Malawania
| Malawania | ||||||||||||
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| Temporal occurrence | ||||||||||||
| Lower Cretaceous (late Hauterivian to Barremian ) | ||||||||||||
| approx. 130.7 to 126.3 million years | ||||||||||||
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| Scientific name | ||||||||||||
| Malawania | ||||||||||||
| Fischer et al., 2013 | ||||||||||||
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Malawania is a genus of ichthyosaurs . Their only species , Malawania anachronus , livedin northern Tethys during the Early Cretaceous Period . In addition to somecharacteristics typicalfor thunnosaurs, it was also characterized by various archaic properties, aswere typicalfor ichthyosaurs of the Triassic . Fossil remains of an animal werefoundnear Chia Gara in northern Iraq in the 1950sand subsequentlyexaminedby Robert Appleby . Their chronostratigraphic origin was disputed for decades. The disagreement about whether the fossil came from the Triassic, the Jurassic or the Cretaceous meant that it could not be described first . Robert Appleby eventually passed away before he could resolve the age issue.
His work was picked up a few years later by Jeff Liston , who succeeded in determining the time of the fossil from the late Hauterivium to Barremium using a dinoflagellate . It was finally described for the first time in 2013 by a research group led by Valentin Fischer . A phylogenetic analysis by the authors showed the genus to be the closest relative of the Jurassic Ichthyosaurus . It was thus possible to show for the first time that both main lines of the Thunnosaurs survived the mass extinction at the end of the Jura and lasted into the Cretaceous.
features
The autapomorphies of Malawania include a posterior extension on the humerus capitulum and an overall very short, trapezoidal humerus. The radius of Malawania anachronus is also about as large as the intermediate medium and the cervical and anterior vertebrae have trapezoidal appendages. The cross-section eight-shaped ribs of the species identify them as thunnosaurs. It had four fingers on each front fin; the broad fins were very similar in structure to those of Ichthyosaurus . The skeleton of Malawania shows some characteristics typical of thunnosaurs as well as some characteristics that can only be found in more original representatives. These include the compact architecture of the front fins and the morphology of the par- and diapophyses , which fuse with the front edges of the vertebral bodies.
Fossil Material, Distribution, and Stratigraphy
The only specimen of the genus, a large block of rock with remains of a front skeleton ( inventory number NHMUK PV R6682), was found in 1952 in a northern Iraqi wadi near Chia Gara (near Amediye ). There it was possibly used as a stepping stone on a mule path. In 1959 it came to the British Museum (Natural History) , where it was examined by Robert Appleby from 1974 . He was particularly interested in the contradiction between the partly clearly Triassic skeletal morphology and the assumed Jurassic origin of the specimen. Appleby's greatest difficulty was assigning the rock to a geological layer. Since the block had probably covered a certain distance after it was uncovered, it was no longer possible to determine which rock outcrop it came from and thus no age could be assigned to it. Appleby was faced with great difficulties because he had to get the opinions of field researchers who had been active in Iraq from London and then compare them with the rock of the block matrix. The corresponding geologists representing this majority view that the block from the Sargelu formation had come, most likely from the aalenischen Rhynchonella - Zone this formation. The field researchers firmly rejected Appleby's assumption that the fossil could come from a Triassic rock layer: the local formations from this time were free of fossils. Appleby etched some fossil pollen out of the rock, photographed it, and sent the images to Norman Hughes for a palynological examination in Cambridge . Hughes unexpectedly came to the conclusion that it must be early Cretaceous rock. An origin from Cretaceous formations had also previously been denied by the field researchers. Corresponding outcrops are far too far away from the site and the local population is not able to transport such large rocks that far. This discrepancy led Hughes to doubt: did the specimens photographed really come from the fossil block? Or was the enormous time difference between his expertise and that of the field researcher based on confusing the sample? To be certain, Hughes took another sample in London to examine it there. However, he could not find any pollen in it, only remnants of wood and cuticle . Finally, he sent the sample to Harold Dunnington , who compared it with material from the Sargelu Formation and concluded that it came from the Rhynchonella zone there. The dating of the matrix did not lead to a clear temporal assignment and Appleby came to the conclusion that he had simply made a mistake when examining the first sample and from then on no longer tried to solve the question of age palynologically. But the morphology of the skeleton also posed problems for him: In some details the fossil resembled Triassic , in other Jurassic ichthyosaurs. Here, too, no clear time allocation could be made. Ultimately, Appleby tended to treat the fossil as a Jurassic ichthyosaur and assign it to the Sargelu Formation, but here, too, the community of field researchers did not agree on the exact temporal location - Upper or Lower Jurassic. Because of the controversy, a manuscript that Appleby submitted to the journal Paleontology in 1979 and was rejected by an editor with reference to the uncertain dating , ultimately failed . Appleby tried to revise the case in the decades that followed and was finally given a commitment to publish in the late 1980s - provided he could determine the age of the block without contradiction. Appleby initially postponed the determination in favor of a monograph on ichthyosaurs, but died shortly before completion in 2004 of pneumonia .
Appleby's manuscripts on ichthyosaurs were viewed by Jeff Liston after his death and checked for usability. The Iraqi fossil block also came into Liston's focus. He studied Appleby's correspondence with various researchers, tried to reconstruct the history of research and in 2008 took new samples which were examined by Stephen Brindley and Riding for microfossils . As in the sample examined by Hughes, mainly decomposed organic material was found, but also pollen, spores and dinoflagellate cysts . The latter enabled the block to be clearly dated this time: They could be assigned to the species Muderongia staurota , which is a key fossil for the late Hauterivium and the Barremium (130.7–126.3 mya ). With this result the age question could be clarified convincingly and the taxonomic classification of the fossil could be tackled. According to this dating, the lower Sarmord Formation or the lower Balambo Formation , which are exposed near Chia Gara and originate from the early Cretaceous period, are considered possible original layers for the specimen .
Systematics
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| Systematic position of Malawania according to Fischer et al. (2013). The sister taxon of the genus is Ichthyosaurus . The authors refrained from grouping together in a family Ichthyosauridae because they regarded this relationship as provisional. |
After the fossil was clearly dated, Liston turned to Valentin Fischer and Darren Naish to take care of its description. Fischer had previously described a number of other ichthyosaurs from the Cretaceous period, which relativized the image of a mass extinction in the group at the end of the Jura. Both he and Naish had already worked with Liston when it came to the first description of the ichthyosaur Acamptonectes . In Liston's eyes, Fischer's previous research work and his eye for the fossil's archaic physique in many details spoke in favor of continuing Appleby's work. Unlike Appleby, Fischer no longer had to rely on the ambiguous expertise of geological field researchers and was not blocked in his work by the idea of ichthyosaur mass extinction, to which Liston attributed the company's success.
The first description was published in 2013 by Fischer, Naish, Liston, Riding, Brindley and Pascal Godefroit . The late Robert Appleby was co-authored in recognition of his preparatory work. Based on the fossil, the authors established Malawania as a new genus with the species Malawania anachronus . The generic name derives from the Kurdish " Malawan“(Swimmer) off. The specific epithet refers to the typical Jurassic characteristics of the species, which for a long time were considered untypical for Cretaceous ichthyosaurs. Appleby originally intended the name " Iraqisaurus kurdistanensis " for the species .
Based on the skeletal morphology, the authors classified Malawania in a phylogenetic analysis as a sister genus of Ichthyosaurus . This means that the genus lies outside the Ophthalmosauridae , to which all ichthyosaurs found to date belong to the Cretaceous period. This enabled fishermen and colleagues to question the notion that only a few lines of Thunnosaurs would have survived the mass extinction at the end of the Jura. They refrained from naming the Ichthyosaurus - Malawania clade as "Ichthyosauridae". However, they gave the sister clade of both genera the name Baracromia , under which the lines of ichthyosaurs are summarized, which come from an early Jurassic radiation of the group.
swell
literature
- Valentin Fischer, Robert M. Appleby, Darren Naish, Jeff Liston, James B. Riding, Stephen Brindley, Pascal Godefroit: A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs . In: Biology Letters . tape 9 , no. 4 , 2013, p. 1–6 , doi : 10.1098 / rsbl.2013.0021 .
Web links
- Jeff Liston: Malawania - swimming against the tide of 'informed' opinion. In: Mr. Wood's Fossils , www.mrwoodsfossils.blogspot.co.uk, May 15, 2013.
- Darren Naish: Malawania from Iraq and the Cretaceous Ichthyosaur Revolution (part II) . In: Tetrapod Zoology , blogs.scientificamerican.com, May 14, 2013.