Paleontological Research in Central Texas
Paleontological research has been conducted in central Texas throughout this century. Dinosaur and other Mesozoic vertebrate bones were reported in the Comanchean Cretaceous rocks near Glen Rose as early as 1887 (Hill 1887; Cope 1894); dinosaur footprints (known to the local Indians as giant turkey tracks) were reported by Shuler in 1917 and 1937. Still, these tracks and fossils were not widely known to the scientific community until Roland T. Bird, a collector from the American Museum of Natural History in New York, rediscovered them while on a fossil hunting expedition in 1938. The story of his Glen Rose adventure was featured in Natural History in 1939. Shortly afterward, Bird had huge slabs of track-bearing limestone removed for display at the New York museum and elsewhere, and paleontologists intensified exploration of these and the surrounding Comanchean deposits (Bird, 1939, 1941, 1944, 1953, 1954; Brown, 1941; Albritton, 1942; Langston, 1960, 1974, 1983; Slaughter, 1969; Slaughter and Hoover, 1963; Perkins, 1971). From their research we now know some of the marine and brackish water fishes, frogs, salamanders, crocodiles, lizards, turtles, ichthyosaurs, dinosaurs, pterosaurs (flying reptiles), small primitive mammals, molluscs, echinoderms, ostracods, and arthropods that inhabited what is now Texas 100 million years ago. Even the microfauna is known: a type of miliolid foraminiferan that characterizes nearshore lagoons, forty-nine genera of pollen and spores, and thirteen genera of spiny organic-walled dinoflagellates and acritarchs (the microplankton that cause "red tide") (Langston, 1983). Gentle ripple marks, mats of algae, animal bore-holes and burrows, in addition to the well-preserved dinosaur trackways, attest to the shallowness of the tidal water. During the Lower Cretaceous, from South Florida to Mexico, a great reef-like organic barrier formed between the open sea and a shallow continental shelf. The shelf was covered by many types of carbonate sediments. Nearshore lagoons and tidal flats had lime mud floors that were sometimes covered by very shallow water and sometimes exposed to air. During periods of exposure, countless dinosaurs and other animals crossed and recrossed the mudflats, leaving imprints of their feet in the soft lime mud.
Even though most of these tracks were quickly obliterated by rainstorms or by the next rising tide, conditions on tidal flats are sometimes suitable for the preservation of trackways. The Comanchean deposits of central Texas are at least as well known for their tracks as for the bony remains of the animals that made them. The rocks containing some of the best dinosaur trackways are composed of alternating layers of terrigenous friable clay marl, soft argillaceous (clayey) limestone, and harder, erosion-resistant limestone. They constitute the 20-foot thick Lower Glen Rose Member of the Glen Rose Formation. This deposit contains most of the alleged mantracks that we examined. Its calcareous or limey layers consist of the remains of countless billions of microscopic skeletal fragments of molluscs, foraminiferans, and algae that lived in the shallow lagoons. Their calcium carbonate skeletons formed the lime mud which later became hardened into limestone. The silty clay-rich layers are composed of lime mud plus a great amount of silt and clay derived from freshwater streams. Periodic, minor floods of these streams washed thin layers of terrigenous silt and clay over the lime mud floors of the lagoons and tidal flats.
In order to be preserved, tracks must be covered by a sediment that contrasts in density and composition with the medium in which they were made. Today we find the best dinosaur tracks near Glen Rose preserved as molds in the hard limestone beds directly underlying beds of much softer terrigenous mudstone or marl. The best of these tracks are simply those that happened to be buried by fine sediments before they could be damaged by other natural processes.
Recent Features in Stone: The Marks of Erosion
We have alluded to the fact that ancient sedimentary rocks contain remnants of actual organisms (fossils) and the traces of their activities (trace fossils). All provide clues to the composition and ecology of life in the past. But exposed surfaces also exhibit features which are recent in originerosional structures. Of course those surfaces may have also been modified by erosion prior to burial. Such primary erosion occurs when the sediments are still fresh and unlithified, whereas modern secondary erosion produces pits, channels and potholes on hard rock. One who isn't a specialist might easily confound primary and secondary erosional features, or might confuse both with trace fossils. Exposed limestone beds (such as those being cut by the Paluxy River) typically show all of these.
Because of its solubility, limestone is severely affected by leaching or karren erosion. Karren erosion occurs on uneven and fissured calcareous beds; it results in the formation of elongated cavities along fractures and depressions which are subjected to increased mineral dissolution by the seepage and pooling of rain water in and around them (Schafersman, 1983; Langston, 1983). It also results in ovoid pitting of exposed surfaces due to the differential dissolution of the cement that holds clastics (sedimentary fragments) together. These elongated cavities and pits are responsible for some of the oddly shaped "manprints" and "toes" that creationists have described, especially on the Park ledge at Dinosaur Valley State Park (viz., Wilder-Smith's Figures 17 and 18, 1968).
Rapidly moving suspension-laden flood water produces elongated erosional channels or cavities running roughly parallel to the direction of river flow, some of which have been mistaken by creationists for mantracks (Schafersman, 1983; Langston, 1983). Some of these may superficially resemble human footprints, more so because they seem to occur "in series," although they differ in shape and size and are separated by uneven distances. Similarly, "insteps," "toes," and "claw marks" can be produced by undercuttingthe differential erosion of soft layers below harder, more erosion-resistant beds. In short, river erosion plays havoc with exposed limestone beds, producing intricate surfaces with numerous depressions. Anyone uncritically looking for shapes of any sort can probably find them. Creationists visiting Glen Rose have displayed vivid imaginations; they have made several human trails, a "Brontosaurus" track, and a "bear" track out of the erosional features on the Park ledge. The surface exposed there is actually a hard, dense limestone or lime wackestone that contains no tracks of any kind (Schafersman, 1983).
For example, Figure 15 shows the feature that was identified by Stanley Taylor and his crew in Footprints in Stone and later by other creationists, including John Morris, as a probable "bear" track. Notice that the so-called claw marks occur in a single soft layer which can be traced along the outline to the left side of the "track" where deep undercutting occurs. Taylor counted five "claw marks"; one can actually count six indentations, but that is irrelevant since none of them is an actual claw mark. These creationists apparently failed to notice that these "claw marks" occur in a single erosion-susceptible bedding plane. They also failed to notice the undercutting that so characterizes erosional features. Actual tracks cannot exhibit such features, since even if a foot sinks down far enough to be partially covered by mud, that mud would slide in or be scooped out by the foot upon removal and would not remain as a rounded overhang. Far from being an enigma that turns the geological time table upside down, this "track" is a simple accident of erosion.
Ancient Features in Stone: Trace Fossils
Not all holes in rocks are due to erosion, of course, and only a stone's throw from the Park ledge are some limestone beds that contain the dinosaur footprints for which the park is famous (see Figure 16). Tracks may be preserved as molds or casts; with burrows they are called trace fossils because they are records or traces of the activities of ancient organisms and not the remains of the organisms themselves. Ichnologists specialize in the study of trace fossils, and ichnology is now recognized as a subdiscipline of paleontology valuable for the solution of traditional paleontological problems (see Sarjeant, 1975, and other chapters in the same book). Some remarkable trace fossils are known from ancient rocks: trails produced by the pectoral fins and tails of walking fish, locomotor traces of other bottom-dwelling fish, swim marks of dinosaurs barely touching the bottom with their toes (Sarjeant, 1975; Coombs, 1980), and so on. Numerous invertebrate burrow and trail marks have been identified (Frey, 1975; Crimes and Harper, 1975), but many have not received proper study. Some animal or moving object undoubtedly made the marks creationists have called "wheel tracks" at the Thayer site near New Braunfels, for example, but proper identification remains to be made.
Ichnologists study trace fossils in order to gain an understanding of the lifeways and habitats of the organisms that produced them. From the form and depth of a track, an ichnologist can tell whether it was made on a hard surface, in shallow water, or in deep water. A group of footprints in series can reveal even more, because relationships between tracks change when gait and speed shift, so track data can be used to reconstruct the gait and speed at which the animal was moving (Alexander, 1976; Thulborn, 1982).
Of particular interest to us is the fact that it is often possible to identify the trackmaker from anatomical features present on the track. (Some caution must be exercised here since we seldom find skeletons of animals that dropped dead in their tracks!) At the very least, the animal family or order to which the trackmaker belonged can usually be identified. Because extinct genera are usually known from the bony remains of far fewer species than actually existed, it is unwise to attempt fossil track identification at the species level. Besides, the feet of species within the same family may be so similar that it becomes nearly impossible to distinguish their tracks even if their foot anatomy is known. Indeed, the footprints made by early members of the hominid family, the australopithecines, are strikingly similar to those of modern humans, despite some differences in the bony anatomy of modern human and australopithecine feet.
Because of the taxonomic problems which invariably arise when traditional taxonomic names are used for the identification of trace fossils, ichnologists have opted for a separate system of classification of tracks. This means that a particular dinosaur such as Acrocanthosaurus may have made particular tracks that are clearly recognized as compatible with the known foot anatomy of this animal, yet paleontologists will avoid asserting that these were the tracks of Acrocanthosaurus. They prefer to give the tracks a new genus nameIrenesauripus in this caseand to note the affinity and probable association of the two. Taxa based on trace fossils are called ichnotaxa; a genus name for a track is thus an ichnogenus. While this produces a proliferation of names, the incompleteness of the fossil record makes this a sound taxonomic practice.
Trace fossils are considered problematic when the identity of their maker has not been determined. In the case of footprints, this may occur because the foot anatomy of the actual trackmaker is unknown (or the fossil species or genus is itself unknown). Or a track may not be recognized as belonging to a particular animal even when that animal's foot anatomy is known, either because track preservation is poor or because the track was made in an unusual manner.
Track preservation depends upon a whole host of conditions (Sarjeant, 1975; Mossman and Sarjeant, 1983):
- First, the medium must be able to hold the impression. This means that it must be fine-grained and cohesivenot so coarse that it fails to register details of the undersurface of the foot, not so wet that it is deformed immediately upon withdrawal of the foot, and not so dry that it is easily damaged by wind. If the water table is so high that water fills the bottom of the imprint, some of the details of surface anatomy will be lost. If the medium is too hard or dry when first stepped on, it will not register the entire undersurface of the foot.
- The medium should be resistant to damage by light rain. Such resistance is enhanced if a track mold made in a moist medium partially dries and hardens before burial.
- 3) The track must be "cast" before it is damaged or obliterated by wind, water, or trampling. That casting medium must differ in consistency and composition from the molding medium so that it will separate easily after lithification has occurred, and it must itself harden and not erode away.
While it may be rare that these conditions are met for any single trackway, it should be obvious that whenever there is a wash of suspension-laden water from freshwater streams over a limey lagoonal mudflat, the exposed trails will vary in condition from excellent to poor; all of these will be preserved.
The form of a track depends not only on the nature of the substrate in which it was originally impressed and the damage to which it was subjected prior to burial, but the manner in which it was originally made. We saw above that a medium may not register the entire undersurface of a foot. It is also true that an animal may not apply the entire undersurface of its foot to the ground, and that this depends in part on how it is moving.
For example, Sarjeant (1971) described some elongated tracks produced by bipedal reptiles in some Permian deposits in Texas. The tracks showed the imprints of two digitsa large deeply impressed fourth digit which bore most of the weight, and a smaller third digit which was only slightly impressed, probably for balance. These tracks were produced by an animal that probably possessed four or five toes all of which might register in walking. In other words, this animal became functionally two-toed when moving at high speeds. This is not at all unusual. Many animals are "plantigrade" (walk on the entire undersurface of their feet) when moving slowly and "semidigitigrade" or "digitigrade" (support their weight on their toes alone) when running. In the case of bipedal reptiles, most of the weight is transmitted through the central digit (middle toe), and this is the digit that will be impressed most deeply. It goes without saying that the toe impressions produced in this manner do not resemble the slow walking tracks produced by the same species. It would be easy for someone who is not a specialist to "read" them as mantracks.
So far we have been describing primary tracks, and we have seen that these may not faithfully record all of the details of the surface anatomy of the bottom of the foot. Features called undertracks and overtracks are also common, and these are even less faithful to dinosaur foot anatomy. They may appear as vague elongated depressions. At the Thayer site near New Braunfels, Texas, overtracks have been mistaken for manprints, and it is highly probable that other "mantrackways" are in reality the undertrackways or overtrackways of bipedal dinosaurs (Milne and Schafersman, 1983; Langston, 1983).
Undertracks were first described by Heyler and Lessertisseur (1963) in thin-bedded European sedimentary rocks. If the layers are sufficiently thin and yielding, the foot of an animal may produce deformation in one or several layers beneath the surface layer. The impressions made in the underlying beds are actually subtrace fossils which are usually very different in form from the true footprint mold (Figure 17), losing detail downward through several layers.
Overtracks and undertracks form easily in thin-bedded algae-bearing deposits. Dinosaurs crossing stacks of algal mats common on tidal flats sometimes made impressions in several layers of spongy algae-filled mud. After the tracks were made, they filled again with algae and mud, conforming at first to the shape of the footprint mold. Several additional layers hence, the original shape of the footprint was lost-replaced by a vaguely elongated or oval depression representing the deepest portion of the original track (the middle toe). Today when the bedding planes separate such that the primary mold can be seen, three toes are quite distinct. But when they split apart such that the "undertracks" or "overtracks" are exposed, the anatomical features of the footprint are obscure. Sometimes only one or two tracks in a trackway will retain their overtrack fillings but the outline of the primary impression will be visible around them. This is true at the Thayer site where oval overtracks were taken to be mantracks.
Trace Fossils, Mantracks, and Mantricks
A variety of Cretaceous trace fossils have been mistaken for mantracks. These include invertebrate burrow casts of Thalassinoides and at least two ichnogenera of dinosaur footprints: Gypsichnites and Irenesauripus. There is another unnamed ichnogenus that is common in these depositsfootprints that were probably made by the sauropod Pleurocoelus. These, too, may have been mistaken for giant human footprints, but this is uncertain. There are many fewer known tetrapod ichnotaxa than there are known taxa based on skeletal remains in the Comanchean Cretaceous deposits of Texas (see Langston, 1974); it is likely that detailed study of the vertebrate footprints will result in the recognition of more ichnotaxa. It is also likely that each ichnogenus listed below represents several species. The tremendous size variation of Gypsichnites footprints is improbable for a single species (although ontogenetic variation must be taken into account).
Thalassinoides is the ichnogenus name given to a particular type of crustacean burrow system (Kennedy, 1975; Bromley, 1975; Curran and Frey, 1977). A Thalassinoides burrow system is essentially composed of horizontal tunnels and Y-shaped branches and polygons that creationists have mistaken for the pads of "saber-toothed tiger" tracks and for spaces between "toes" of a mantrack (Schafersman, 1983).Similar burrows are known to be made today by thalassinidean shrimp and other organisms that live in shallow muddy estuarine or lagoonal environments (Curran and Frey, 1977).The trace fossil itself is common in shallow-water or supratidal carbonate rocks (Kennedy, 1975); complex burrow systems may form on carbonate substrates when deposition ceases for a while (Bromley, 1975).
The Comanchean dinosaurs belong to two orders: Saurischia (lizardhipped) and Ornithischia (bird-hipped) dinosaurs. The former includes two suborders of relevance hereTheropoda and Sauropoda; the latter includes the suborder Ornithopoda. These suborders have famous representatives that were not in fact present in the Lower Cretaceous. Tyrannosaurus was a large Upper Cretaceous carnivorous theropod; the herbivorous Apatosaurus (formerly known as Brontosaurus) was a Jurassic sauropod. The famous duckbilled, plant-eating Iguanodon was an ornithopod that has no known representative in central Texas, but is known from Lower Cretaceous deposits elsewhere and may have had a representative, as yet undiscovered, in central Texas at the time (Langston, 1974, 1983).
The animals whose remains have been found in Comanchean deposits were thus relatives of the better known dinosaur genera. They include the carnivorous theropod Acrocanthosaurus, the herbivorous ornithopod Tenontosaurus, and the sauropod Pleurocoelus (mistakenly called Brontosaurus in early accounts). See Figure 18.
Acrocanthosaurus was a "three-toed" bipedal dinosaur that actually possessed four toes. The fourth toe, a small clawed hallux, was located somewhat high in the rear of the shank. It probably made the long, slender dinosaur tracks with sharp heel impressions and occasional claw impressions that are common in central Texas (Langston, 1974, 1983). These are given the ichnogenus name Irenesauripus. As we have seen, tracks with elongated heel marks fit prominently in the mantrack controversy.
The foot anatomy of another "three-toed" bipedal dinosaur, Tenontosaurus, fits Gypsichnites better than it fits any other known Comanchean tracks, although known skeletal remains of adult Tenontosaurus are too small to have produced the largest of the Gypsichnites footprints. Gypsichnites tracks range in length from 12 to 24 inches, are broader than those of Irenesauripus, and, in contrast to those of Irenesauripus, show no hallux impression (Langston, 1983). Tenontosaurus is known to have possessed a hallux, but, as Langston suggests, it is possible that the hallux was positioned high enough in the foot to have rarely touched the substrate in normal locomotion. (Think of the dinosaur as walking slightly tip-toe, not always touching the heel to the ground; their feet were very much like those of modern birds.) Langston believes that Gypsichnites tracks may represent several species of ornithopod dinosaurs; what is certain is that they were made by bipedal dinosaurs and that overtracks of some of them have been mistaken for mantracks.
Pleurocoelus was most probably responsible for the large quadrupedal sauropod footprints that made Glen Rose famous when Roland T. Bird discovered them in 1938. The local sauropod hindfoot tracks were made by an animal with four forward-facing toes and another, the dew claw, at the rear. Lee Mansfield believes that the roughly elongated shape of the hindfeet plus the distinctive front toe impressions of this animal gave rise to the local giant mantrack legend (Mansfield in Cole, 1984).It is easy to modify a Pleurocoelus footprint by adding an extra toe at the front. A chisel and some coffee grinds (to smooth out rough edges) will do it. The new "mantracks" that we have seen are not these, however.
Creationist mantrack claims should be evaluated within the context of what is known about life and environments in central Texas during the Lower Cretaceous Period. For example, the notion that tracks in the Glen Rose area were made by animals and humans fleeing a worldwide cataclysmic flood becomes patently absurd in light of the known (quiet) sedimentary environment responsible for the build-up of the Glen Rose Formation. Furthermore, when creationists label certain elongate channels or depressions "mantracks," they ignore hypotheses that explain these features far more adequately. Some of the important facts that creationists fail to take properly into account are:
- Primary erosion can distort dinosaur tracks prior to their fossilization, thereby making them less distinct.
- Secondary erosion of exposed rock surfaces can further obscure genuine tracks that are already fossilized, as well as create elongate channels that may appear to be "in series" (due to uniform river flow). Differential erosion of softer and harder rock layers can form irregularities that may be mistaken for "insteps," "claw marks," or "toe imprints."
- Erosion by leaching can occur in depressions, fissures or cracks in rocks into which rain water seeps. Such erosion can widen cracks and depressions to form features that may superficially resemble tracks.
- Under certain conditions, fossilized burrows of small invertebrates may seem to resemble details of the imprint of a human or other footprint.
- In thin-bedded deposits, animals sometimes leave vague imprints of their feet in the layers of sediment directly underlying the surface into which the original footprints were impressed. In addition, thin layers of sediment washed over the tracks may conform vaguely to the shape of the primary tracks. When hardened, the "undertracks" and "overtracks" formed in this manner will be less distinct than the actual primary footprints, losing detail progressively upward or downward. When these, instead of the primary tracks, are exposed, they can give a false impression of details of the foot anatomy of the trackmaker.
- The nature of the medium in which an animal leaves its tracks affects the appearance of the tracks. Thus, either on a very muddy or on a very dry medium, details of foot anatomy will be lost and individual toe or "heel" impressions may superficially resemble human footprints.
Anyone making scientific claims must first consider alternative hypotheses and attempt to rule out the less parsimonious of them. Extraordinary, sensational claims require extraordinary proof. In this case, creationists have shown remarkable u, nwillingness to consider the simplest and, often, seemingly obvious alternative explanations. As we have shown, there are numerous mechanisms by which elongated features are formed in rock. These explanations fit the evidence far better than the notion that giant humans and dinosaurs lived together on the Texas mudflats during the Cretaceous!