Search PubMed⌕ Search

Biomedical subjects

O Rieppel

Publications and source records attributed to O Rieppel.

13 recordsLinked to original sources
PubMed2001

Turtles as hopeful monsters.

A recently published study on the development of the turtle shell highlights the important role that development plays in the origin of evolutionary novelties. The evolution of the highly derived adult anatomy of turtles is a prime example of a macroevolutionary event triggered by changes in early embryonic development. Early ontogenetic deviation may cause patterns of morphological change that are not compatible with scenarios of gradualistic, stepwise transformation.

Animals↗
PubMed2001

The development of the skull in Acrochordus granulatus (Schneider) (Reptilia: Serpentes), with special consideration of the otico-occipital complex.

The skull of Acrochordus has been characterized by the absence of a crista circumfenestralis (a synapomorphy shared by all snakes), and by the absence of a recessus scalae tympani that in other squamates forms by subdivision of the embryonic metotic fissure. These traits have variably been identified as either plesiomorphic or paedomorphic. The study of the development of the osteocranium in a series of cleared and stained embryos of Acrochordus shows a close correspondence to the development of the skull in other snakes. The exception is the formation of the postorbital from two ossification centers. The significance of this observation, which might suggest the embryonic fusion of a postfrontal with a postorbital, remains enigmatic, as it is based on one side only of the skull of a single specimen. By contrast, a rudimentary and modified crista circumfenestralis can be identified in the skull of Acrochordus. Furthermore, absence of a recessus scalae tympani is not due to an undivided fissura metotica, but results from the obliteration of the anterior part of the metotic fissure instead. With respect to this character, Acrochordus is neither plesiomorphic, nor paedomorphic, but autapomorphic. This interpretation of the skull of Acrochordus is compatible with the sister-group relationships this genus shares with colubroid snakes.

Animals↗
PubMed2001

The origin of snakes: limits of a scientific debate.

The origin of snakes has recently become a palaeontological 'hot potato'. Two groups of scientists have come to very different conclusions from the studying the same fossils. Are there tests capable of rejecting wither one of these alternative conclusions or will human interpretation always allow such differences of opinion?

Animals↗
PubMed2000

A fossil snake with limbs.

A 95-million-year-old fossil snake from the Middle East documents the most extreme hindlimb development of any known member of that group, as it preserves the tibia, fibula, tarsals, metatarsals, and phalanges. It is more complete than Pachyrhachis, a second fossil snake with hindlimbs that was recently portrayed to be basal to all other snakes. Phylogenetic analysis of the relationships of the new taxon, as well as reanalysis of Pachyrhachis, shows both to be related to macrostomatans, a group that includes relatively advanced snakes such as pythons, boas, and colubroids to the exclusion of more primitive snakes such as blindsnakes and pipesnakes.

Animals↗
PubMed1999

Turtle origins.

Explore the source record for details and available documents.

Animals↗
PubMed1996

Testing homology by congruence: the pectoral girdle of turtles.

The recent analysis of the homologies of the elements in the highly derived pectoral girdle of turtles is used as an example to clarify how character congruence tests conjectures of homology in cladistic analysis. Character congruence does not test alternative conjectures of similarity i.e. alternative character definitions and/or descriptions. It will only test the proposition that an observed relationship of similarity is homologous or not, and hence indicate whether this observed relationship of similarity may be parsimoniously explained by common ancestry, or must rather be treated as homoplasy. To test alternative hypotheses of homology, or alternative conjectures of similarity, the data matrix must be constructed such as to allow the evaluation of these alternatives on the basis of parsimony.

Animals↗
PubMed1993

[Limbs of tetrapods--an actual problem of evolution].

The tetrapod limb is among the most intensively studied developmental systems in tetrapods. Earlier models based on a symmetrical determination of the early limb bud were recently questioned by new observations documenting a predominance of the postaxial axis during limb development. These findings are at variance with observations on limb reduction in a number of fossil reptile clades. Recent investigations corroborate the hypothesis of a decoupling of the patterns and sequence of chondrification and ossification in limb development. Ontogenetic repatterning may establish new topological patterns of connectivity among constituent elements in a biological structure as cartilage is replaced by bone. To view chondrification and subsequent ossification of cartilaginous precursors as decoupled processes bears directly on conjectures of homology of limb elements, and also requires the distinction of two separate mechanisms of limb reduction: truncation of morphogenesis and failure of ossification.

Animals↗
PubMed1985

[Pattern and process: complementarity in biological thinking].

Evolutionary theory is said to explain order in nature by a historical process. An analysis of the writings of E. Geoffroy-Saint-Hilaire shows, however, that pattern and process in nature are not based on simple, i.e. unreflected observation, but rather constitute alternative and incompatible ways of interpretation of observations according to certain methodological rules. It is concluded that pattern and process are complementary views of nature, pattern providing process with a direction, process giving an explanation for pattern.

Animals↗
PubMed1979

A functional interpretation of the varanid dentition (Reptilia, Lacertilia, Varanidae).

A cinéradiographic analysis of the feeding movements in Varanus bengalensis produced the following results. The mouth is opened by raising the head (upper jaw) rather than by lowering the lower jaw. Starting from the resting position, the muzzle unit is elevated around 9 degrees relative to the rest of the skull during jaw opening; the quadrate swings anteriorly around 21 degrees. During jaw closure, the snout is depressed around 15 degrees relative to the rest of the skull, hence 6 degrees beyond the resting position. The quadrate swings backwards around 27 degrees. Amphikinesis is interpreted as allowing a stronger posterior recurvature of the maxillary teeth in Varanus. This increases the holding effect of the teeth without increasing their length, an adaptation of Varanus to capture relatively large prey. The formation of plicidentine (dentine infolding) in the teeth of Varanus increases the surface of attachment of the teeth on the supporting bone. Moreover, the dentine lamellae take up tensile and compressive stresses along their long axes upon axial or vertical loading of the teeth. The slope of pleurodonty is modelled so as to minimalize shear stress on the surface of ankylosis upon axial or vertical loading of the teeth.

Animals↗
PubMed1978

The throat musculature of Sphenodon, with comments on the primitive character states of the throat muscles in lizards.

The throat musculature of Sphenodon is described and figured, and compared with that of lizards. In many features of its throat musculature, Sphenodon may well represent a primitive lepidosaurian pattern, comparable to some extent to the chelonian structure of throat musculature. To infer primitive character states within extant lizards, one cannot simply extrapolate from the condition observed in Sphenodon. In fact, it must be assumed that a rapid change of many features of the throat muscles occurred during the early evolution of lizards in order to provide the basis for further differentiation with extant lacertilian infraorders.

Animals↗
PubMed1976

[The orbitotemporal region in the cranium of Chelydra serpentina Linnaeus (Chelonia) and Lacerta sicula Rafinesque (Lacertilia)].

An analysis of the ontogenesis of the chondrocranium gives the following homologies in the skull of Lacerta and Chelydra: (see article). It is shown that the extracranial position of the cavum epiptericum in Lacerta corresponds to the stage of maximal development of the chondrocranium in Chelydra. But further development brings about a dermal sidewall in the orbitotemporal region, b,ilt up by descending parietal flanges which incorporate the epipterygoid. This not only allows the pila antotica to degenerate but also incorporates the cavum in the cranial cavity.

Anatomy, Comparative↗