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29°08′00″N 103°10′00″W / 29.13333°N 103.16667°W / 29.13333; -103.16667

Talley Mountain

Dinosaur macrofossils

Paleontologist Julia T. Sankey has reported the presence of upper Aguja Formation bonebeds near Talley Mountain. ^[1] These bonebeds contain "[a]ssociated skeletons and islated bones of A. mariscalensis, the hadrosaur Kritosaurus, ankylosaurs and tyrannosaurids". ^[1] Sankey interpreted the bonebeds as originating at "watering holes" where dinosaurs would congregate during a drought. ^[1] When the animals died their bones would have been "exposed and trampled" before being carried away and buried by floodwaters. ^[1] The Talley Mountain bonebeds dinosaurs like Agujaceratops and Kritsaurus are better known that less common constituents of the fauna, like the theropods. ^[2] This disparity in understanding is more prnunced in the big bend area tan in northern Campanian fossil sites. ^[2]

Vertebrate microfossils

Sankey als notes that not much is known about the dinosaur nests and babies of the formation despite the abundance of remains uncovered through screening sediment and collection of eggshell at the surface at microsites. ^[2] Such microsites have also produced thousnds of fossils from diverse groups of animals including " sharks and rays, amphibians, lizards, turtles, crocodilians, pterosaurs, and mammals". ^[2]

The Talley Mountain bonebed microvertebrate sites have yielded more than 3,000 fossils, from 38 groups animal. ^[2] These tiny fossils have been used to help understand the age and ancient environment of the strata. ^[2] Baby dinosaur teeth recovered from the Talley Mountain bonebeds have been referred to hadrosaurs, ceratopsians, pachycephalosaurs, Saurornitholestes, Richardoestesia, and tyrannosaurids. ^[2]

In addition to the utility of their abundant fossil yields, the Big Bend microvertebrate sites are also useful because they are mre abundant themselves than bonebeds containing large fossils. ^[3] Their greater abundance allows them t be sampled from "numerous different stratigraphic intervals" giving insight in the local environments changes through time. ^[3] Sankey relied on this approach for previous research that tracked the abundance of shark and ray teeth through five microsites across different strata. ^[4] Frm the 3,349 fossils she observed a gradual decline in shark and ray tooth abundances. ^[4] This corresponded with a eustatic sea-level drop that occurred at the boundary between the Campanian and Maastrichtian. ^[5]

In the 1930s, crews working for the Works Progress Administration excavated quarries near Talley Mountain. ^[6] In the third of these quarries ankylosaurs were found to comprise 29% of 82 prepared specimens. ^[6]

In a 2005 paper, Sankey and others described 13 teeth or fragments of teeth left by tyrannosaurids in the Aguja and Javelina Formations. ^[7] These teeth were collected during field work performed in 2002. ^[7] Tyrannosaurid teeth have rounder cross sections than other kinds of carnivorous dinosaur. ^[7] In 2001 Sankey examined some unidentifiable theropod teeth from the Talley Mountain microsite. ^[7] Since aspects of their morphology didn't resemble other known theropods, she was not able to refer them to a specific group. ^[7] However, in her 2010 paper, she reported similar, but more completely preserved, teeth from the Rattlesnake Mountain bonebeds that enabled her to identify them as tyrannosaurid. ^[7] The confusion may have been caused by changes in tyrannosaur tooth anatomy known to occur as the animals mature. ^[7] Sankey notes that some of these teeth were "clearly from juveniles". ^[7] She also observes that "even fragmentary" tooth fossils like the tip of a tooth a fragment preserving both sides can give clues about the size of the animal that lost them. ^[7] Some of the Rattlesnake Mountain bonebed tyrannosaurid teeth are more blade like than is typical for the group, possibly due to the apparent young age of the animals, as this feature has been observed in tyrannosaur teeth previously. ^[7] Some of the smaller teeth recovered from the Rattlesnake Mountain bonebeds may have been left by hatchlings. ^[7] Sankey notes that "[f]inding juvenile tyrannosaurid teeth along with small theropod and other dinosaur teeth" is common in Late Cretaceous strata. ^[7]

Sankey 2008 microfossils

In 1997 Lehman postulated the idea of a Kritosaurus fauna covering the southern region of western North America. ^[8] The Big Bend region was part of this biogeographic province during the late Campanian and early Maastrichtian. ^[8] The Kritosaurus fauna includes southern Colorado and the area of western North America to the south of it. ^[8] The environment of southern western North America during the Late Cretaceous were warm woodlands with open canopies. ^[8] Fossil pollen from this biogeographic province comprises what is called the Normapolles palynoflora. ^[8] The northern province was cooler, wetter and forest canpoies that were less penetrable by light. ^[8] These differing environmental conditions, especialy temperature and rainfall divied western North America into distinct faunas. ^[8] From the late Campanian to early Maastrichtian, the Big Bend region of Texas was at about 35 degrees north of the equator and located near the Western Interior Seaway. ^[8]

The northern biogeographic province has been more thoroughly studied than its southertn counterpart and is therefore much better understood. ^[8] Big Bend fossils are important for understanding this period of time because it is one of the southernmost sources of Cretaceous fossils in North America. ^[8] Big Bend has features distinguishing it from more northern deposits like greater distance from areas that were of higher elevation during the Late Cretaceous, and lower rates of sedimentation. ^[8] Big Bend was also signifcantly drier than the ancient environments of northern western North America. ^[8] The region was drier because the Western Interior Seaway was regressing and mountains were rising to the west. ^[8] These relatively dry conditions have influenced the fossil record by leaving dinosaur bonebeds with geological evidence for droughts severe enough to completely dry out the local marshes. ^[8] 61-62 The local upper Aguja and Javelina Formations preserve many channel lag deposits of conglomerates containing nodules formed in soil and cemented by carbonates. ^[9]

62

The dinosaur park formation of mid to late Campanian Alberta was deposited by curving channels of flowing water in a wet lowland with high rates of sedimentation and subsidence. ^[10] The Big Bend area had lower rates of sedimentation and fossils due to the lesser amount of subsidence experienced by the Tornillo Basin. ^[10] Alberta did not start experiencing dry conditions until late in the Maastricthian. ^[10]

62-63

Apart from two brief episodes of greenhouse-fueled warming, the Maastrichtian age had the coolest and most variable climate of the Cretaceous period. ^[11]

63

Microsite research is especially important in deposits that where larger fossils are uncommon, like those of the Big Bend Region. ^[12] Much of what science knows about the southern province of Cretaceous Western North America and the western region of Texas in particular comes from microsites. ^[12] What larger bones are present in western Texas deposits are generally easy to see, so the local turtles, crocodilians and larger dinosaurs are easy to study. ^[12] However almost nothing about the area's Late Cretaceous mammals would be known without microsite research, which are generally recovered through screenwashing. ^[12] Likewise, major discoveries about the Late Cretaceous lizards of Texas have been ade at microsites. ^[12] The lizards found there are very different from those of northern western North America. ^[12] Microsites are also the primary source of information on Big Bend's carnivorous dinosaurs like tyrannosaurids, as well as juvenile dinosaurs and dinosaur eggs. ^[12]

Microsite research is important because it deals in large quantities of fossils that permits statisticak analysis of ancient ecosystems. ^[12] Microsites in Alberta reveal changing ecosystem make-ups in response to changing environmental conditions over time. ^[12]

The Aguja Formation is widespread within the Big Bend region of Texas. ^[13] It thins along its eastward axis and ranges in thickness from 285 to 135 m. ^[13] The Aguja consists of sandstones interbedded with shale and lignite. ^[13] The Aguja Formation represents a variety of ancient environments including marine, paralic, and inland floodplain. ^[13] The upper shale member of the Aguja Formation formed during the last period of sedimentation prior to the Laramide and also during the last withdrawl of th eWestern INterior Seaway from the region. ^[13] This was called regression 8 by Kauffman in 1977. The lower part of the upper shale has carbon-rich mudstones, thin lignite beds, large siderite ironstone concretions. ^[13] These formed in environments like distributary channels, levees, crevasse splays, and poorly drained interdristributary marshes and bays. ^[13] The upper part of the member has variegated mudstones, sandstones with conglomeratic lags of caliche nodules were deposited by flowing water within a floodplain in a delta and an inland floodplain. ^[13] The inland floodplain deposits contain several different paleosols from the upper Aguja into the Javelina formation that overlies it. ^[13] The stages of development of these paleosols were connected to the rise and fall of the Western Interior Seaway. ^[13]

63-65

Deinosuchus riograndensis is a typical large vertebrate from the lower Aguja, as is Agujaceratops maricalensis, an indeterminate species of Kritosaurus, however, becomes more common higher in the Aguja Formation. ^[14] All of these taxa are known from associated skeletons. ^[15] Paleomagnetic data from the Aguja limits the possible ages for the microsites Sankey studied. ^[15] The sites correlate to the base of chronozone 32, which means they are between 71 to 75 million years old, or from the late Campanian to the early Maastrichtian. ^[15] Other paleomagnetic research shows the upper Aguja to be early Maastrichtian in age. ^[15]

Sankey studied the microsites of Talley Mountain in order to better understand the make up of the local fauna and how the abundances of different kinds of animals changed over time. ^[16]

Sankey studied five microsites in sadstone conglomerates consisting of clay, nodules that formed in soil, and small bones and teeth. ^[17] She acquired 1,753 kg of rocks. ^[17] She broke down the rocks using 10-25% acetic acid solution and sifting the product through 1 mm mesh. ^[17]

The fossils Sankey collected are housed in the LSU Museum of Natural Science Vertebrate Paleontology collections. ^[18] 65-66 Sankey identified individual fossils by comparing them to previous museum collections. ^[19] 66 The five microsites Sankey studied spanned a 20 m stratigraphic interval near Talley Mountain. ^[20] All of the microsites were deposited by streams. ^[20] They contained many bits of clay, nodules that formed in the soil, small bones, small teeth, and larger fragments of wood and bones. ^[20] She obtained a total of 3,349 identifiable fossils. ^[20] Bony fish comprised 68% of the specimens, cartilaginous fish cromprised 14%, crocodylomorphs comprised 11%, dinosaurs 3%, amphibians 2%, squamates 1%, and turtles were present but not counted toward the total. ^[20] The site positioned lowest within the stratigraphic column is near the quarry from which Deinosuchus riograndensis was excavated by the American Museum of Natural History. ^[20] Other nearby fossils sites include the agujaceratops mariscalensis and Kritosaurus species bonebeds. ^[20] The sites represent successively drier inland environments as one travels up the stratigraphic column. ^[20] The lowest of the sites was deposited in an interdistributary marsh to bay environment while the top site was depoisted in an inland floodplain. ^[20] Evidence for this transition is found in the descreasing portion of bony fishes, sharks, and rays in contrast to the gradual increase in the percentage of fossils representing dinosaurs and mammals. ^[20]

The lowest microsites preserve two different kinds of shark. ^[21] The presence of these shark fossils provide evidence for the incursion of marine life into the depositional environment. ^[21]

Scapanorhynchus texanus, a mitsukurinid shark found in the lowest site, composes less than 1% of the sites' shark fossils. ^[21] Its presence stands out as unexpected given that modern mitsukurinids tend to live 200-700 m below the surface, however modern mitsukurinids are known to enter shallower water at times, especially at night. ^[21] An indeterminate species of Hybodus is also present and like S. texanus represents less than 1% of the Talley Mountain microsite fossils. ^[21] Scapanorhynchus texanus had been reported from the lower Aguja formation before Sankey's research, but this was the first documentation of Hybodus in the formation. ^[21] Some of the fishes of the lowest Talley Mountain microsite indicate water varying in salinity from brackish to freshwater. ^[21] 66-67 Lissodus selachos was one such brackish-to-fresh water species. ^[22] This species is the most abundant shark taxon in the five microsites Sankey examined, although its share diminished over time, from 82% in the lower site to 1% in the highest. ^[22] Previous research had suggested that L. selachos could live in brackish or freshwater. ^[23] L. selachos is a common shark in the Terlingua fauna, which was deposited in an estuary. ^[23] It is also found in an upper Aguja site with a freshwater depositional environment. Squatirhina americana is another shark likely capable of living brackish or shallow marine water known from the Talley Mountain microsites, although it is rare, being less than 2% of the sites' fossils. ^[23] An ancient sawfish, Ischrhiza avonicola is also present in the microsites, but only represents less than 1% of their fossils. ^[23] I. avonicola probably lived like modern sawfish by feeding on bottom-dwelling fish and shellfish where rivers flow into the sea in warm to tropical regions. ^[23] Most modern sawfish live in brackish to estuary conditions, although in Central America a freshwater species is known. Onchopristis dunkeli is another species from the Talley Mountain microsites that probably lived like a modern sawfish. ^[23] Like I. avonicola, it was a rarity in Sankey's collection, representing less than 3% of the fossils. ^[23] The specimens Sankey uncovered extended its known age range into the late Campanian. ^[23] Ptychotrygon is another rare sawfish-like species that only represented less than 1% of the microsites' fossils. ^[23]

Dasyatid stingrays are present but rare in the Talley Mountain microsites, representing less than 4% of its fossils. ^[23] Some modern stingrays are known to live in freshwater, and most generally can tolerate estuarine conditions or the freshwater of rivers. ^[23] Stingrays live in shallow waters off the coast of subtropical to tropical regions where they feed on small fish, crustaceans, and molluscs on the bottom. ^[23]

Lepisosteus occidentalis, a gar, is the most common fossil in the Aguja Formation. ^[23] Likewise, it is the most common fossil in the Talley Mountain microsites. ^[23] Like other aquatic taxa, its abundance decreases upward through the stratigraphic column. ^[23] At the lowest site it comprises 65% of the fossils found, but at the highest it represents only 29%. ^[23] The modern alligator gar, Lepisosteus spatula, probably has an analogous lifestyle, living in Gulf Coast streams in salinities ranging from freshwater to brackish. ^[23]

Two different kinds of amphibians are known from the Talley Mountain microsites; Albanerpeton and Scapherpeton, which represent less than 5% of the microsites' fossils. ^[24] Both of these amphibians are regarded as aquatic forms, and provide likely evidence for the influence of freshwater ecosystems on the depositional environment. ^[24] Amphibian fossils are also known in the Terlingua fauna. ^[24] Amphibian fossils are known from the uppermost Aguja Formation. ^[24]

Trionychid turtles are among the other aquatic animals present in the Talley Mountain microsites. ^[25] However, Sankey didn't include the trinoychid turtle remains while calculating the relative abundance of taxa because the number of trionychid fossils counted would be highly dependant on how one treated the individual fragments. ^[25] 67-69 " Aspideretes" is a trionychid and the most abundant turtles of the Aguja Formations upper shale member. ^[26] However, the Talley Mountain turtles fossils can't be identified confidently. ^[26]

69

Crocodylians are more abundant in the microsites than turtles and represent 10-25% of the fossils. ^[27] Sankey identified Brachychampsa and goniopholids in the microsites. ^[27] Goniopholis cf. kirtlandicus is known from the Terlingua fauna. ^[27] Some crocodylian teeth were present that Sankey could not identify, implying even greater diversity than had been previously known. ^[27] Deinosuchus teeth were found nearby the microsites, but Sankey didn't count these among the microsite fossils because they were just taken by hand from the surface instead of extracted from the rock by screen washing like the rest of her microfossils and she didn't want to introduce methodological inconsistency to her work. ^[27] Deinosuchus is the largest crocodylian found in the Aguja Formation. ^[27] Deinosuchus is common in both the middle shale and the lowest interval of the upper shale member. ^[27] The type specimen of Deinosuchus riograndensis was found near Talley Mountain. ^[27] 71 Most of the taxa from the Talley Mountain microsites are aquatic, however there is still a wide variety of terrestrial taxa present. ^[28] The lizard fossils found near Talley Mountain are different than the lizards found in similarly-aged rocks in the northern region of western North America. ^[28] Lizard fossils represent 2% or less of the Talley Mountain microfossils. ^[28] Sankey reported the first known occurrence of Chamops within Big Bend and the southernmost example of the genus Peneteius. ^[28]

Dinosaur teeth of varying levels of completeness have been recovered from the Talley Mountain microsites and have provided significant amounts of information about the kinds of dinosaurs that used to live in the Big Bend region. ^[29] Dinosaur microfossils comprise 2-8% of the Talley Mountain microfossils. ^[29] Known dinosaurs from the Talley Mountain microsites include hadrosaurids, ceratopsians, pachycephalosaurids, tyrannosaurids, Saurornitholestes cf. lagnstoni, Richardoestesia isosceles, and two different kinds of theropods that Sankey couldn't identify. ^[29] So many of the dinosaur teeth preserved in the Talley Mountain microsites seem to be from very young individuals that it's probable that many kinds of dinosaurs were nesting in the local area. ^[29] There are no known fossil dinosaur eggs or nests known from Big Bend, although eggshell fragments are known from upper Aguja Formation strata cropping out at Rattlesnake Mountain. ^[29]

Mammals comprise 1-5% of the Talley Mountain microsite fossils, although Sankey regards their contributions to the local ecology as significant. ^[30] They provide important biostratigraphic reference points. ^[30] The local mammals are markers for the Judithian land mammal ages and include multituberculates like Cimolomys sp., Mesodma sp. cf. Cimexomys, cf. Paracimexomys, as well as marsupials like Alphadon cf. halleyi. ^[30]

The five microsites span a 20m stratigraphic interval. ^[31] The large sample of fossils allowed Sankey to track the changes undergone by the local environment during a relatively short duration of time. ^[31] She concluded that the environment of deposition became gradually more and more inland over time. ^[31] The lowest microsite preserves fossils characteristic of interdistributary marsh-bay environments, while the uppermost site was deposited on an inland floodplain. ^[31] Fossils of aquatic vertebrates like gars, sharks, and rays, decline in abundance gradually higher in the stratigraphic column from 79% at the lowest site to 61% at the highest site. ^[31] Terrestrial animals like mammals and dinosaurs, however increase in abundance gradually. ^[31]

Although microsites have benefits, they have preservational biases. Because these deposits formed in streams, the microfossils were separated from the larger bones of turtles, dinosaurs, and the larger crocodylians. ^[31] 71-73 Microsites also less frequently preserve the fragile bones of animals like amphibians and lizards because they would have been destroyed by the action of the water while the bones were being moved. ^[32] 73 Sankey describes the microfossil discoveries as "flesh[ing] out" the ecosystems of the Big Bend area during the Late Cretacoeus. ^[33] She observed that before the microfossils were examined science's knowledge of the local ancient fauna were based primarily on individual finds of large vertebrates like Deinosuchus riograndensis, Agujaceratops mariscalensis, and Kritosaurus sp. ^[33] The Talley Mountain fauna is similar to the vertebrate microsite at Terlingua, another deposit from the lower portion of the upper Aguja Formation. ^[33] This provides more evidence for the distinctness of the Aguja Formation's fauna. ^[33]

Less is known about the Cretacoues vertebrates of the southern region of western North America than in the northern regions. ^[34] Since the Big Bend area is one of the southernmost sources of Cretaceous fossils in North America, the Talley Mountain microfossils help fill an important void in scientific knowledge. ^[34]

Sankey sees support for the distinctness of the southern fauna in the Talley Mountain microfossils since the preserved taxa differ at the generic or specific level compared to northern taxa. ^[34] They also suggest that southern province dinosaur were less diverse than those of northern faunas. ^[34] The dinosaur communities of the late Campanian Big Bend area more closely resembles those of southern Alberta during the late Maatrichtian than the fauna of southern Alberta that existed at the same time the Big Bend deposits were forming. ^[34] Examples of features shared by late Campanian Big Bend and late Maastrichtian southern Alberta include Richardoestesia isosceles being more common than R. gilmorei, Troodon and Dromaeosaurus being absent or rare, and pachycephalosaurids were present to common. ^[34]

Sankey attributed differences in the fauna mostly to climate since the Big Bend area became drier earlier than the northern region of western North America. ^[34] Droughts so severe they dried out entire marshes were the likely instigating factor for the dinosaur bonebeds of the Aguja Formation. ^[34] These droughts left tell-tale nodules of ancient caliche in the soils that are now sedimentary rocks. ^[34] After these droughts ended, large floods, possibly even flash floods would have washed away dirt and bones, leaving them in streams that deposited the conglomerate of Talley Mountains modern vertebrate microsites. ^[34] At the same time, Alberta was a wet plain near the coast of the Western Interior Seaway and did not begin to have dry weather until the late Maastrichtian. ^[34]

Understanding the way earth's climate changed during the Late Cretaceous climate is important because it helps scientists distinguish between earthly or extraterrestrial factors on the mass extinction at the Cretaceous-Paleogene boundry. ^[35] 73-74 Around 74 million years ago, during the late Campanian, earth's climate began to cool and sea level dropped. ^[36] 74 This sea level drop precipitated the withdrawl of the Western Interior Seaway. ^[37] During the Maastrichtian two brief greenhouse episodes triggered small extinction events before the mass extinction ended the Cretaceous. ^[37] Upper Aguja Formation microsites have produced abundant fossils that provide significant data about the southernmost Cretaceous faunas of North America. ^[38] Since the recovered taxa are distinct at the generic or specific level they reinforce previous suggestions that the southern portion of North America are distinct from the northern part. ^[38] These faunal differences can mostly be attributed to the warmer drier climate of the southern US during the late Campanian. ^[38] However, by the late Maastrichtian even the northern regions of western North America were experiencing that sort of climate. ^[38] The warmer drier climate also lessened dinosaur biodiversity in the southern region relative to the north ^[38]. Similar trends may one day be seen in future discoveries even farther south, like in Mexico. ^[38]

Footnotes

References

• Sankey, J.T. 2008. Vertebrate paleoecology from microsites, Talley Mountain, upper Aguja Formation (Late Cretaceous), Big Bend National Park, Texas, USA; pp.61-77. In: The Unique Role of Vertebrate Microfossil Assemblages in Paleoecology and Paleobiogeography, J.T. Sankey, and S. Baszio (eds). Indiana University Press (Bloomington).
• Sankey, J.T. 2010. Faunal composition and significance of high diversity, mixed bonebeds containing Agujaceratops mariscalensis and other dinosaurs, Aguja Formation (upper Cretaceous), Big Bend, Texas; pp. 520–537. In: New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, M. Ryan, B. Chinnery-Allgeier, and D. Eberth (eds). Indiana University Press (Bloomington).

External links

• Handbook of Texas Online: Talley Mountain