 |
| Map of the main Middle to early Upper Triassic outcrops in the state of Rio Grande do Sul, southern Brazil, highlighting the occurrence of Aleodon (A) and its skeletal reconstruction with available bones (in yellow) based on all known specimens (made by VDPN). |
At one time it became quite tedious writing yet another article about yet another of the transitional fossils that creationists dogma says aren't there. Who wants to read more or less the same thing several times a week with only the details changed? The problem of course is that every fossil is transitional because each represents a snapshot in time of a species that is evolving and changing over time.
But this latest one is interesting in that it is clearly transitional not between one genus and another or even between one family and another but between two or three major taxons. It is transitional between the reptiles and the mammals and comes close to the stem amniote that had earlier split from the amphibians to give rise to the land-based egg-laying tetrapods. This group went on to give rise to reptiles, mammals, dinosaurs and birds. The current consensus in palaeontology is that the amniotes probably radiated from a single ancestral species that had evolved an egg able to be laid out of water, so breaking the dependence of the amphibians on water.
 |
| Specimen UFRGS-PV-0146-T, skull in ventral view with accompanying line drawing. Scale bar equals 50mm. Light gray indicates most affected areas with breakages. Abbreviations: bs/ps basisphenoid/parasphenoid; C, upper canine; co, condyle of exoccipital; I, upper incisor; ju, jugal; max, maxilla; pal, palatine; PC, upper postcanine; pf, paracanine fossa; po, postorbital; pt, pterygoid; sq, squamosal. |
 |
| Specimen UFRGS-PV-0146-T, jaws in lateral view with accompanying line drawing. Scale bar equals 50mm. Abbreviations: c, lower canine; cre, canine reconstruction; d, dentary; pc, lower postcanine; pdc, postdentary bone complex. Dark gray indicates the right jaw and light gray indicates broken bone. |
The scientific importance of this fossil is not the fact that it is transitional, of course. These things are generally accepted without comment by the science community because the facts of evolution and the evolutionary relationships between the taxons is established beyond reasonable doubt. It is interesting in that it may be the first carnivorous proto-mammal found in South America, and indeed the first such proto-mammal found outside Africa.
At this geological period, Africa and South America were part of the same landmass, Gondwana, so the significance is in how widespread this group had become. The debate revolves around whether these fossils are those of a group known as the Chiniquodontidae or those of an Aleodon a morphologically similar genus of probainognathian cynodont, a taxon which evolved in the Triassic period, co-existed with dinosaur precursors and other archosaurs and eventually gave rise to mammals.
The authors of this paper, a team led by Agustín G. Martinelli from Laboratório de Paleontologia de Vertebrados, Departamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul (UFRGS), Agronomia, Porto Alegre, Rio Grande do Sul, Brazil have reexamined these fossils and concluded that they are Aleodon rather than Chiniquodontids.
Abstract
In this contribution we report the first occurrence of the enigmatic African probainognathian genus Aleodon in the Middle-early Late Triassic of several localities from the state of Rio Grande do Sul in southern Brazil. Aleodon is unusual among early probainognathians in having transversely-expanded postcanine teeth, similar to those of gomphodont cynognathians. This genus was previously known from the Manda Beds of Tanzania and the upper Omingonde Formation of Namibia. The Brazilian record of this genus is based upon multiple specimens representing different ontogenetic stages, including three that were previously referred to the sectorial-toothed probainognathian Chiniquodon theotonicus. We propose a new species of Aleodon (A. cromptoni sp. nov.) based on the specimens from Brazil. Additionally, we tentatively refer one specimen from the upper Omingonde Formation of Namibia to this new taxon, strengthening biostratigraphic correlations between these strata. Inclusion of A. cromptoni in a phylogenetic analysis of eucynodonts recovers it as the sister-taxon of A. brachyrhamphus within the family Chiniquodontidae. The discovery of numerous specimens of Aleodon among the supposedly monospecific Chiniquodon samples of Brazil raises concerns about chiniquodontid alpha taxonomy, particularly given the extremely broad geographic distribution of Chiniquodon. The discovery of Brazilian Aleodon and new records of the traversodontid Luangwa supports the hypothesis that at least two subzones can be recognized in the Dinodontosaurus Assemblage Zone.
Martinelli AG, Kammerer CF, Melo TP, Paes Neto VD, Ribeiro AM, Da-Rosa ÁAS, et al. (2017)
The African cynodont Aleodon (Cynodontia, Probainognathia) in the Triassic of southern Brazil and its biostratigraphic significance.
PLoS ONE 12(6): e0177948. https://doi.org/10.1371/journal.pone.0177948
© 2017 The authors.
Published open access.
Reprinted under a Creative Commons Attribution 4.0 International Licence (CC-BY 4.0)
This paper is of course concerned with the minutiae of precise taxonomic relationships and whether one closely related mammal-like reptile was the same or different to another closely related mammal-like reptile. The fact that there is room for doubt is exactly what would be expected of a specimen from close to the point of diversion of these taxon when we would not expect to see major morphological difference, given the small-step-at-a-time nature of evolution.
Where creationists frauds manage to pull the wool over the eyes of their dupes is in first selling them (literally) the idea that the theory of evolution requires one species to give birth to another or to proceed quickly via an intermediate (which was somehow required to leave a fossil record of itself) as though it was trying to be something different, so they have redefined the terms 'microevolution' and 'macroevolution' and present these as the result of different processes. They never say how these supposed different processes are supposed to differ of course, and this deliberate misrepresentation of science then enables them to fool their dupes into believing the non-existence of these imaginary half one thing, half another fossils is a fatal flaw in the theory.
In fact, transitional specimens which are difficult to place exactly in one taxon or another, as discussed in this paper, are precisely what the Theory of Evolution predicts.
'via Blog this'
No comments :
Post a Comment
Obscene, threatening or obnoxious messages, preaching, abuse and spam will be removed, as will anything by known Internet trolls and stalkers, by known sock-puppet accounts and anything not connected with the post,
A claim made without evidence can be dismissed without evidence. Remember: your opinion is not an established fact unless corroborated.