The Evolution of Dinosaurs: Much Conjecture, Little Evidence

The evidence for dinosaur evolution was reviewed, along with the various theories of dinosaur evolution and the evidence for their support. Dinosaurs are commonly believed to have evolved from a small, crocodile-like animal; however, a review of the known fossils provides no evidence for dinosaur evolution from non-dinosaurs, despite the excellent and abundant dinosaur fossil record. This fi nding is very signifi cant because the bones of many of the averageto larger-sized dinosaurs discovered to date are usually fairly well preserved due to their large size and thickness. Dinosaurs appear abruptly in the fossil record and disappear just as suddenly. The fossil fi ndings for several major dinosaur species also were reviewed. So far, based on the many thousands of nearly complete skeletons, plus multi-thousands of partially complete skeletons, around 400 to 700 different dinosaur species have been identifi ed (Novacek, 1996). Furthermore, a large collection of teeth and even some soft tissues have been preserved (Hwang, 2005; Lingham-Soliar, 2008). Since abundant fossil evidence exists, if dinosaurs evolved from some primitive precursor, good fossil evidence for their evolution from their earlier ancestors should have been uncovered by now. However, the extant fossil evidence does not support their evolution from lower forms of life. The popular meaning of the term “dinosaur” is “terrible lizard” because of their size and assumed ferociousness. They were all terrestrial reptiles—members of the archosauria clade that had scaly skin and hatched their young from eggs. A few were enormous in size, but most were around the size of bulls, and a few were as small as chickens. Dinosaurs were not only huge, but they also “were the fi rst land animals ... designed for speed and agility” (Haines, 1999, p. 14). Most were excellent runners on land, mostly up on their toes due to their hip and ankle construction. Yet, in spite of the abundant fossil record, our “knowledge of dinosaurs is very fragmentary and much that has been written remains speculation,” and “many authors have failed to differentiate between speculation and fact” (Croft, 1982, p. 9). Although much has been learned since these words were * Jerry Bergman, Ph.D., Biology Department, Northwest State College, [email protected] Accepted for publication June 27, 2009 120 Creation Research Society Quarterly written, it is still true that we know comparatively little about dinosaurs, partly because most of our knowledge is based on footprints, bones, teeth, and a few body parts such as scales. These parts make up only about ten percent of the animal (Croft, 1982). The many major unknowns include their specifi c diet (although, judging on structures such as teeth, most types are classifi ed as herbivores or carnivores) and whether they were ectotherms, or cold-blooded (the common view in the past), or endotherms, warm-blooded (the view that much accumulated evidence supports) (ex. DeYoung, 2000, pp. 94–98). Dinosaur Taxonomy Dinosaurs are part of the archosauria (ancient lizard) clade that includes thecodontians saurischians, ornithschians, crocodilians, and the fl ying pterosaurs (Weishampel et. al, 2007). The only members of the archosauria clade still alive today are crocodiles and alligators (Parker, 2000). Dinosauria is divided into two signifi cantly distinct dinosaur families, those with birdlike hips that point downward and toward the tail, the ornithischians, and those with lizardlike hips that point downward and to the front, the saurischians. The saurischians include some small, slightly built reptiles and others that are large fi erce animals believed to have evolved before dinosaurs. So far, all “attempts to relate these two types of dinosaurs to the Triassic pseudosuchians” are problematic because “there appears to be a puzzling overlap in time between the two groups,” and, so far, “possible evolutionary links between them obstinately refuse to appear” (Cox, 1976, p. 314). The saurischia are divided into the theropods (beast feet), which walked on two three-toed birdlike feet with sharp claws, and the sauropods (lizard feet), which walked on four feet and had small heads, long necks, and bulky bodies such as apatosaurs (Cranfi eld, 2002). The ornithischians were a very large and varied group (Parker, 2000). This classifi cation also has come under fi re. Forster (2000, 46) wrote that “most paleontologists now feel that we simply need to stop considering the Dinosauria as being composed of only the Saurischia and Ornithischia.” Among the reasons is that paleontologists know almost nothing “about the early evolution of these creatures, and in particular, the evolution of the dinosaurs before the saurishian-ornithischian split” (Forster, 2000, p. 46). The taxonomy in paleontology that formed the basis of modern taxonomy was problematic from the beginning of the discovery of dinosaurs. As E.D. Cope and O.C. Marsh vied for the glory of fi nding spectacular dinosaurs and mammals in the American West, they fell into a pattern of rush and superfi ciality born of their intense competition and mutual dislike. Both wanted to bag as many names as possible, so they published too quickly, often with inadequate descriptions, careless study, and poor illustrations. In this unseemly rush, they frequently gave names to fragmentary material that could not be well characterized and sometimes described the same creature twice by failing to make proper distinctions among the fragments ... both Cope and Marsh often described and offi cially named a species when only a few bones had been excavated and most of the skeleton remained in the ground (Gould, 1991, p. 87). In spite of years of intensive effort, major disagreement still exists among the experts on dinosaur classifi cation, which is one reason why determining their phylogeny is so diffi cult for paleontologists. The most recent taxonomy proposal is not based on evolution or fossil trees but cladistic analysis using 107 anatomical traits (Weishampel et. al, 2007). The fact is, how “closely related one fossil animal is to another is very much a matter of opinion” (Horner and Lessem, 1993 , p.128), and this is one reason why so much disagreement exists about their phylogeny. Another problem is about half of all putative species are known only by “a few teeth or bone scraps” (Horner and Lessem, 1993, p. 128). The Origin of Dinosaurs Dinosaurs were abundant in number and variety around the world by the Late Triassic (Forster, 2000, p. 49). Their variety and abundance coupled with a lack of any empirical evidence for their evolution has resulted in many phylogeny proposals. One of the most common phylogeny theories today is that dinosaurs evolved from an alligator-like reptile. Haines (1999) wrote that there is still much controversy about how and when dinosaurs evolved. But the most popular current theory has dinosaurs fi rst appearing as small, twolegged carnivores in the mid-Triassic, around 235 million years ago with a combination of features that marked them as different (p. 14). The Archosaura reptiles (from which some believe the dinosaurs have descended) are thecodonts that fi rst appeared in the fossil record during the Triassic (Benton, 1984). Thecodonts, a term meaning “socket-toothed,” were large, heavy crocodile-like animals that crawled low to the ground and on all four legs. They had long jaws and tails similar to crocodiles, and for this reason some argue that they were only a type of primitive crocodile. Other experts argue that thecodonts were an offshoot or branch of the line that led to the dinosaurs. The theory is that a thecodont’s (or some other Archosaur’s) limb position evolved to allow the dinosaur precursors to walk in a more upright position until they eventually could walk on their back legs, becoming the dinosaurs that we know today Volume 46, Fall 2009 121 from the fossil record. This speculation is not directly based on evidence but is the most plausible conjecture postulated for dinosaur evolution because all other possibilities are even less tenable. No fossil evidence exists for this widely accepted theory, or for any of the other less accepted theories. Another candidate for the earliest direct dinosaur ancestor is a housecat-sized animal named Lagosuchus, believed by evolutionists to have lived 235 million years ago in Argentina (Horner and Lessem, 1993). Some paleontologists speculate that “Lagosuchus or one of its relatives may have been the ancestor of the dinosaurs” because they possessed “many of the features thought to be present in [the] oldest dinosaurs” (Forster, 2000, p. 44). From the fragmentary remains recovered so far, Forster (2000) concludes that Lagosuchus is “probably not the ancestor” of dinosaurs but “is at least closely related to the ancestors of the dinosaurs” (p. 45). Others argue that yet some other Archosaur that appeared in the late Permian, many of which strongly resemble crocodiles, were their ancestor (Richardson, 2003, pp. 40–41). One theory popular for years is that some amphibian crawled out of the water, adapted to land, and eventually evolved into the Crocodylotarsi (crocodile ankle) that later evolved into the dinosaurs and the Ornithosuchia (bird-crocodile), which became the crocodilians (ex. Forster, 2000, p. 44). Furthermore, the thecodontians are theorized to have given rise to theropods, which gave rise to the saurischians, then the sauropods, camosaurs and coelurosaurs (Croft, 1982). The thecodontians also gave rise to the ornithischians, which gave rise to the ornithopods, and stegosaurs. From these groups evolved pachycephalosaurs, hadrosaurs, ceratopsians, and ankylosaurs (Croft, 1982). In 1990 three widely accepted hypotheses of carnosauria (meat-eating dinosaurs) origins existed. One hypothesis was that prosauropods were direct descendants from certain thecodontians. Another hypothesis is that carnosaurs were one monophyletic group called theropoda, which evolved from Podokesauridae (Weishampel, 1990). Another theory is that carnosaurs evolved from a primitive coelurosaur-like animal, a group of birdlike dinosaurs. These many theories are all unconstrained by fossil evidence but rather rely on morphological comparisons and conjecture. Consequently, the imaginations of Darwinists are allowed great freedom in developing hypotheses. Some evolutionists reject all of these theories, concluding that dinosaurs evolved from some “unspecifi ed quadrupeds” (Weishampel, 1990 p. 193). The earliest known ornithischian dinosaur is Pisanosaurus, known by only one poorly preserved badly weathered fragmentary skeleton discovered by Galileo Scaglia in Argentina (Forster, 2000, p. 46). Only some jaw parts, a shoulder blade fragment, parts of the hind leg, and a few vertebrae were found. Based on the small, blunt teeth that lie side by side in the jaw, it was fi rst judged to be a very early ornithischian (Forster, 2000). Although the teeth are characteristic of ornithischians, and not either herrerasaurids or saurischian dinosaurs, some paleontologists are not convinced that Pisanosaurus is even an ornithischian dinosaur. The fact that it was a small, lightly built creature only as large as a medium-sized dog indicates that it may not be a dinosaur at all, but rather an extinct animal of some other type. It is not known if it walked on two or four legs, but evidence suggests that it may have been bipedal (Forster, 2000). So much controversy over dinosaur origins exists that some argue for diphyletic (having two separate) origins, others for three or four or more separate origins from different stem archosaurs (Fastovsky and Weishampel, 2005). In the 1970s a revolution in dinosaur origins occurred, uniting saurischians and ornithischians, two very different animals, as well as birds, into one clade, an idea fi nally widely accepted by the mid 1980s. Also, the group class Thecodontia has now been abandoned by many paleontologists. Although the monophyletic view now dominates, evidence for “multiple roots of Dinosauria might still exist and in fact may be more obvious now that the cover of ‘Thecodontia’ has been blown” (Fastovsky and Weishampel, 2005, p. 91). The reason for these disagreements is because these theories are based largely on speculation, not fossil evidence (Fastovsky and Weishampel, 2005).