Hylonomus
| Hylonomus | ||||||||||||
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Life picture of Hylonomus lyelli |
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| Temporal occurrence | ||||||||||||
| Westfalium A ( Bashkirium , Upper Carboniferous ) | ||||||||||||
| 315 million years | ||||||||||||
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Hylonomus | ||||||||||||
| Dawson , 1860 | ||||||||||||
| Art | ||||||||||||
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Hylonomus is an extinct, lizard-likegenus of reptiles . She lived in the Upper Carboniferous around 315 million years ago in the Canadian province of Nova Scotia . Hylonomus is certainly one of the oldest representatives of the amniotes , the "real" land vertebrates. The type species Hylonomus lyelli is currently the only recognized species.
etymology
The name Hylonomus is the Latinized version of the ancient Greek ὑλο-νόμος, which means something like 'forest dwellers'. The name refers to the fact that the type material was recovered from the inside of a fossil tree stump, which in turn is part of an entire fossil forest. Described Hylonomus and the way Hylonomus lyelli from schottischstämmigen Canadian paleontologist Sir John William Dawson (1860). Dawson chose the specific epithet lyelli in honor of Sir Charles Lyell , one of the most famous geologists of the 19th century, with whom he had investigated the later site of Hylonomus in 1852.
Find location and taphonomy
So far the only place where Hylonomus has been found is the headland Coal Mine Point in the world-famous cliff of Joggins in Nova Scotia, Canada. The corresponding section of the cliff consists of 315 million year old sand , silt and clay stones , including thin coal seams , from the Westfal A ( Bashkirium ), which are grouped together under the name Joggins Formation. A special feature of this locality is the preservation of tetrapod fossils inside the upright fossil stumps of club moss trees ( Sigillaria , Lepidodendron ). The material used to describe Hylonomus was also discovered in such a tree stump. Even the more complete Hylonomus specimens are mostly completely disarticulated, i.e. In other words, they are no longer in the original (anatomical) context, but the skeleton has fallen apart and the bones have got mixed up. However, the bones themselves are relatively well preserved.
This very special and comparatively rare form of conservation could have come about as follows: During floods as a result of the break of a natural river dike (Levee), large quantities of sand were washed into the swamp forest in a short time, which buried it several meters high near the break. The trees could not stand this and died. The upper parts of the trees rotted away and only the buried, sand-enclosed lower parts of the trunks remained. These stumps were hollow and over time, leaves and other organic matter would collect and rot in them. This attracted detritus-eating arthropods (most likely millipede-like forms or insects ). It is therefore possible that individual individuals of Hylonomus either accidentally fell into the hollow stumps while hunting these arthropods and did not come out again and ultimately perished there, or that they moved relatively safely and naturally in the hollow trunks, also as a habitat and died a natural death in it. After the sediment was flooded in again, the cavities in the stumps were also filled, creating the conditions for the stumps and their contents to be converted into fossils.
features
Hylonomus is a typical generalized representative of the early amniotes: it was relatively small (the reconstructed total length of the body, including the tail, is approx. 20 cm, with a skull length of a little more than 3 cm) and resembled a modern lizard in its habitus.
The structure of the skull of Hylonomus essentially corresponds to that of other early amniotes, especially that of the somewhat younger representative Paleothyris from the upper middle carbon of Nova Scotia and that of other basal eureptiles e.g. B. from the early Permian of Texas ( Protorothyris , Protocaptorhinus ): The posterior side wall of the skull has no opening ( anapsid skull ) and on the posterior edge of the dorsal (upper) roof of the skull there are still bone elements that most “higher” eureptiles do not are more present (supratemporal, tabular, postparietal). The lacrimale probably extended over the entire length of the snout to the rear edge of the outer nostril. The elements of the roof of the mouth and the anterior part of the parasphenoid (a base of the cranial skull) are covered with numerous small teeth (denticles) and the pterygoid (a palatal bone) has, as is typical for early amniotes, a clearly pronounced transverse process. The stapes (Columella) is relatively massive and most likely did not serve to perceive high-frequency sounds. The individual elements of the jaw each had 35 to 40 small pointed teeth. Two enlarged fang-like (caniniform) teeth in the maxillary at the 6th and 7th position are striking. The preserved dentalia also have two enlarged teeth, but in the 5th and 12th or 6th and 13th positions.
The holotype of Hylonomus has 26 trunk and cervical vertebrae (presacral vertebrae), with one or two more probably still present in the living animal. The maximum number of tail vertebrae preserved per individual is 30. In the living animal, it could have been up to 50. The vertebral anatomy corresponds to that of a typical early amniote with compact vertebral bodies made up of large pleurocentres and greatly reduced hypocenters. Neural arches and vertebral bodies are firmly fused together. In contrast to captorhinids, the neural arches are not thickened ("swollen") and the spinous process is generally low. The first two cervical vertebrae form an atlas-axis complex typical of amniotes. Sacral vertebrae could not be identified on the basis of the fossil material, but the pelvic girdle was probably attached to the spine by a single sacral vertebra.
The pelvis is basically three-rayed (triradiate) and has a similar design to that of paleothyris with an upward-backward, narrowly drawn out ilium ("iliac blade") and a rather flat pubis and ischium ("pubioischiadic plate"). The tarsus shows the compact structure with astragalus and calcaneus, which is characteristic of amniotes. In contrast to basal synapsids (" pelycosaurs ") but similar to captorhinids, the distal tarsus has only one large central center (in "pelycosaurs" there are two smaller distal centralia). The phalangeal formula cannot be reconstructed due to the disarticulation of the material, but it is conspicuous that the terminal (most distal, last) phalanges are longer than the subterminal (penultimate) phalanges, whereas in captorhinids and other early amniotes the terminal phalanges are always longer than the subterminal. The total length of the foot roughly corresponds to the combined length of the upper and lower leg.
Way of life
Hylonomus , like his younger relative Paleothyris , lived in a tropical coal swamp forest . His body size and pointed teeth suggest that he fed on small arthropods and probably actively hunted them.
Systematics
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Dawson (1863) classifies Hylonomus as the only one of the tetrapod genera of the "coal measures" (coal-bearing layers) Nova Scotia in his order Microsauria , which he in turn subordinates to the class Reptilia. He thus apparently regards Hylonomus as an "amniot". However, he also classifies all other Carboniferous tetrapods Nova Scotia among the Reptilia, but as representatives of the order Labyrinthodontia . Neither the microsauria, which today are assigned to the lepospondyli , nor the labyrinthodontia, the representatives of which are now mainly assigned to the temnospondyli , are reptiles or amniotes according to today's view. It is therefore doubtful that the "Reptilia" in the sense of Dawson represent the same concept as in later times, not to mention today. The term “amniota” was not even defined at this point. In addition to Hylonomus lyelli , Dawson (1860) described two other species: H. wymani (in honor of Jeffries Wymans , who was involved in the description of the first tetrapod fossils found in Joggins) and H. multidens .
In a first revision of the tetrapods from Joggins, Steen (1934) considers Hylonomus to be lepospondyles due to its vertebral anatomy. In addition, Steen considered the type specimens of H. wymani and H. multidens to be non-diagnostic and thus both species de facto as nomen dubium - an assessment that is generally accepted to this day.
Carroll (1964) identified Hylonomus again as a reptile due to its combination of features (cranial and postcranial), but this time as a reptile according to a relatively modern view and thus actually as an amniot. Since then, Hylonomus, together with the Protoclepsydrops, which also comes from Joggins, has been the geologically oldest amniot. He was also seen as a so-called "trunk reptile". As such, it was placed in the Captorhinomorpha, which was considered the “most primitive” reptile group at the time, and within which it was again placed in the Protorothyrididae or Romeriidae family .
From more recent, cladistic- based kinship analyzes (e.g. Müller & Reisz, 2006), the view finally emerged that Hylonomus is original, but less original than previously assumed. These cladistic analyzes showed that the genus already lived in different main lines after the amniotes were split up. Accordingly, Hylonomus is a basic representative of the so-called Eureptilia , a line to which the Diapsids and thus all recent reptile groups as well as the birds belong.
The fact that representatives of "higher" lines of the sauropsids (monophyletic reptilia including birds) already lived in the Upper Carboniferous suggests that the separation of the sauropsid and synapsid lines could have occurred in the earliest Upper Carboniferous and the origin of the amniotes thus possibly as far as the Lower Carboniferous goes back.
Others
Due to the importance of Hylonomus for the understanding of the development history of the terrestrial vertebrates and because the holotype and the only previously known specimens of the genus were found in Nova Scotia, the species Hylonomus lyelli is one of the landmarks of Nova Scotia as a "provincial fossil".
literature
- Robert L. Carroll: The earliest reptiles. Journal of the Linnean Society (Zoology). Vol. 45, No. 304, 1964, pp. 61-83, doi : 10.1111 / j.1096-3642.1964.tb00488.x
Individual evidence
- ^ Henry George Liddell, Robert Scott: A Greek-English Lexicon . Eighth Edition. Harper Brothers, New York 1897, p. 1600, right ( archive.org )
- ^ A b c John W. Dawson: On a Terrestrial Mollusk, a Chilognathous Myriapod, and some New Species of Reptiles, from the Coal Formation of Nova Scotia. Quarterly Journal of the Geological Society. Vol. 16, 1860, pp. 268–277 ( full text on BHL )
- ^ Charles Lyell, John W. Dawson: On the Remains of a Reptile ( Dendrerpeton Acadianum , Wyman and Owen) and of a Land Shell discovered in the Interior of an Erect Fossil Tree in the Coal Measures of Nova Scotia. Quarterly Journal of the Geological Society. Vol. 9, 1853, pp. 58-67 (including J. Wyman: Notes on the Reptilian Remains. Pp. 64-66, and R. Owen: Notes on the above-described Fossil Remains. Pp. 66-67) ( Full text on BHL )
- ^ Sarah J. Davies, Martin R. Gibling, Michael C. Rygel, John H. Calder, Deborah M. Skilliter: The Pennsylvanian Joggins Formation of Nova Scotia: sedimentological log and stratigraphic framework of the historic fossil cliffs. Atlantic Geology, Vol. 41, No. 2-3, 2005, pp. 115-142 ( online )
- Jump up ↑ JH Calder, AC Scott, AC Milner: The Tree Hollow Fauna of Joggins: Ockham's Razor Fells the Pitfall Theory. North American Paleontology Convention, Dalhousie University, Halifax, Nova Scotia, Canada, June 19-25, 2005, Programs and Abstracts. Paleobios. Vol. 25, No. 2 (Supplementum), 2005, p. 28 ( PDF 128 kB)
- ^ A b John William Dawson: Air-Breathers of the Coal Period: A Descriptive Account of the Remains of Land Animals Found in the Coal Formation of Nova Scotia with Remarks on their Bearing on Theories of the Formation of Coal and of the Origin of Species . Dawson Brothers, Montreal 1863, 81 p. ( Scanned microfiche on archive.org )
- ^ Roy Lee Moodie: The coal measures Amphibia of North America. Carnegie Institution of Washington Publication, No. 238.Washington 1916 ( archive.org ), pp. 75 ff.
- ^ Margaret C. Steen: The Amphibian Fauna from the South Joggins. Nova Scotia. Proceedings of the Zoological Society of London. Jhrg. 1934 (Vol. 104), pp. 465-504, doi : 10.1111 / j.1096-3642.1934.tb01644.x
- ↑ Johannes Müller, Robert R. Reisz: The Phylogeny of Early Eureptiles: Comparing Parsimony and Bayesian Approaches in the Investigation of a Basal Fossil Clade. Systematic Biology. Vol. 55, No. 3, 2006, pp. 503-511, doi : 10.1080 / 10635150600755396
- ↑ Provincial Fossil Hylonomus lyelli The Nova Scotia Legislature (Note: contains incorrect information on the etymology of the generic name)