Posted: Feb 28, 2010 2:17 pm
by theropod
This is the final (for now) segment regarding dinosaur paleontology from our former home.

I made a statement last evening (2/9/10) that I would present an essay on the evolutionary pathways which lead to the emergence of dinosaurs. Hopefully I will be able to show, with the assistance of peer reviewed literature, that dinosaurs did not magically appear in the fossil record. It seems there are some people that hold to the view that because the fossil record is incomplete this implies that the fossil we do find are not indicative of a valid evolutionary pathway. Nothing could be further from the truth, and is not possibly based on anything but conjecture. This conjecture, as colorful as any imaginative writing in history, does not reflect the facts, and only serves to confuse the lay person. It is my intent to provide hard evidence that at no point was magic, or supernatural causes, involved the rise of dinosaurs.

The fossil record is a fragile set of data points based on the remains of once living creatures. The act of fossilization is neither perfect nor assured for all creatures that die. Once an organism has become a fossil there are no guarantees that natural forces will remain constant so that the fossil will be retained across an extended stable temporal setting. Plate tectonics subduct great masses of fossil bearing strata on a continuous basis, and destroy any fossils contained therein. Erosion of strata is also constant and greatly contributes to removing fossils from the geologic column via mechanical reduction and disruption. Even when the fossils are not lost due the effects of erosion but rather redeposited the "founding condition" information is lost and with it a great deal of vital data. Other strata which contain fossils are buried so deeply that any efforts to study them is either cost prohibitive or physically impossible to access. In some cases forests have overgrown fossil bearing strata and make field work a fools errand. This leaves us with the study of what fossil bearing strata remains. Obviously the older a given depositional setting is the more challenging it becomes to recover the fossils, because of the forces cited earlier, as we examine progressively older deposits. These challenges to paleontology have been well understood and in fact studied carefully so as to gain a predictive prospective prior to starting field work.

Many people unfamiliar with the methodology paleontologists employ have a misconception that all we do is dig up bones and make assumptions. Nothing could be further from the facts. Paleontology is as much a "true" science as any other scientific endeavor with more attention paid to comparative anatomy than most medical practitioners. Minute details of a femoral articulation, for example, across several species allows for construction of a cladistic analysis that focuses on the differences and similarities. These studies have allowed us another avenue to better understand how the forces of evolution influenced the fossils we recover.

Before there were any creatures that can be classified as true dinosaurs there had to have been progenitors, according to the theory of evolution. This is true for the first dinosaurs as it is for all creatures. According to our current understanding of the pathway from which dinosaurs emerged this event happened sometime over 230 million years ago. Note that I attempted to use references with the most recent work I could locate so as to present our most up-to-date understanding of the subject.

Please consider the following publication which was published only a few months ago.

Biological Reviews
Volume 85 Issue 1, Pages 55 - 110

Published Online: 6 Nov 2009
DOI: 10.1111/j.1469-185X.2009.00094.x
Journal compilation © 2010 Cambridge Philosophical Society

"The origin and early evolution of dinosaurs"

Max C. Langer, Martin D. Ezcurra, Jonathas S. Bittencourt and Fernando E. Novas


The oldest unequivocal records of Dinosauria were unearthed from Late Triassic rocks (approximately 230 Ma) accumulated over extensional rift basins in southwestern Pangea. The better known of these are Herrerasaurus ischigualastensis, Pisanosaurus mertii, Eoraptor lunensis, and Panphagia protos from the Ischigualasto Formation, Argentina, and Staurikosaurus pricei and Saturnalia tupiniquim from the Santa Maria Formation, Brazil. No uncontroversial dinosaur body fossils are known from older strata, but the Middle Triassic origin of the lineage may be inferred from both the footprint record and its sister-group relation to Ladinian basal dinosauromorphs. These include the typical Marasuchus lilloensis, more basal forms such as Lagerpeton and Dromomeron, as well as silesaurids: a possibly monophyletic group composed of Mid-Late Triassic forms that may represent immediate sister taxa to dinosaurs. The first phylogenetic definition to fit the current understanding of Dinosauria as a node-based taxon solely composed of mutually exclusive Saurischia and Ornithischia was given as "all descendants of the most recent common ancestor of birds and Triceratops". Recent cladistic analyses of early dinosaurs agree that Pisanosaurus mertii is a basal ornithischian; that Herrerasaurus ischigualastensis and Staurikosaurus pricei belong in a monophyletic Herrerasauridae; that herrerasaurids, Eoraptor lunensis, and Guaibasaurus candelariensis are saurischians; that Saurischia includes two main groups, Sauropodomorpha and Theropoda; and that Saturnalia tupiniquim is a basal member of the sauropodomorph lineage. On the contrary, several aspects of basal dinosaur phylogeny remain controversial, including the position of herrerasaurids, E. lunensis, and G. candelariensis as basal theropods or basal saurischians, and the affinity and/or validity of more fragmentary taxa such as Agnosphitys cromhallensis, Alwalkeria maleriensis, Chindesaurus bryansmalli, Saltopus elginensis, and Spondylosoma absconditum. The identification of dinosaur apomorphies is jeopardized by the incompleteness of skeletal remains attributed to most basal dinosauromorphs, the skulls and forelimbs of which are particularly poorly known. Nonetheless, Dinosauria can be diagnosed by a suite of derived traits, most of which are related to the anatomy of the pelvic girdle and limb. Some of these are connected to the acquisition of a fully erect bipedal gait, which has been traditionally suggested to represent a key adaptation that allowed, or even promoted, dinosaur radiation during Late Triassic times. Yet, contrary to the classical "competitive" models, dinosaurs did not gradually replace other terrestrial tetrapods over the Late Triassic. In fact, the radiation of the group comprises at least three landmark moments, separated by controversial (Carnian-Norian, Triassic-Jurassic) extinction events. These are mainly characterized by early diversification in Carnian times, a Norian increase in diversity and (especially) abundance, and the occupation of new niches from the Early Jurassic onwards. Dinosaurs arose from fully bipedal ancestors, the diet of which may have been carnivorous or omnivorous. Whereas the oldest dinosaurs were geographically restricted to south Pangea, including rare ornithischians and more abundant basal members of the saurischian lineage, the group achieved a nearly global distribution by the latest Triassic, especially with the radiation of saurischian groups such as "prosauropods" and coelophysoids.

In the above paper we can see that those examining the evidence have valid reason to suggest that while every single fossil required to establish an evolutionary pathway have not been found leading to true dinosaurs the fossils we do have show just such a relationship. This evidence is based on sound scientific principles, such as the cladistic analysis. In order to refute these findings one needs to show how there cannot be, or never was, any relationship between the dinosaurs and the creatures that are so closely resembling them. Such refutation will require the same diligence and careful examination as was performed in the gathering of data presented above. Without sound data in objection we are left with opinion, which is not a valid method to overturn our current understanding. Considering the lateral supporting evidence, for creatures outside of the dinosaurs, that evolution acts across populations of organisms we would also have to refute all the other instances of speciation we have in support of evolution by common descent.

Of course there are many questions remaining in regarding the issue of just how the dinosaurs emerged. The entire point is that science is an continuous process and paleontology is no different. Those of us that understand the principles of the scientific method can comprehend the problems facing such research. Considering how far back in time we are searching for answers it becomes clear that great accomplishments have been made in this area.

Consider the following:

"Dinosaurian Precursors from the Middle Triassic of Argentina: Lagerpeton Chanarensis"
Paul C. Sereno and Andrea B. Arcucci
Journal of Vertebrate Paleontology, Vol. 13, No. 4 (Jan. 14, 1994), pp. 385-399

We describe the holotype and referred material of the unusual Middle Triassic dinosaur precursor, Lagerpeton chanarensis. The autapomorphies of Lagerpeton chanarensis include the convex and fluted puboischial flange, hook-shaped femoral head, elongate femoral fourth trochanter, fused astragalocalcaneum, tall posterior ascending process on the astragalus, and functionally didactyl pes. Among ornithodirans, Lagerpeton chanarensis is more closely related to dinosaurs than are pterosaurs. We list tarsal and pedal synapomorphies shared by Lagerpeton chanarensis and other dinosauromorphs that are absent in pterosaurs and crurotarsal archosaurs. These include an increase in the relative size of the central metatarsals and subparallel orientation of pedal digit V. There is no evidence that Lagerpeton and other small-bodied dinosauromorphs in the Los Chañares fauna, namely "Lagosuchus" lilloensis, comprise a monophyletic group. The digitigrade pes and the elongate and erect hind-limbs of basal pterosaurs and dinosauromorphs suggest that an obligatory bipedal posture had evolved in early ornithodirans. In extant vertebrates, obligatory bipedal posture is often associated with either cursorial or saltatory locomotor habits. Although these two locomotor patterns are difficult to distinguish on skeletal traits alone, the anteriorly inclined dorsal neural spines, small pelvic girdle, fused proximal tarsals, and very narrow didactyl pes in Lagerpeton chanarensis suggest that it may have been a saltator.

In this paper the authors make note of a species that displays a set of characteristics which very closely approximates the set of skeletal features displayed within dinosaurs. Of particular importance is the puboischial/hip joint morphology. This area is one of a set of diagnostic features that determines a dinosaur as a dinosaur. This feature sets dinosaurs apart form lizards and a great many creatures that shared their world. Quite interestingly is the focus on the bipedal stance observed in Lagerpeton chanarensis in particular and in the Lagosuchus in general which shows this was not a "one time" freak.

Let's look at one final paper:

Science 20 July 2007:
Vol. 317. no. 5836, pp. 358 - 361
DOI: 10.1126/science.1143325

"A Late Triassic Dinosauromorph Assemblage from New Mexico and the Rise of Dinosaurs"

Randall B. Irmis, Sterling J. Nesbitt, Kevin Padian, Nathan D. Smith, Alan H. Turner, Daniel Woody, Alex Downs


It has generally been thought that the first dinosaurs quickly replaced more archaic Late Triassic faunas, either by outcompeting them or when the more archaic faunas suddenly became extinct. Fossils from the Hayden Quarry, in the Upper Triassic Chinle Formation of New Mexico, and an analysis of other regional Upper Triassic assemblages instead imply that the transition was gradual. Some dinosaur relatives preserved in this Chinle assemblage belong to groups previously known only from the Middle and lowermost Upper Triassic outside North America. Thus, the transition may have extended for 15 to 20 million years and was probably diachronous at different paleolatitudes.

Here the authors make note of a temporal span in which the transitional evolution of dinosaurs occurred. Note that they examined the data from several depositional settings. Their work suggest very strongly that the rise of dinosaurs was not sudden nor was this transition taking place at the same rate at all locations. This alone destroys any notion of a special creation for the dinosaurs. The fact that dinosaur ancestors remained extant during this transition also tells us that, in accordance with our understanding of evolution, replacement of species is not required for new and novel life forms to arise. In this case the question, "If dinosaurs came from Lagerpeton chanarensis why were there still Lagerpeton chanarensis after dinosaurs evolved?"

I feel the matter of the evolutionary transitional steps leading to true dinosaurs, while not complete, have been shown to be supported by the evidence. It is not a matter of faith, or assumption, that more specimens and a finer detail of the fossil record will emrge. Paleontology is not a static effort and with each new discovery we learn more about the forces of evolution acting in the past. At no point is there any suggestion from the evidence that dinosaurs suddenly appear in the fossil record, and indeed ancestral fossils have been found. The association of how these ancestors of dinosaurs relate has been shown. Some of those ancestral organisms were not immediately replaced within the biosphere and the rate of dinosaur diversification following their emergence is well documented. Both major branches of the dinosaurs were well established by the end of the Triassic, and they did not show up in the fossil record by some act of magic.


For those interested in a more technical reporting of this matter see the link below. This paper deals more with the post emergence evolutionary forces than the other references cited above, but is still relevant and well worth the read.

"The first 50 Myr of dinosaur evolution: macroevolutionary pattern and morphological disparity"