[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

[dinosaur] Perinatal aristonectine plesiosaur from Antarctica + German rhynchocephalians




Ben Creisler
bcreisler@gmail.com

A new paper:

José P. O’gorman, Marianella Talevi and Marta S. Fernández (2016)
Osteology of a perinatal aristonectine (Plesiosauria; Elasmosauridae).   
Antarctic Science (advance online publication) 
DOI: http://dx.doi.org/10.1017/S0954102016000365 
http://journals.cambridge.org/action/displayJournal?jid=ANS



Perinatal specimens give valuable information about the first stages of vertebrate ontogeny. Here, the morphology and palaeohistology of an aristonectine perinatal specimen from Seymour Island (Isla Marambio), López de Bertodano Formation are analysed. The palaeohistological analysis shows incomplete endochondral ossification (retention of a calcified cartilaginous core in the medullary region), predominance of primary bone tissue without secondary remodelling, lack of primary or secondary osteons and of growth marks in the cortical bone, and open vascular spaces not surrounded by a thin coat of lamellar bone tissue. General lines of morphological changes were inferred from comparing the fossil with an adult aristonectine specimen indicating i) a tendency of relatively high and broad posterior cervicals to decrease during ontogeny, ii) a decrease of relative size of the dorsolateral process and an increase of the glenoid ramus and iii) the existence of two separate stages in propodial growth divided into an initial elongation followed by a distal expansion. The presence of a perinatal specimen in the James Ross Archipelago indicates that the region was used as a breeding area by the aristonectines during the last part of the Cretaceous.


====


An older paper in German that may be of interest:

Oliver W. M. Rauhut & Adriana López-Arbarello (2015)

Zur Taxonomie der Brückenechse aus dem oberen Jura von Schamhaupten. [On the taxonomy of rhynchocephalians from the Late Jurassic of Schamhaupten]

Archaeopteryx 33: 1-11

https://www.researchgate.net/publication/301517359_Zur_Taxonomie_der_Bruckenechse_aus_dem_oberen_Jura_von_Schamhaupten_On_the_taxonomy_of_the_rhynchocephalian_from_the_Late_Jurassic_of_Schamhaupten

 

The Rhynchocephalia (tuataras) are an ancient lineage of lepidosaurian reptiles. Today, they are only represented by a single genus, but the clade was diverse taxonomically and ecologically during the Mesozoic. One of the most important Mesozoic units that have yielded rhynchocephalians are the Late Jurassic laminated limestones of southern Germany. These layers are especially noteworthy as they usually contain complete skeletons and not only isolated elements, as it is the case with most other Mesozoic rhynchocephalian localities. Unfortunately, however, the taxonomy of the rhynchocephalians from these units has not been satisfactorily established so far, which hampers studies of their evolutionary importance. From the Late Kimmeridgian limestones of Schamhaupten, Bavaria, a single, complete specimen of a rhynchocephalian has been reported so far. This specimen has been identified as Leptosaurus pulchellus, thus accepting the proposed synonymy of the genera Kallimodon (with the type species Kallimodon pulchellus) and Leptosaurus (type species Leptosaurus neptunius). Detailed comparisons between the specimen from Schamhaupten with the type specimen of Kallimodon pulchellus and published descriptions of Leptosaurus neptunius resulted in the recognition of important differences, however. Differences to the type of Kallimodon pulchellus include the morphology of the maxillary teeth, the phalangeal formula of the manus, and the shape of the posterior process of the second sacral rib. An important difference with the type of Leptosaurus neptunius is the higher number of premaxillary teeth in the specimen from Schamhaupten (four versus two), despite a significantly larger body size, whereas there is rather a tendency to reduce the number of premaxillary teeth through fusion during ontogeny in rhynchocephalians. Thus, these differences indicate that the specimen from Schamhaupten can neither be referred to Kallimodon pulchellus, nor to Leptosaurus neptunius, indicating that rhynchocephalian diversity in the Late Jurassic of southern Germany was higher than currently recognized. However, a formal description of this specimen as a new taxon has to await a thorough revision of all of the available materials (and thus an evaluation of all proposed taxa) from these deposits.