Placodus
| Placodus | ||||||||||||
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Placodus gigas , skeleton reconstruction in the State Museum for Natural History Stuttgart |
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| Temporal occurrence | ||||||||||||
| Anisium to Ladinium | ||||||||||||
| 247.2 to 235 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Placodus | ||||||||||||
| Agassiz , 1833 | ||||||||||||
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Placodus is a genus of diapsid reptiles from the Central Triassic (approx. 247.2 to 235 mya ) of Central Europe and China. The only currently recognized species are Placodus gigas and Placodus inexpectatus . The genus is assigned to an extinct group of marine diapsids, the Sauropterygia (fin lizard).
description
Above: Replica in the Natural History Museum in Leiden (premaxillary probably replaced, the spatula-shaped incisors or their tooth sockets are missing).
Below: Historical drawing from 1876, without premaxillary and without showing the bone sutures (rotated 90 ° counter-clockwise compared to the illustration above). The “plaster teeth” can be clearly seen in the figure below. Note that the maxillary teeth are much smaller than the palatal teeth.
skull
Placodus has a robustly built, wide, triangular skull that is largely closed and about 20 cm long. There is no lower temple window . This comparatively compact skull had developed from a diapsid skull with two temporal windows, but is already so specialized that no closely related non-placodont animal diapsid form can be determined. The squamous bone (squamosum) is large and offered a lot of surface for strong jaw adductors . The relatively wide outer edge of the dental, the tooth-bearing lower jawbone, as well as the extremely strong, high coronoid process in the rear area of the lower jaw branch served the same purpose. In addition, the symphysis, the contact surface at the front ends of the two lower jaw branches, is very long and very ossified. At the rear end of both branches of the lower jaw there is a long retro-articular process pointing backwards. Modern crocodiles also have similar extensions . They serve as levers for the muscles to open the jaw.
Dentition
The dentition consists of three spatula-shaped, protruding teeth on the premaxillary, four teeth on the maxillary, three or four teeth on the dental and a total of six large palatal teeth . The maxillary and palatal teeth are, as for the placodontia , the “plaster tooth lizards ”, generally typical, relatively flat and wide. The palatal teeth are significantly larger than the maxillary teeth, actually form a regular dental plaster and have a thick layer of enamel . The teeth in the lower jaw are also significantly larger than the maxillary teeth and have occluded both with them and with the palatal teeth .
Postcranial skeleton
Its trunk skeleton, which is around 1.5 meters long, shows that Placodus was only moderately adapted to an aquatic way of life. The trunk, equipped with 28 vertebrae (plus three sacral vertebrae), is only slightly longer than that of terrestrial reptiles, and the 40 to 50 caudal vertebrae do not have any greatly elongated neural and hemal processes , as is the case with the oar tails of some Permian and Triassic marine reptiles ( e . B. Hovasaurus ) is the case. The skeleton of the extremities is only incomplete. The humerus is very similar to that of Cyamodus , another genus of placodon animals. Only a few elements of the carpal and tarsus are known, which is probably due to the fact that this area of the extremities was only weakly ossified . The phalangeal formula of the hand is given as 2-3-4-5-3 (4?), That of the foot cannot be reconstructed. The hand and probably the foot were not designed as paddles. However, the shoulder and pelvic girdles were not as tightly connected to the axial skeleton as would be expected in a purely terrestrial reptile. Above the neural processes of the trunk spine, a series of osteoderms ran down the back. The underside of the body was reinforced by well-developed abdominal ribs .
Way of life
Placodus lived on the coasts of the Muschelkalk Sea , an epicontinental European tributary of the western Tethys , as well as on the coasts of the Far Eastern Tethys. Its way of life was roughly similar to that of today's marine iguanas . However, Placodus was not a herbivore, but probably ate hard-shelled invertebrates such as mussels and arm pods . He picked them up from the substrate with the help of the protruding premaxillary teeth and cracked them with the strong, flat teeth that sat on the rest of the jawbone and the palatine bones ( durophagia ). Numerous small foramen and pits in the surface of the premaxillary and maxillary suggest that the snout was very sensitive to touch.
Occurrence
Known remains of Placodus were geographically restricted to Central Europe until the 2000s and stratigraphically to the Muschelkalk group . Located in the Bavarian State Collection for Palaeontology and Geology deposited type specimen of Placodus gigas and numerous other pieces come from the Upper Muschelkalk around Bayreuth . The type locality is the Oschenberg . Other finds within today's territory of Germany come from a. from Steinsfurt near Heidelberg (complete copy of the Senckenberg Museum ), Bad Sulza in Thuringia, Freyburg an der Unstrut , Rüdersdorf near Berlin and from Helgoland . Outside Germany, Placodus is known from the shell limestone of the Paris basin near Lunéville , from the Gogolin formation, the oldest unit of Polish shell limestone, from Gogolin in Upper Silesia and from Winterswijk in the Netherlands. The only specimen that was only discovered in the 2000s but has survived very completely from a site outside Europe comes from the upper part of the Guanling Formation (overlapping in time with the Muschelkalk group) in the Guizhou Province in southern China.
Systematics
Since the first description of the type species Placodus gigas by Louis Agassiz in 1833, * numerous other species have been described, all of which, however, were declared to be representatives of the type species as part of a revision of the genus by Olivier Rieppel in 1995. The species Placodus inexpectatus , discovered in China, was only added in 2008 .
Placodus forms the group Placodontia together with a number of similar, purely Triassic marine reptiles . According to a cladist analysis from 2000, he has a very basic position within this group . A presumed direct relationship with Paraplacodus from the border bitumen zone of Monte San Giorgio , which was taken into account by classifying both genera in the Placodontoidea group, was not confirmed in this analysis. Instead, Paraplacodus, as the most basic representative of the group, is the sister taxon of all other Placodontia, the most basic representative of which is Placodus . The placodon animals themselves are considered to be the most basic clade of the Sauropterygia , a purely marine diapsid group that also contains the plesiosaurs .
Remarks
- *Agassiz and the discoverer of the skull, Georg Graf zu Münster , assumed at the time that it was the remains of a durophage fish. It was not until Richard Owen realized in 1858 that the fossil bones of Placodus belong to a reptile.
Web links
literature
- Olivier Rieppel: The genus Placodus : Systematics, Morphology, Paleobiogeography, and Paleobiology. Fieldiana Geology, New Series, No. 31, 1995, doi: 10.5962 / bhl.title.3301 .
- Michael J. Benton: Paleontology of the Vertebrates (translation of the 3rd edition of "Vertebrate Paleontology" from 2005 by Hans-Ulrich Pfretzschner). Pfeil, Munich 2007, ISBN 978-3-89937-072-0 , p. 164.
- Robert L. Carroll: Paleontology and Evolution of the Vertebrates (translation of the English edition "Vertebrate paleontology and evolution" from 1988). Thieme, Stuttgart 1993, ISBN 3-13-774401-6 , pp. 263-265.
Historical writings
- Tilly Edinger: The placodon animals. 2. The central nervous system of Placodus gigas Ag. Treatises of the Senckenbergische Naturforschenden Gesellschaft, vol. 38, issue 4, Frankfurt am Main 1925, plate XXIV, pp. 311–318
- Fritz Drevermann: The placodon animals. 3. The skeleton of Placodus gigas AGASSIZ in the Senckenberg Museum . Treatises of the Senckenbergische Naturforschenden Gesellschaft, vol. 38, issue 4, Frankfurt am Main 1933, 16 plates, pp. 323–364
- Friedrich von Huene: The Placodontier. 4. On the way of life and kinship of Placodus . Treatises of the Senckenbergische Naturforschenden Gesellschaft, Vol. 38, Issue 4, Part 2, Frankfurt am Main 1933, pp. 365–382
Individual evidence
- ↑ a b Da-Yong Jiang, Ryosuke Motani, Wei-Cheng Hao, Olivier Rieppel, Yuan-Lin Sun, Lars Schmitz, Zuo-Yu Sun: First Record of Placodontoidea (Reptilia, Sauropterygia, Placodontia) from the Eastern Tethys. Journal of Vertebrate Paleontology. Vol. 28, No. 3, 2008, pp. 904–908, doi : 10.1671 / 0272-4634 (2008) 28 [904: FROPRS] 2.0.CO; 2 (alternative full text access : UC Davis ( memento from August 15, 2015 in the Internet Archive ))
- ^ O. Rieppel: The genus Placodus . 1995 (see literature ), p. 2 ff.
- ↑ Olivier Rieppel: Paraplacodus and the phylogeny of the Placodontia (Reptilia: Sauropterygia). Zoological Journal of the Linnean Society. Vol. 130, No. 4, 2000, pp. 635-659, doi : 10.1111 / j.1096-3642.2000.tb02204.x