fossil record that is now unnecessary. The duplicated control regions and gene rearrangements of the Platysternon mtDNA probably resulted from the duplication of part of the genome and then the subsequent loss of redundant genes. Although it is possible that having two control regions may provide some advantage, explaining why the control regions would be maintained while some of the duplicated genes were eroded, examples of this are rare. So far, duplicated control regions have been reported for mt genomes from just 12 clades of metazoans, including Platysternon.

Tail Kinematics of Juvenile Common Snapping Turtles during Aquatic Walking

Chelydrids (including snapping and big-headed turtles) are unusual among extant turtles in possessing long, robust tails. In other lineages of quadrupedal reptiles, long tails perform critical functions during both terrestrial and aquatic locomotion, and the tails of Common Snapping Turtles have been shown to help stabilize juveniles as they ascend terrestrial slopes. However, Common Snapping Turtles live primarily in aquatic habitats, and the function of the tail in these environments has not been examined. The first step to evaluating the role of the chelydrid tail in water is to evaluate its pattern of motion; therefore, we collected high-speed digital video of tail kinematics from juvenile Common Snapping Turtles (Chelydra serpentina) during aquatic walking. Common Snapping Turtles hold the tail off the substrate and move it as a nearly rigid strut during aquatic walking, cyclically flexing it side to side by 11–12° from the body midline. These motions occur one-quarter cycle out of phase with the motions of the limbs; thus, the timing of tail movements suggests that they are likely not a passive consequence of hind-limb retraction and are likely controlled by one (or a combination) of tail muscles, rather than ipsilateral hind-limb retractors. The potential for tail movements to contribute to aquatic thrust in Common Snapping Turtles is uncertain. However, Common Snapping Turtle tail movements resemble those of salamanders and lizards in many respects, suggesting that Common Snapping Turtles might retain primitive tetrapod or sauropsid features of tail motor control despite possessing a radically divergent body plan.

Buccal oscillation and lung ventilation in a semi-aquatic turtle, Platysternon megacephalum

Movements of the hyobranchial apparatus in reptiles and amphibians contribute to many behaviors including feeding, lung ventilation, buccopharyngeal respiration, thermoregulation, olfaction, defense and display. In a semi-aquatic turtle, Platysternon megacephalum, x-ray video and airflow measurements from blowhole pneumotachography show no evidence that above water hyobranchial movements contribute to lung inflation, as in the buccal or gular pump of amphibians and some lizards. Instead, hyobranchial movements produce symmetrical oscillations of air into and out of the buccal cavity. The mean tidal volume of these buccal oscillations is 7.8 times smaller than the mean tidal volume of lung ventilation (combined mean for four individuals). Airflow associated with buccal oscillation occurs in the sequence of inhalation followed by exhalation, distinguishing it from lung ventilation which occurs as exhalation followed by inhalation. No fixed temporal relationship between buccal oscillation and lung ventilation was observed. Periods of ventilation often occur without buccal oscillation and buccal oscillation sometimes occurs without lung ventilation. When the two behaviors occur together, the onset of lung ventilation often interrupts buccal oscillation. The initiation of lung ventilation was found to occur in all phases of the buccal oscillation cycle, suggesting that the neural control mechanisms of the two behaviors are not coupled. The pattern of occurrence of both buccal oscillation and lung ventilation was found to vary over time with no obvious effect of activity levels.

Comparative Physiology of the Vasomotor Effects of Neurohypophysial Peptides in the Vertebrates

The neurohypophysial peptides are vasopressor or depressor in action depending on the species. Isotocin, mesotocin and oxytocin constrict the branchial vessels in fish and induce a reflex vasodilation in the systemic vasculature. The vasodilation has persisted in some higher vertebrates and is particularly prominent in the snakes and birds where vasotocin and arginine vasopressin also are vasodepressor but are much less potent than mesotocin and oxytocin. In other vertebrates including fish, vasotocin and vasopressin are pressor and exert their effects mainly on the peripheral resistance. The newt, toads and soft-shell turtle gave pressor responses to all neurohypophysial peptides, with vasotocin showing the highest potency. The frogs, big-headed turtle and lizards were intermediate with vasotocin being pressor, mesotocin being pressor and oxytocin exhibiting a dual effect.