The ontogeny of the shell in side‐necked turtles,with emphasis on the homologies of costal and neural bones

Although we are starting to understand the molecular basis of shell development based on the study of cryptodires, basic comparative ontogenetic data for the other major clade of living turtle, the pleurodires, are largely missing. Herein, the developmental and phylogenetic relation between the bony shell and endoskeleton of Pleurodira are examined by studying histological serial sections of nine specimens of three different species, including an ontogenetic series of Emydura subglobosa. Emphasis is given to the portion of the carapace in which ribs and vertebral spinous processes become part of the carapace. Central questions are how neurals and costals are formed in pleurodiran turtles, whether costals and neurals are of endoskeletal or exoskeletal origin, and what ontogenetic factors relate to neural reduction of some Pleurodira. The neurals and costals do not develop as independent ossification centers, but they are initial outgrowths of the periosteal collar of endoskeletal ribs and neural arches. Slightly later in development, the ossification of both shell elements continues without a distinct periosteum but by metaplastically ossifying precondensed soft‐tissue integumentary structures. Through ontogeny, ribs of the turtles studied are closely associated with the hypaxial intercostalis musculature while epaxial interspinalis musculature connects the neural arches. We here propose an alternative structural hypothesis for the neural reduction and, ultimately, the complete loss of the neural series. The complete reduction of neurals in Emydura spp. may be linked to heterochrony, accompanied by a restricted influence of epaxial musculature and epidermal–dermal interaction in shell bone formation. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Morphogenetic and constructional differences of the carapace of aquatic and terrestrial turtles and their evolutionary significance

The postembryonic development of the turtle carapace was studied in the aquatic Еmys orbicularis and the terrestrial Тestudo graeca. Differences in the structure of the bony shell in aquatic and terrestrial turtles were shown to be associated with varying degrees of development of epidermal derivatives, namely, the thickness of the scutes and the depth of horny furrows. Sinking of the horny furrows into the dermis causes local changes in the structure of the collagen matrix, which might precondition the acceleration of the ossification. Aquatic turtles possess a relatively thin horny cover, whose derivatives are either weakly developed or altogether absent and thus make no noticeable impact on the growth dynamics of bony plates. Carapace plates of these turtles outgrow more or less evenly around the periphery, which results in uniform costals, relatively narrow and partly reduced neurals, and broad peripherals extending beyond the marginal scutes. In terrestrial turtles (Testudinidae), horny structures are much more developed and exert a considerable impact on the growth of bony elements. As a result, bony plates outgrow unevenly in the dermis, expanding fast in the zones under the horny furrows and slowly outside these zones. This determines the basic features of the testudinid carapace: alternately cuneate shape of costals, an alternation of broad octagonal and narrow tetragonal neurals, and the limitation of the growth of peripherals by pleuro-marginal furrows. The evolutionary significance of morphogenetic and constructional differences in the turtle carapace, and the association of these differences with the turtle habitats are discussed.     

A geometric morphometric assessment of the effects of environment and cladogenesis on the evolution of the turtle shell

In the largest group of extant turtles, the Testudinoidea, the acquisition of an aquatic or terrestrial way of life has occurred within two clades, allowing the study of homoplasy linked to environment (commonly named convergence). Here we appraise the respective importance of two sources of morphological variation: a major cladogenetic event and a major environmental shift (aquatic vs. terrestrial). The repeatability of the same evolutionary process (environmental change) allows an assessment of the weights of both natural selection and phylogenetic constraints on several morphological features of the shell. These sources of morphological variance on the complex shell structure were studied using geometric morphometrics. We depict the morphological variation of three parts of the turtle shell: epidermal carapace, bony carapace, and plastron. In the three structures, we found that both phylogeny and environment were significant sources of morphological variation, and geometric morphometrics allowed the pattern of morphological variation due to each effect to be assessed. The assessment of the homoplasy due to environment and of the pattern of morphological variability suggests that the carapace has undergone similar morphological changes between aquatic and terrestrial environments within the two clades. The radiation of the Testudinoidea is interpreted as an adaptive radiation.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 485–501.     

Developmental basis of limb homology in Pleurodiran turtles,and the identity of the hooked element in the chelonian tarsus

Although Pleurodiran turtles represent an important component of extant turtle radiation, our knowledge of the development and homology of limb bones in turtles rests mostly upon observations made on derived members of the Cryptodiran clade. Herein, we describe limb development in three pleurodirans: Podocnemis unifilis Troschel, 1848, Podocnemis sextuberculata Cornalia, 1849 and Phrynops hilarii (Dumeril and Bibron, 1835), in an effort to contribute to filling this anatomical gap. For earlier stages of limb development, we described the Y‐shaped condensation that gave rise to the zeugopodial cartilages, and differentiation of the primary axis/digital arch that reveals the invariant pattern common to tetrapods. There are up to four central cartilaginous foci in the carpus, and the proximal tarsale is formed by the fusion of the fibulare, intermedium, and centrale 4. Digital development is similar for the five digits. Changes in toe V occur predominantly in the distal tarsale 5. Ontogenetic reduction of phalanges is observed in toe V of Podocnemis. Based on these results, we suggest that the hooked element present in the chelonian tarsus, and traditionally recognized as a modified fifth metatarsale, is actually the fifth distal tarsale. Additionally, our data on limb development of pleurodiran turtles supply more taxonomically comprehensive information to interpret limb configuration within the chelonian clade. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 845–866.     

Phylogenetic interpretation of ontogenetic change: sorting out the actual and artefactual in an empirical case study of centrarchid fishes

Hypothesized relationships between ontogenetic and phylogenetic change in morphological characters were empirically tested in centrarchid fishes by comparing observed patterns of character development with patterns of character evolution as inferred from a representative phylogenetic hypothesis. This phylogeny was based on 56–61 morphological characters that were polarized by outgroup comparison. Through these comparisons, evolutionary changes in character ontogeny were categorized in one of eight classes (terminal addition, terminal deletion, terminal substitution, non-terminal addition, non-terminal deletion, non-terminal substitution, ontogenetic reversal and substitution). The relative frequencies of each of these classes provided an empirical basis from which assumptions underlying hypothesized relationships between ontogeny and phylogeny were tested. In order to test hypothesized relationships between ontogeny and phylogeny that involve assumptions about the relative frequencies of terminal change (e.g. the use of ontogeny as a homology criterion), two additional phylogenies were generated in which terminal addition and terminal deletion were maximized and minimized for all characters. Character state change interpreted from these phylogenies thus represents the maxima and minima of the frequency range of terminal addition and terminal deletion for the 8.7 × 10³⁶ trees possible for centrarchids. It was found for these data that terminal change accounts for c. 75% of the character state change. This suggests either that early ontogeny is conserved in evolution or that interpretation and classification of evolutionary changes in ontogeny is biased in part by the way that characters are recognized, delimited and coded. It was found that ontogenetic interpretation is influenced by two levels of homology decision: an initial decision involving delimitation of the character (the ontogenetic sequence), and the subsequent recognition of homologous components of developmental sequences. Recognition of phylogenetic homology among individual components of developmental sequences is necessary for interpretation of evolutionary changes in ontogeny as either terminal or non-terminal. If development is the primary criterion applied in recognizing individual homologies among parts of ontogenetic sequences, the only possible interpretation of phylogenetic differences is that of terminal change. If homologies of the components cannot be ascertained, recognition of the homology of the developmental sequence as a whole will result in the interpretation of evolutionary differences as substitutions. Particularly when the objective of a study is to discover how ontogeny has evolved, criteria in addition to ontogeny must be used to recognize homology. Interpretation is also dependent upon delimitation within an ontogenetic sequence. This is in part a function of the way that an investigator ‘sees’ and codes characters. Binary and multistate characters influence interpretation differently and predictably. The use of ontogeny for determining phylogenetic polarity as previously proposed rests on the assumptions that ancestral ontogenies are conserved and that character evolution occurs predominantly through terminal addition. It was found for these data that terminal addition may comprise a maximum of 51.9% of the total character state change. It is concluded that the ontogenetic criterion is not a reliable indicator of phylogenetic polarity. Process and pattern data are collected simultaneously by those engaged in comparative morphological studies of development. The set of alternative explanatory processes is limited in the process of observing development. These form necessary starting points for the research of developmental biologists. Separating ‘empirical’ results from interpretational influences requires awareness of potential biases in the course of character selection, coding and interpretation. Consideration of the interpretational problems involved in identifying and classifying phylogenetic changes in ontogeny leads to a re-evaluation of the purpose, usefulness and information conveyed by the current classification system. It is recommended that alternative classification schemes be pursued.     

Ontogenetic shifts in resource allocation: colour change and allometric growth of defensive and reproductive structures in the Caribbean spiny lobster Panulirus argus

Resource allocation theory predicts a disproportionately large allocation of resources to defensive structures during early ontogeny in organisms that are subject to more intense predation at smaller than at larger body sizes. We tested this prediction on the Caribbean spiny lobster Panulirus argus, which exhibits a negative relationship between predation risk and body size with a high natural mortality of smaller individuals. Independent allometric growth analyses demonstrated that numerous defensive structures (e.g. orbital horns, segments supporting the antenna, the tail fan) display negative allometric growth throughout ontogeny. We interpret these findings as lobsters investing disproportionately more resources to defensive structures when small to improve survivorship. Similarly, we observed an ontogenetic shift in lobster colour pattern; small individuals (< 23 mm carapace length) that inhabit nursery grounds (preferably among red algae) displayed a disruptive pattern (camouflage), whereas larger juveniles displayed a bicolour pigmentation typical of adult lobsters. This shift in colour pattern further suggests that small lobsters employ cryptic coloration throughout their asocial algal stage. However, this cryptic coloration offers no advantage when lobsters grow larger and start dwelling in crevices. Other structures linked to reproduction (e.g. female pleopods and male pereopods) experienced either isometric or positive allometric growth throughout ontogeny. Our results support one of the main predictions of resource allocation theory and demonstrate ontogenetic shifts in defensive structures and coloration concomitantly with changes in lobster mortality risk mediated by size‐dependent predation risk. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Patterns of sexual size dimorphism in Chelonia: revisiting Kinosternidae

Rensch's rule, a macroevolutionary pattern in which sexual size dimorphism (SSD) increases with body size in male‐biased SSD species, or decreases with female‐biased SSD species, has been investigated in many vertebrates because it indicates whether SSD is being driven by sexual selection or a different force (i.e. fecundity or natural selection). Evidence in turtles has shown some conflicting results, which may be explained by the different phylogenies used in the analyses. Because the newly available well‐resolved phylogeny of family Kinosternidae provides evidence for the ancient monophyly of Staurotypidae and Kinosternidae and their recognition as separate families (previously Staurotypidae was considered as a subfamily within Kinosternidae) and introduced the genus Cryptochelys for the monophyletic leucostomum clade, we revisit the pattern of SSD and body size in Kinosternidae. By contrast to what had been proposed, we found that the Kinosternidae as formerly recognized (i.e. including Staurotypus and Claudius) and the restricted Kinosternidae both follow a pattern consistent with Rensch's rule. Our analysis with published body size data did not change our results, confirming the importance of the phylogeny used in macroevolutionary studies. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 806–809.     

Are diminutive turtles miniaturized? The ontogeny of plastron shape in emydine turtles

Miniaturization, or the evolution of a dramatically reduced body size compared to related lineages, is an extraordinarily widespread phenomenon among metazoans. Evolutionary biologists have been fascinated by miniaturization because this transition has occurred numerous times, often among close relatives, providing a model system for studying convergent evolution and its underlying mechanisms. Much of the developmental work describing the ontogeny of miniature species suggests that paedomorphosis is the predominant avenue of miniaturization. Nevertheless, specific alterations to ontogeny appear highly variable, so that even related lineages with similar miniaturized traits produce those similarities via distinct ontogenetic paths. One major vertebrate group that has been overlooked in research on miniaturization is turtles. In the present study, we examined patterns of shape change in the plastron (the ventral part of the shell) over the course of ontogeny in a small clade of turtles (Emydinae) aiming to investigate whether two independently evolved diminutive members of the clade (Glyptemys muhlenbergii and Clemmys guttata) should be considered as miniaturized. We employ geometric morphometric methods to quantify the patterns of shape change these potentially miniaturized species and their relatives undergo during ontogeny, and use molecular phylogenetic trees to reconstruct ancestral conditions and provide information on the polarity of shape changes. We find that differing changes in ontogenetic parameters relative to ancestral conditions accompany the evolution of small size in emydines: G. muhlenbergii changes the duration of ontogeny and rate of shape change, whereas C. guttata changes growth rate. The observed ontogenetic repatterning of these species is reminiscent of changes in ontogeny and life history often found in miniaturized taxa. However, we conclude that C. guttata and G. muhlenbergii are not truly miniaturized because they still produce typical adult shell morphologies, and larger emydines display comparable ontogenetic flexibility. Because no emydines carry juvenile shell features forward into adulthood, we speculate that few, if any turtles, will show paedomorphic shell traits without corresponding changes in defensive strategy because such shells may offer insufficient protection. © 2013 The Linnean Society of London     

Evolution and homology of the astragalus in early amniotes: new fossils, new perspectives

The reorganization of the ankle in basal amniotes has long been considered a key innovation allowing the evolution of more terrestrial and cursorial behavior. Understanding how this key innovation arose is a complex problem that largely concerns the homologizing of the amniote astragalus with the various ossifications in the anamniote tarsus. Over the last century, several hypotheses have been advanced homologizing the amniote astragalus with the many ossifications in the ankle of amphibian-grade tetrapods. There is an emerging consensus that the amniote astragalus is a complex structure emerging via the co-ossification of several originally separate elements, but the identities of these elements remain unclear. Here we present new fossil evidence bearing on this contentious question. A poorly ossified, juvenile astragalus of the large captorhinid Moradisaurus grandis shows clear evidence of four ossification centers, rather than of three centers or one center as posited in previous models of astragalus homology. Comparative material of the captorhinid Captorhinikos chozaensis is also interpretable as demonstrating four ossification centers. A new, four-center model for the homology of the amniote astragalus is advanced, and is discussed in the context of the phylogeny of the Captorhinidae in an attempt to identify the developmental transitions responsible for the observed pattern of ossification within this clade. Lastly, the broader implications for amniote phylogeny are discussed, concluding that the neomorphic pattern of astragalus ossification seen in all extant reptiles (including turtles) arose within the clade Diapsida.     

Homology and heterochrony: the evolutionary embryologist Gavin Rylands de Beer (1899-1972)

The evolutionary embryologist Gavin Rylands de Beer can be viewed as one of the forerunners of modern evolutionary developmental biology in that he posed crucial questions and proposed relevant answers about the causal relationship between ontogeny and phylogeny. In his developmental approach to the phylogenetic phenomenon of homology, he emphasized that homology of morphological structures is to be identified neither with the sameness of the underlying developmental processes nor with the homology of the genes that are involved in the development of the structures. De Beer's work on developmental evolution focused on the notion of heterochrony, arguing that paedomorphosis increases morphological evolvability and is thereby an important mode of evolution that accounts for the origin of many taxa, including higher taxa.     

Generation of a second eye by embryonic transplantation of the antero‐ventral hemicephalon

Vertebrate ocular morphogenesis requires proper dorso‐ventral polarity within the optic vesicle, and loss of dorso‐ventral polarity results in failure of optic cup formation and domain specification, as shown by a reverse transplantation of the optic vesicle. We have shown previously that the ocular development depends not only on the signal within the antero‐ventral optic vesicle but also on the extraocular signals. In the present study, using embryonic transplantation of a discrete portion of the embryonic chick brain, we demonstrate formation of a second eye from the antero‐ventral hemicephalon when it was transplanted in the antero‐dorsal hemicephalon of the host embryo. The transplant consists of an antero‐ventral quadrant of the optic vesicle and the surrounding part of the anterior cephalon. The original dorso‐ventral polarity of the transplant was once cancelled and re‐established in accordance with that of the host embryo. Neither dorsal nor ventral cephalic halves in isolation did not develop into entire eye structures under the culture condition; the dorsal halves developed merely into the retinal pigmented epithelium and the ventral halves into the neural retina alone. The present study clearly suggests that extraocular dorsal and ventral signals counterbalance each other to specify the polarity of the optic vesicle.     

Retinal Anatomy of hatchling sea turtles: Anatomical specializations and behavioral correlates

The eyes of three species of sea turtle hatchlings (loggerheads, green turtles, and leatherbacks) possess visual streaks, areas of densely packed ganglion cells running along the antero‐posterior retinal axis. These probably function to provide heightened visual acuity along the horizon. The vertical extent and absolute concentration of cells within the streak, compared to the rest of the retina, differ among the species. Leatherbacks have an additional specialized region (area temporalis) that might enhance their ability to detect prey below them in the water column. Green turtles and loggerheads, but not leatherbacks, show compensatory eye reflexes that keep the visual streak horizontal. Species differences in retinal structure and eye reflexes probably reflect their unique specializations in visual ecology and behaviour.     

Embryonic development of the skeleton of Amphisbaena darwini heterozonata (Squamata: Amphisbaenidae)

An assemblage of amphisbaenian embryos has allowed us to characterize the external morphology of the developing embryos as well as the chondrification and ossification sequences of their skeletal elements. The external characterization of embryos serves as an incomplete developmental table. In contrast to the condition in other squamates, the premaxilla seems to arise azygously from the beginning or to represent very early fusion during embryogenesis. The tabulosphenoid forms from two cartilages to which are added extensive membranous ossifications. The two parietals engage in medial fusion at the midline, where the anterior process of the synotic tectum ossifies and forms the sagittal crest. The lateral element‐X does not ossify until very late in embryogenesis and is interpreted as an epiphysial ossification. The compound mandibular bone arises from the ossification of the posterior part of Meckel's cartilage and the fusion of at least two dermal centers, interpreted as surangular and splenial. The vertebral column shows an antero‐posterior gradient of vertebral differentiation. The number of vertebrae is fixed from the beginning of their differentiation. The remnants of pectoral and pelvic girdles are represented by cartilaginous rods. Some reproductive data obtained during the collection of data could be compared with those from the literature. J. Morphol. 239:1–25, 1999. © 1999 Wiley‐Liss, Inc.     

Seasonal Patterns of Adrenomedullary Hormones and Glycemia in the Soft-Shelled Turtle Lissemys punctata punctata

The aim of the present study was to acertain the seasonal pattern of adrenomedullary hormones and of glycemia in Lissemys turtles. Both the norepinephrine and epinephrine concentrations as well as blood glucose levels varied seasonally which began to rise from February, became maximum during April–May (early summer), declined during June–September (late summer) and were extremely low subsequently (October–January). The seasonal adrenomedullary hormonal and glycemic cycles however do not coincide with the annual ovarian cycle, thereby indicating that the adrenomedullary and glycemic cycles are out of phase with the ovarian cycle in turtles. The possible mechanisms of seasonality of the adrenal medulla and glycemia are discussed.     

Similarity

Recent debates concerning conflicting hypotheses of higher-level phylogeny such as the sister-group relationships of tetrapods, turtles, birds and snakes, serve as examples in the analysis of the nature of morphological evidence as it is currently used in phylogeny reconstruction. We note a recent shift of emphasis towards ever-larger data matrices, which may come at the cost of detailed character analysis and argumentation. Because the assessment of morphological characters necessarily entails a conceptual element of abstraction, there is also a threat that preconceived notions of phylogeny influence character analysis. Because the test of congruence does not address character analysis in itself, we argue that character hypotheses, i.e. primary conjectures of homology, need to be testable, and potentially refutable, in their own right. We demonstrate the use of classical criteria of homology (topological relations and/or connectivity, in conjunction with the subsidiary criteria of special similarity and intermediate forms) in the test, and refutation, of morphological characters. Rejection of the classical criteria of homology in the test of morphological character hypotheses requires the formulation of alternative methods of test and potential falsification of morphological characters that have so far not been proposed. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 59–82.     

Chondrogenesis of the limbs and mesopodial ossification of Podocnemis expansa Schweigger, 1812 (Testudines: Podocnemidae)

We address the chondrogenic formation of the limbs and the mesopodial ossification pattern of the Pleurodira Podocnemis expansa, to resolve the homology of these elements as well as the pattern of connection of the autopodial elements and the origin of the digital arch. Embryos and juveniles of P. expansa were cleared and stained for cartilage and bone. The fore‐ and hind‐limbs were also studied histologically. We describe the development of the stylopodium and zeugopodium originating from a Y‐shaped cartilaginous condensation, and the differentiation of the primary axis and the digital arch in the initial stages of limb development. The most pronounced changes were observed in the chondrogenic pattern and ossification of the mesopodium, although development of the digits is similar and we found no ontogenetic reduction such as that described for other Testudines. In this study, as in previous research involving several groups of reptilian sauropsids, we found an inconsistent pattern between the chondrogenic formation and mesopodial ossification of the limbs, indicating that these developmental events are dissociated. In summary, the chondrogenic and ossification sequences of these elements do not follow the same pattern. In addition, the differences found between P. expansa and other species to which it was compared clearly indicate that these events follow more than one pattern in Testudines. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Developmental pathways,homology and homonomy in metameric animals

Following Wagner's (1989) distinction between historical and biological concepts of homology, we analyze homology problems of metameric animals in the light of a biological concept. In identifying homology, we refer to the common informational background which two structures share. Therefore, homology relationships are matters of degree; they are ‘perfect’ only when there is full identity of informational background between the structures under comparison. Homonomy (serial homology) is not fundamentally different from other kinds of homology. We regard the differences between epimorphically and anamorphically developed segments as minor; therefore, the two kinds of segments are largely homologous. The morphogenetic processes giving rise to segmental structures are regarded as not necessarily hierarchical. We contrast the phylogenetic pattern of hierarchically nested homologies with a largely non-hierarchical pattern of homologous structures within the individual organism. This topological difference adds to heterochrony in generating the widespread mismatch of ontogeny and phylogeny.