Heleosaurus
| Heleosaurus | ||||||||||||
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Drawing of the holotype of Heleosaurus scholtzi |
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| Temporal occurrence | ||||||||||||
| Middle Perm | ||||||||||||
| 265.1 to 259.9 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Heleosaurus | ||||||||||||
| Broom , 1907 | ||||||||||||
| Art | ||||||||||||
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Heleosaurus is an extinct genus of varanopid synapsids from the Middle Permian of South Africa . Fossils of the up to half a meter long animal come with high probability from the Tapinocephalus assemblage zone of the Beaufort group of the South African Karoo main basin . There is only one species , Heleosaurus scholtzi , which was first described in1907 by the South African paleontologist Robert Broom . As one of the few representatives of the South African varanopids, Heleosaurus plays an important role in determining the relationships within the monophylum of the varanopids. Like all varanopids, Heleosaurus was a carnivore.
classification
Heleosaurus scholtzi was described as an early diapsides reptile by Robert Broom in 1907 . Broom put the kind with outwardly similar taxa to the Permian and Triassic Eosuchia , which were thought to the evolutionary basis for living diapside reptiles like tuatara , lizards , snakes and crocodiles were and their fossil ancestors. For a long time heleosaurus than the family of Younginidae belong. The integrated armoring, the s-shaped, diapsid-like curvature of the femur and the serration of the teeth were cited as reasons for this classification.
After doubts about this view had arisen as early as the 1980s, a better understanding of the anatomy and evolution of the Varanopids led to a reassessment of the taxonomic classification of Heleosaurus .
The specimens of Helosaurus come from the Tapinocephalus assemblage zone of the upper Abrahamskraal Formation of the South African Karoo Basin , which is dated to an age of approx. 270 million years.
description
The only species, Heleosaurus scholtzi , differed from other varanopids by the characteristic ornamentation of the angular and surangular lower jaw bones . Despite the serration of the marginal teeth, which appear compressed on the lip and tongue sides ( labiolingual ), the largest ones were not elongated as in other varanopids . The abdominal surface of each vertebra had a transverse, broad crest which was divided by a longitudinal notch. This feature distinguishes Heleosaurus from Elliotsmithia .
Individual specimens of Heleosaurus had a head body length of about 30 centimeters. Including the tail, Heleosaurus measured about 50 centimeters. The snout was low and sloping, the cheekbone (os jugale) and the os quadratojugale were covered with small, squashed bumps or nodules. Heleosaurus shared this trait with the closely related genus Mesenosaurus . As with Mesenosaurus , a particularly large tubercle sat on the underside edge of the jugale at the level of the postorbital process. The os quadratojugale was elongated and reached far below the temporal window so that it almost touched the maxilla . A narrow notch along the hem for the attachment of the scale leg . This indicates that the dandruff leg front (anterior), as with most other small Varanopiden far (under the lateral lateral handed) Temporalfenster.
The ploughshare was long and narrow, with a single row of small teeth on its medial edge. Here, too , Heleosaurus shows strong similarities with Mesenosaurus . The arrangement of the two bones of the palatine bone , which forks near the internal naris, is similar to that of Mesenosaurus . The palatine bone thus reached too far back (posteriorly) for a second temporal opening to find space, as in the diapsids. This finding supports the assumption that Heleosaurus belongs to the Varanopids, i.e. was a synapsis.
Heleosaurus had slender ribs that were only connected to the vertebrae by a joint , i.e. they had only one rib head. The ends of the ribs towards the trunk (proximal) were also narrow. The heads of the presacral ribs were triangular and broadened. Mycterosaurus and Archaeovenator had similar characteristics . Below the ribs, very narrow abdominal ribs (gastralia) can be seen, which are only about a third of the width of the ribs and about 20 millimeters long.
An important feature of Heleosaurus were osteoderms (skin bone plates) that lie along the back of the spine . The bone plates had a square or slightly rounded shape and were arranged in rows with up to five plates. The genus Elliotsmithia had similar osteoderms. The diameter of the bone platelets decreases from the center to the sides. Permian diapsids such as Youngina also had osteodermic armor, which was one reason for the original classification of Heleosaurus as a diapsis.
The morphology of the femur is very similar to that of Mycterosaurus and Varanops . The bone is slender, elongated and has an S-shaped curvature. The condition of the shaft and the rolling mound is very different from that of Youngina.
Paleoecology
An aggregation of Heleosaurus fossils indicates that Heleosaurus operated brood care. In South Africa, for example, the well-preserved remains of five individuals - one adult and four young animals of the same size - were found close together. Due to the good and almost complete state of preservation as well as the narrow, one-way arrangement of the fossils, Botha-Brink and Modesto rule out the formation of fossil aggregation through alluvium or other transport processes after the death of the animals and assume that the individuals together in a building or shelter died and fossilized. This thesis is further supported by the fact that Heleosaurus was a very rare representative of the Middle Permian fauna of South Africa, so an accidental accumulation of the carcasses can be considered unlikely.
This would be the oldest evidence of parental care for pelycosaurs and amniotes at all, around 140 million years before this could be demonstrated for the dinosaur genera Psittacosaurus and Oryctodromeus . The earliest evidence of social behavior can only be found in the remains of Tropidostoma and Diictodon from the Upper Permian. Brood care meant an enormous survival advantage in a Paleozoic environment dominated by therapsids. Botha-Brink and Modesto see parallels in the brood care of Heleosaurus to the Australian genus Egernia from the skink family .
Individual evidence
- ↑ a b c d e f g h i R. R. Reisz, SP Modesto: Heleosaurus scholtzi from the Permian of South Africa: a varanopid synapsid, not a diapsid reptile. In: Journal of Vertebrate Paleontology. 27 (3), 2007, pp. 734-739.
- ^ R. Broom: On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. In: Transactions of the South African Philosophical Society. 18, 1907, pp. 31-42.
- ^ RL Carroll: Eosuchians and the origin of archosaurs. In: CS Churcher (ed.): Athlon: Essays on Paleontology in Honor of Loris Shano Russell. Miscellaneous Publications of the Royal Ontario Museum, Toronto 1976, pp. 58-79.
- ↑ a b c J. Botha-Brink, SP Modesto: Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. In: Journal of Vertebrate Paleontology. 29 (2), 2009, pp. 389-400.
- ↑ a b c Jennifer Botha-Brink, Sean P. Modesto: A Mixed-Age Classed 'Pelycosaur' Aggregation from South Africa: Earliest Evidence of Parental Care in Amniotes? In: Proceedings: Biological Sciences. Vol. 274, No. 1627, 2007, pp. 2829-2834.