The Diminutive Cretaceous Fauna of Rufe Snow by George Hansen

Note: part 1 of this article appeared in the November issue; it is included here for continuity.

When commercial development threatens to cover a favorite collecting site under buildings and asphalt, I would argue it's time to make an exception to the amateur's unwritten rule against public disclosure about its location. I first noticed evidence of this about six months ago at an excellent spot in northeast Fort Worth, Texas. Initially, survey flags peppered the nearly forty acres. Now, drilling for soil samples is in progress. I expect that soon the area will be swarming with concrete delivery trucks. This southwest corner of the triangle formed by Rufe Snow Drive, Loop 820 and Industrial Park Boulevard has been a popular one in the region for quite a while. So, I believe it is important to document what I can uncover about its fauna, stratigraphy, and paleoenvironment for those who have been fortunate to have visited and collected there.

The attraction to this exposure comes from the diversity, small size, and excellent preservation of the fossils found there. Ammonites, crabs and other small crustaceans, parts of brittle stars and asteroid starfishes, an echinoid, shark teeth and bony fish vertebrae, pelecypods and gastropods make up the more common fossils. Micromorphic ammonites, pelecypods, and gastropods are replaced by limonite, hematite, and pyrite.

Though this particular paleontological window may be closed in the near future by the fates of enterprise, a descriptive article on it is not without merit. My bit of reading on and small experience with the Cretaceous of north central Texas has led me to other, similar exposures in the region. I hope that what is shared here may provide perspective to those who are exploring these other areas for the exquisite fossils they yield.

Cretaceous Setting

The Texas Cretaceous is divided into the lower Comanche series and upper Gulf series. Within each of these are several groups. The Comanche series includes in ascending order the Trinity, Fredericksburg and Washita; the Gulf includes the Woodbine, Eagle Ford, Austin, Taylor and Navarro groups. The Rufe Snow exposure is situated within the lower Cretaceous Washita group.

The Cretaceous Period began in Texas with seas advancing upon the Wichita Paleoplain from the south and east. Simultaneously advancing seas from the north eventually met with this "Gulf"sea during Eagle Ford time. Maximum transgression during this and the subsequent Austin time resulted in seas covering much of the midwestern interior of the United States. Afterward, this broad inland sea lane retreated once again, and since then marine incursions in this region have been restricted to continental margins. The southern extent of the Cretaceous in the state is marked by limestone of reef origin containing rudistids, corals, and sponges. To the north these phase into terragenic sediments: the rudistids eventually disappear and are replaced by oysters as the dominant faunal component. As it had in the Paleozoic, the North American craton continued to straddle the equator. The marine environment covering much of the continent was shallow and warm.

Stratigraphy and Paleoenvironment

The Washita in north Texas has been known for many years as a stratigraphic record of cyclical deposition, which was due to periodic changes in rates of sedimentation and subsidence. The lower Washita forms a north-south, approximately fifteen-mile-wide band through the middle of Tarrant County. Older formations are exposed generally toward the west side of this outcrop and the younger ones toward the east. Alternating layers of limestone, marl, clay and sandstone in north Texas grade to the Georgetown Limestone in central Texas. During the lower Cretaceous, a shallow basin formed in the ocean floor where north Texas (Fort Worth) is now situated. Basinal subsidence, coupled occasionally with tectonic uplift to the north, alternatively lowered and raised the level of the basin floor relative to the neighboring landmass to the northwest. Terragenic sediments originating from river deltas to the northeast (or northwest at other times) filled the basin quickly, producing further subsidence. In central Texas the lower Washita ocean floor formed a shelf and was less affected by this cycle. As a result, waters there were cleaner and limestone is the significant depositional component. Within the north Texas basin waters cycled between muddy and relatively clear as subsidence and tectonic activity alternately waxed and waned between periods of instability and stability. Coeval sandy facies to the northeast, near the Red River, suggest the shoreline was proximal (within 50 - 75 miles) to the basin.

This cyclical depositional process recurred at different times and in different locations. Repeatedly, small pyritic fossils appear in clay and marl facies around the state. These fossils are so distinctive they caught the attention of early paleontologists. Adkins listed Arca, Engonoceras, Scaphites, Turrilites, Hamites, Cinulia, Nerinea, Lunatia, Turritella, starfishes and numerous, small, non-pyritic crustacea, especially crabs, all as particularly characteristic of rapid clay and marl deposition. This phenomenon was not limited to the lower Washita or Texas either. Similar fossils have been collected from the Del Rio Clay in central and southwest Texas and in the several parts of the lower Cretaceous of Europe and north Africa.

The strata at Rufe Snow are intriguing because three distinct facies are exposed within a relatively narrow range of elevation. From west to east across the 250-yard-wide stretch of the exposure, the elevation rises only about six feet. Lower, flat areas along Industrial Park Boulevard are yellow to dark-brown clay. Proceeding east approximately 75 yards, near the middle of the clay exposure, one finds concentrations of dark-red, ironstone, nodular concretions scattered over much of the area. The elevation is just a bit higher than Industrial Park, and rain has washed shallow gullies into the clay surface. This nodular exposure continues eastward in a band about 50-75 yards wide. A fortunate consequence of the drilling is the exposure of subsurface gray-blue clay that would not have been evident otherwise. Near the highest point of elevation a marly limestone ridge marks a transition to an upper platform of limestone which continues eastward and disappears under a parking lot about 100 yards away.

Just below the ridge, rain gullies in the clay are much deeper than in the flatter areas to the west. Index fossils found in the limestone ridge indicate this exposure is the Pawpaw-Main Street contact. The north side of the clay exposure is bounded by lower, limestone flat. This may be part of the ridge strata.

According to Blake and Reid and references they cited, one of the Washita depositional cycles reached maximum subsidence toward the lower-to-middle Pawpaw. Near the basin floor, water circulation was probably restricted. Throughout the time period of this formation the sedimentation rate continued to be high. When creatures died they were quickly buried under a rapidly depositing, anoxic sediment. The exposure at Rufe Snow marks a transition in the environment of these fauna -- cloudy, muddy water cleared toward the end of the Pawpaw allowing a higher concentration of carbonate deposits to form the limestone now seen as the ridge. The diminutive nature of the fauna found only in the lower clay facies may indicate that during rapid deposition, the environment in which these animals found themselves was relatively stagnant, was cloudy with silt, water circulation was restricted, and oxygen levels were low. When the silt cleared, dissolved carbonate and oxygen increased and larger, clear-water fauna were allowed to achieve greater sizes because key nutritional requirements were again being met. During the subsequent Main Street formation, subsidence and sedimentation rates were relatively low. The concentration of limestone in this formation is consequently higher than the Pawpaw.

The Fauna

The photographs shown here were taken with a camera attached to a stereomicroscope, set near its lowest magnification power (10X). With this it was not possible to capture complete images of fossils larger than about the size of a dime. To get the best possible detail, the magnification was adjusted to fill as much of the image field as possible while minimizing peripheral focal distortion and maximizing depth of field.

On the whole the Pawpaw does not contain many fossils, but they do occur abundantly in localized areas. In Tarrant County the limonite, dwarfed mollusks are a distinctive feature of these special exposures. Specimens are small, usually less than 15-25 mm in the largest dimension. Ammonites exposed on the surface are frequently broken along suture lines, and their pieces are by far the most common specimens found. However, anyone spending 2 to 3 hours at Rufe Snow will leave with several complete specimens of Engonoceras and Mantelliceras. While intact specimens are generally less than 25 mm in size, some ammonite fragments suggest sizes as great as 40-75 mm were achieved. Complete Turrilites and Mariella are rare, but finding specimens with three or more small whorls from older parts of ammonites is fairly common. Many of the very small "normal" ammonites may also be the inner whorls of larger disarticulated fossils. Sometimes molluscs are found in the ironstone nodules, and now and then they can be found with original pearlescent shell material still evident in patches on a fossil's surface. Long after these creatures were buried, iron sulfide filled their cavities, duplicating fine details in all their parts. After the Cretaceous seas withdrew, weathering processes acted on the pyrite, converting it to limonite, and ultimately hematite. As sediments were removed again and fossils weathered to the surface, other chemical processes etched them along growth sutures, weakening these parts, making them susceptible to mechanical stresses associated with displacement. This is a lengthy way to say there is a reason pieces are more common than complete ammonites.

The starfish found at Rufe Snow have been beautifully illustrated in the literature. I can only hope the photos I am showing here provide a useful supplement to that work. I have found pieces of an asteroid arm that were only recently assigned to the Benthopectinids. The newly described fossil is Alkaidai sumralli, named generically after the star Alkaid in the constellation Ursa Major and specifically after Colin Sumrall who found and prepared the holotype. The Benthopectinids are generally considered to be off shelf or basinal, but enough specimens, both modern and extinct fossils, have been found in neritic facies to lead us to believe they were not very sensitive to depth. Regrettably, attempts to resolve adequate identifying features were not achieved with my one specimen of a central ophiuroid disc, so I cannot say more about this fossil other than to show a photo of arm segments.

Unlike southern Tarrant County exposures of the Pawpaw and the Lake Waco pit (Del Rio Fm.) in McLennan County, where asteroids and brittle stars have been found nearly intact, those at Rufe Snow are usually broken into pieces and scattered. The pieces tend to accumulate on the lower, flat clay surface next to Industrial Park Boulevard. To find them, one has to get close to ground because the specimens are very small. Knee pads or a hip pad are very useful to keep the search a comfortable one. All told, the frequency that these pieces turn up entices me to think a whole fossil may be found, hopefully before interment under asphalt and concrete. Meanwhile the composites that I am assembling from disparate animal parts are thoroughly satisfying.

Adkins reported eight genera of echinoids from the Pawpaw of north Texas: Goniophorus, Peltastes, Enallaster (now Washitaster), Hemiaster, Epiaster, Holaster, Holectypus, and Goniopygus. These are listed as rare to abundant, depending upon the location. Echinoids are rare at Rufe Snow -- I have only found incomplete parts of three specimens of Washitaster, seemingly covered in a fine layer of rust.

The crustaceans are tenaciously difficult to find, but are one of the more interesting components of the assemblage found at Rufe Snow. For one thing they can be found retaining significant amounts of their original carapace, but are frequently limonitic steinkerns. They are tiny, but in most instances, good detail of the shell can still be resolved under a hand lens or microscope. Crabs are 15 mm or smaller in their longest dimension. Sometimes the appendage structure is found nearly intact on the ventral carapace. Other decapod crustaceans are also found. With the exception of one lobster I found, which is about 50 mm long, these other crustaceans are also small. For many years the professional literature neglected the taxonomy of these crustaceans. However, I have heard that a paper by Rodney Feldman at Kent State University is due to be published in in the professional literature, perhaps the Journal of Paleontology, sometime in the next year.

Shark teeth and bony fish vertebrae are exposed from time to time at Rufe Snow. Two genera from the three I have found at Rufe Snow are shown in Fig. 19. This photo does not compare to those shown in Welton and Farish's book on Texas Cretaceous sharks. For those who wish to pursue this area further, their book is an excellent resource.

About two miles to the west an exposure of the complete Pawpaw exhibits greater faunal diversity. One interesting fossil commonly found there, but not at Rufe Snow, is the free-floating, minute crinoid, Poecelocrinus dispandus (Peck 1958). The family to which it belongs only occurs from the lower Cretaceous to the upper Cretaceous Eagle Ford. This unique fossil was beautifully illustrated and described by Nestell and others in the Journal of Paleontology in 1977.

More genera have been reported in the literature than I have found at Rufe Snow. This is due largely to the limited time I have spent collecting there -- in total less than 20 hours. Listings in the publications appear to be composites of several locations. Environmental variations within small geographic or stratigraphic distances were probably significant and could account for differences in reported assemblages from different locations. A detailed stratigraphic analysis coupled with accurate faunal counts at several sites could reveal a grading of eurytopy to stenotopy for various taxonomic levels. Now that I have spent so much time focusing on the diminutive nature of the fossils in the Pawpaw of Tarrant County, I would like to leave the reader with a bit of doubt. It is entirely possible these animals weren't as dwarfed as has been thought. Early paleontologists were attracted to the novelty of the size and the beauty of the fossils. But, many of the pieces I have found suggest sizes were significantly larger. An exhibit in the Geology Department at the University of Texas in Arlington points out that not all Pawpaw fossils were small and shows several specimens to prove the point. I have also seen specimens of Pawpaw ammonites and crustaceans from private collections that need to be measured in centimeters (i.e., 7.5 cm) rather than in millimeters. While this is certainly far from the gigantic proportions of other fossils found in the marine Cretaceous deposits of Texas, it is entirely possible that the 'diminutive' nature of these fossils is an artifact of their long established reputation.

Acknowledgements

Special thanks are due to four friends who generously and enthusiastically offered their assistance and knowledge during preparation of this manuscript. Mr. Tony Newman was very kind with the loan of his instruments and patient help to obtain the photographs. Mr. Frank Dalton offered his kind assistance and attention to detail to translate the photos to digital images. Messrs. Scott Kelley and Rob Reid, III were delightful as they unselfishly shared their enthusiasm and tremendous knowledge of taxonomy and the Texas Cretaceous Pawpaw paleoenvironment. I feel fortunate to have friends such as these.

References and Other Articles of Interest

Adkins, W.S., 1918 (1920) The Weno and Pawpaw Formations of the Texas Comanchean, University of Texas Bulletin 1856.

---- 1928 Handbook of Texas Cretaceous Fossils, University of Texas Bulletin 2838.

Blake, D.B. and Reid, R., III. Some Albian (Cretaceous) asteroids (Echinodermata) from Texas and their paleobiological implications. J. Paleo. 72 (3), 512-552, 1998.

Cason, R.R. & Vincent, J.W. Paleoenvironments of the Denton, Weno and Pawpaw Formations, Washita Group, Central Texas. Meeting of the South-Central Section of the Geological Society of America and Associated Societies, Waco, Texas, March 29-31, 1987. Abstract only.

Emerson, B.L., Emerson, J.H., Akers, R.E. and Akers, T.J., Texas Cretaceous Ammonites and Nautiloids, Texas Paleontology Series Pub. No. 5, Paleontology Section, Houston Gem and Mineral Society, Houston, Texas 1994.

Finsley, C., A Field Guide to Fossils of Texas, 2nd Edition, Texas Monthly Field guide Series, Gulf Publishing Company, Houston, Texas 1996.

Geologic Atlas of Texas, Dallas Sheet, Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 1972.

Geologic Atlas of Texas, Austin Sheet, Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 1974.

McGill, D.W. Washita Formations, North Texas, correlated to Georgetown Limestone, central Texas. in Comanchean (Lower Cretaceous) Stratigraphy and Paleontology of Texas, L. Hendricks, ed., Society of Economic Paleontologists and Mineralogists, Permian Basin Section, Publication No. 67-8, Midland, Texas 1967, pp. 218-239.

Meyer, J.P. Hunting Cretaceous crabs. Lapidary Journal, September 1991, pp. 61-68.

Peck, R.E. Lower Cretaceous crinoids from Texas. J. Paleo. 17 (5), 451-75, 1943.

Renfro, H.B., United States Geological Highway Map Series, Map No. 7: Geological Highway Map of Texas, Published by The American Assoc. of Petroleum Geologists, Tulsa, Oklahoma.

Scott, R.W., Root, S.A., Tenery, J.H., and Nestell, M. Morphology of the Cretaceous microcrinoid Poecilocrinus (Roveacrinidae). J. Paleo. 51 (2), 343-49, 1977.

Sellards, E.H., Adkins, W.S. and Plummer, F.B., The Geology of Texas, Vol. 1, Stratigraphy, Bureau of Economic Geology Bulletin 3232, The University of Texas at Austin, Austin, Texas 1932.

Shimer, H.W. and Shrock, R.R., Index Fossils of North America, M.I.T. Press, Cambridge, Massachusetts 1994.

Slocki, S.F. Physical Stratigraphy of the Georgetown Limestone Equivalents in Tarrant, Denton and Cooke Counties, Texas. in Comanchean (Lower Cretaceous) Stratigraphy and Paleontology of Texas, L. Hendricks, ed., Society of Economic Paleontologists and Mineralogists, Permian Basin Section, Publication No. 67-8, Midland, Texas 1967, pp. 183-216.

Welton, B.J. and Farish, R.F. The collector's guide to fossil sharks and rays from the Cretaceous of Texas. Before Time, Lewisville, Texas 1993.