Paleontological inventory of Paleozoic, Late Mesozoic, and Cenozoic plant, invertebrate, and vertebrate fossil species from Big Bend National Park, Texas, USA – over a century of paleontological discovery

The extraordinary paleontological record from Big Bend National Park (BIBE), Texas chronicles nearly 120 million years of largely uninterrupted deposition through Late Cretaceous, Paleogene and Neogene time. Therefore, the park records one of the most complete and continuous fossil records of its kind in North America, if not the world. Paleontologists have collected and studied fossils from BIBE for over a century and nearly 1400 fossil species have been reported thus far. The BIBE paleontological record includes type specimens representing 44 scientifically valid species (five plants, nine invertebrates, and 30 vertebrates). Numerous other reported specimens are very likely new to science but have yet to be formally named. The present catalog presents the currently known assemblage of fossil plant, invertebrate, and vertebrate species from BIBE within a single, comprehensive record with significant references for each. This work is designed and written to be a research and resource management tool for scientists and non-scientists alike.


Introduction
For more than 100 years, paleontological researchers have made some of North America's most important fossil discoveries in the Big Bend region of West Texas, USA -many of those in what is now Big Bend National Park (BIBE) (Fig. 1). Many other 'fossil' parks within the National Park Service (NPS) system contain strata which represent a relatively brief geologic interval providing a snapshot of the paleoenvironment represented in the rocks (e.g., Petrified Forest, Dinosaur, and Florissant national parks). On the other hand, BIBE's fossils come from a geologically long (ca. 120 Ma) and mostly uninterrupted series of strata which make it possible to study the succession of paleocommunities over geologic time. This is especially important in that the significance of fossil resources is directly related to degree of scientific information provided by the environmental contexts in which they are preserved. In fact, Big Bend National Park contains more than fossilized plants and animals; it contains a succession of "fossilized" aquatic and terrestrial ecosystems spanning ca. 120 Ma of Earth's history. Aside from the sheer number of fossil species discovered within the park, Big Bend is also known for several iconic fossil species including the largest flying creature known -the giant pterosaur Quetzalcoatlus northropi (Lawson), the colossal titanosaur Alamosaurus (Gilmore) and the hyper-giant alligatoroid Deinosuchus riograndensis (Colbert and Bird).
The updated taxonomic catalog herein is derived from a seminal paleontological inventory of Big Bend National Park produced by Wick and Corrick (2015). The present fossil inventory represents the most significant portion of that earlier work. It involves a comprehensive listing of all reported fossil species (currently around 1300) having been discovered in BIBE by professional paleontologists and academic researchers so that the astonishing number and variety of fossil taxa from BIBE are included in a single published reference. Along with the taxonomic tables are brief descriptions of the park's geologic history and formations so that the reader has a convenient point of reference. Each reported species is accompanied by at least one (or more) significant references so that researchers can use them as a springboard for further research.
The original 2015 (unpublished) catalog was developed as an internal NPS document so that NPS interpretive and law enforcement personnel, resource managers, and qualified permitted academics might better explain, protect, manage, and research the diversity and significance of the park's fossil resources. Hence, it was written using uncomplicated language so that it could be better understood by readers with variable levels of interest and expertise. That approach is maintained here. Whatever the case, it must be noted that this catalog (like all projects of its type) remains a work-inprogress. New discoveries will undoubtedly add to the park's paleobiodiversity and new explorers will emerge over the coming decades to expand upon what we have discovered thus far. It must also be noted that several fossil species relevant to the BIBE paleontological story have been discovered just outside of the park in the same geologic formations exposed within it. These were also included in the present catalog under the assumption that these species are very likely present in the park as well but have yet to be found there.
Relevant references involving the various individual species reported here is provided within each of the taxonomic lists and so specific references are not included within the preliminary text. Repositories and accession numbers for the specimens representing the species listed in the catalog can be found in their respective referenced works. Furthermore, understanding the changing landscape of Big Bend is critical to understanding its paleontological story. The reader is, therefore, strongly encouraged to review Blakey and Ranney (2018) as their work provides an excellent and coherent geotectonic synthesis involving the changing landscape of western North America during Late Cretaceous, Paleogene, and Neogene time. Finally, in order to better understand the geologic context of the park, as well as the stratigraphic and geospatial relationships of the formations outlined in this report, the reader is encouraged to visit https://pubs.usgs.gov/sim/3142/ for the online version of the latest geologic map of BIBE produced by the U.S Geological Survey (Turner et al. 2011). Overview of Big Bend geologic history and paleoenvironments Paleozoic era Fossils from Big Bend National Park are widespread within Mesozoic and Cenozoic strata which are well-exposed throughout the park. Paleozoic strata are not well exposed within the park and are largely confined to its northern margins and so fossils from this time are not well known. These older rocks were laid down some 330-285 million years ago then subsequently deformed during the Ouachita orogeny. They appear in the configuration that we see today as the subsequent result of Laramide compression, faulting, and erosion during more recent times (e.g., Page et al. 2008). Those fossils that that have been found (e.g., conodonts, graptolites, and brachiopods) suggest deposition generally within deep-water, basinal marine habitats. Within the park, the Paleozoic and Mesozoic stratigraphic sequences are separated by a significant unconformity representing a depositional hiatus and/or erosion during Triassic, Jurassic, and early Cretaceous time.

Late Cretaceous system
Around 120 million years ago, a warm, shallow sea (the Western Interior Seaway) bisected North America dividing the continent in half from today's Gulf of Mexico to the Arctic Ocean (Blakey and Ranney 2018), providing the setting for deposition of limy, marine muds and calcareous oozes. Today, these limestones and shales preserve the remains of sea-dwelling invertebrates such as urchins, foraminifera, and mollusks. Within and around BIBE, these strata create the sheer walls of Santa Elena, Mariscal, and Boquillas Canyons, almost the entire range of the Dead Horse Mountains, as well as the magnificent cliffs of the Sierra Ponce and Sierra del Carmen in nearby Mexico. Strata from this interval comprise the Lower Cretaceous, Comanchean Series (marine carbonate) rocks of the Glen Rose, Telephone Canyon, Del Carmen, Sue Peaks, Santa Elena, Del Rio, and Buda formations (Maxwell et al. 1967;Busbey and Lehman 1989;Turner et al. 2011).
Approximately 90 million years ago, the shallow Cretaceous seaway began a gradual retreat to its present location -today's Gulf of Mexico. Calcareous marine muds, and silty clay containing more terrigenously-derived sediments were deposited on the nearby shallow, marine shelf along with the remains of giant bivalves, oysters, sharks, fish, ammonites, and mosasaurs. Gulfian Series limestones and shales of the flaggy Boquillas Formation and soft bentonitic clays of the Pen Formation were deposited during this time (Maxwell et al. 1967;Cooper et al. 2017).
Around 78 million years ago, Big Bend was situated upon the shore of the ancient seaway (Blakey and Ranney 2018). A complex of coastal rivers, meandering streams, estuaries, and marshlands developed in the tropical climate. Alternating periods of marine transgression and shoreline progradation are responsible for the cyclic deposition of the sandstones, mudstones, and shales contained within the Aguja Formation's complex ensemble of inter-tonguing facies (Lehman 1985). These deposits have yielded fossilized trees, oysters, turtles, crocodiles, dinosaurs, and mammals. This was a time of remarkable Figure 1. Generalized stratigraphic column (A) exposed within Big Bend National Park, Texas, USA (B). Approximate absolute stratigraphic ages are based upon biostratigraphic and radiometric information from multiple sources (Maxwell et al. 1967;Lehman et al. 2006;Befus et al. 2008;Tiedemann 2010;Cooper and Cooper 2018). Chart modified from USGS (public domain). diversity within the ancient ecosystem of ancient BIBE as marine, brackish, and fresh-water subaquatic habitats were situated very near to each other as well as to better drained, terrestrial floodplain environs.
Some 70 to 65 million years ago, Laramide tectonism began uplifting the proto-Rocky Mountains to the west. As a result, the Late Cretaceous shoreline had retreated well to the east of today's park (Blakey and Ranney 2018). This new tectonic regime resulted in significant changes involving deposition and resultant lithology compared to deposits of the older Aguja Formation (Lehman et al. 2018). The most significant of these changes was the development of the Tornillo Basin across the Big Bend region (e.g., Lehman 1986) (Fig. 1). During this time, a river-floodplain environment dominated the deposition of fluvial sands and muds within the Tornillo Basin which are preserved within the Javelina and Black Peaks Formations within the Park. Today, fluvial channel sandstones, colorful overbank mudstones, and thin lacustrine facies can be found in many areas of BIBE which harbor the remains of many creatures including dinosaurs, pterosaurs, and many types of smaller reptiles, as well as conifer trees and flowering plants. The climate had changed since Aguja time and it was becoming cooler and more seasonal (e.g., Linnert et al. 2014). Dinosaurs reached their largest sizes during this time (e.g., Woodward 2005;Woodward and Lehman 2009).
The end of the Cretaceous Period was also a time of great change for life on Earth. Although there are several hypotheses for the extinction of the dinosaurs some 66 million years ago, their disappearance at the end of the Cretaceous gave rise to the 'Age' of mammals. Whether caused by climate change, disease, or the impact of a large meteor in the Yucatan of Mexico, this extinction event occurred during deposition of the Black Peaks formation in BIBE, one of the few public lands in North America which contain strata that span the Cretaceous-Paleogene (K-Pg) extinction boundary.

Paleogene system
Around 63 million years ago (Paleocene time) the dinosaurs were gone. However, ancient mammals survived the K-Pg extinction event (as did avian dinosaurs -the birds) and began to evolve on the ancient river floodplains in BIBE. Although this was the same river system which originated millions of years earlier during Javelina Formation time, the Rocky Mountains continued their unrelenting uplift (e.g., Lehman 1986;Blakey and Ranney 2018). Therefore, the fluvially-derived Black Peaks Formation continued to be deposited even further inland as the sea continued its slow retreat to the east. Bright maroon and somber grey/black 'candy-striped' paleosol (ancient soil) horizons characterize this portion of the Black Peaks section and signal a time when silty, fluvial muds were deposited on a stable, well-developed inland floodplain (Lehman et al. 2018). Huge trees that lined these sandy drainages and were often undercut by the currents, causing them to topple into the river where they became oriented to the paleocurrent direction (now informally called the "log jam sandstone" interval of the Black Peaks). The fossils of these trees show no growth rings, whereas those from the surrounding floodplain (conifers) do have them (Wheeler and Lehman 2005;. This circumstance suggests that the climate afforded constant growth for only those trees along the river and that others went seasonally dormant as rainfall became scarce. Mammals thrived; however, they were small during this time with the largest being only the size of a medium-to small-sized dog. 55 million years ago during early Eocene time, the Tornillo Basin continued to aggrade with fluvial sediments of the Hannold Hill Formation in BIBE (Maxwell et al. 1967;Beatty 1992) (Fig. 1). These deposits consisted of coarser sands deposited in higher gradient river channels. The bright purple and peach-colored paleosol horizons we see now are today's expressions of the confined, muddy overbank deposits emplaced during deposition. Interestingly, some Hannold Hill exposures exhibit striking evidence for compressional deformation during deposition (Lehman and Busbey 2007) which records the final "push" of the Laramide Orogeny in Big Bend as well as the conclusion of basinal development within BIBE. As a result, the Hannold Hill Formation is limited to the northeast portion of the park as the Tornillo Basin, by this time, was almost completely infilled elsewhere by fluvial deposits. At long last, the ancient river system which had long coursed through the basin had reached its closing stages. Paleogene time saw an explosion of new species; mammals diversified in BIBE and became larger (e.g., Wilson 1967).
During mid-Eocene time (about 46 million years ago), the Laramide Orogeny had almost reached its culmination and the Big Bend region was now elevated several thousand feet. Erosion then became the dominant regime, stripping away much of the Hannold Hill, Black Peaks, Javelina, and Aguja formations throughout BIBE and surrounds. All of these strata (as well as the fossils preserved within) might have been lost. However, what remained of them was then covered by deposits laid down by a new river system that developed atop the ancient, infilled basin. As a result, sediments of the Canoe Formation were laid down unconformably on the previously eroded surface. These new rocks were made up of thick fluvial channel sands and gravels (the Big Yellow Sandstone in the park) as part of a braided river system (Maxwell et al. 1967;Rigsby 1986). Mammals had flourished and were now of many types and sizes. Turtles also inhabited the new river corridor which was lined with conifers and flowering plants.
Approximately 42 million to 32 million years ago (during middle Eocene to early Oligocene time) Big Bend experienced a strikingly different depositional regime as widespread volcanism commenced. Strata deposited during this time differ markedly across the region as the result of the changing loci and composition of various igneous intrusions, lavas, ash-falls, as well as the fluvial volcaniclastics derived from them via weathering (Maxwell et al. 1967). Within BIBE, these deposits became the Chisos Formation, a colorfully diverse collection of tuffs, conglomerates, fluvially re-worked ash deposits, stream channel sandstones, and variegated mudstones situated between ensembles of extrusive lavas (e.g., the Alamo Creek Basalt). Portions of the Chisos Formation are locally fossiliferous whereas others are completely devoid of fossils. It is generally believed that the volcanism involved here was subduction-related and that a temporary shallowing of the angle of subduction of the Farallon plate (descending eastward, below the western edge of North America) resulted in the emplacement of various plutons and volcanoes far inland from the margin of subduction along western North America. Because of their complexity, these deposits are named differently in different areas (e.g., Canoe and Chisos formations within BIBE and Devil's Graveyard Formation outside of the park to the northwest) (e.g., Maxwell et al. 1967;Wilson and Runkel 1989). Whatever the case, similarities involving their geologic make-up and fossil evidence suggest that these formations are broadly coeval. Although the Devil's Graveyard Formation is very fossiliferous, these taxa were not included in the present catalog as that formation does not crop out within the park.
During early Oligocene time (around 32 to 26 million years ago), volcanism continued with a series of eruptions in what is today BIBE (Maxwell et al. 1967;Lehman and Busbey 2007). Higher in section, the un-fossiliferous South Rim Formation (along with the so-called "Burro Mesa" Formation of Turner et al. 2011) capped the Chisos Formation with a series of thick, brightly colored rhyolitic lavas which are particularly striking along the Ross Maxell Scenic Drive in the western part of the park. The Chisos Mountains within BIBE were fully formed by this time and, along their flanks, new and even larger mammals replaced older forms. Volcanic deposition in the region ended some 26 million years ago (Henry et al. 1989). As a result, erosion again resumed.

Neogene system
By the end of Oligocene time (around 20 million years ago), the Rocky Mountains stood in bold relief above the western plains. Compressional stresses involved in mountain-building finally eased across the North American continent resulting in a 'relaxation' of continental crust. As a result of this trans-continental stretching, rift zones developed which, over time, allowed large bodies of rock to slide downward along active faults, producing a horst-and-graben topography. This created the North American, Basin and Range Province which spans southern Canada to northern Mexico including Big Bend. The Big Bend region saw the development of several grabens and resultant bolsons including one within the central part of today's BIBE (from the Sierra del Carmen to the east to the Mesa de Anguila to the west). This graben formed a "sunken block" of strata, down-dropped several thousand feet by faulting (Maxwell et al. 1967;Lehman and Busbey 2007). As a result, two half-bolsons formed in BIBE (one on either flank of the eroding Chisos Mountains): the Delaho Bolson in the west and Estufa Bolson in the east (Stevens andStevens 1985, 1989). During Miocene and Pliocene times, these bolsons slowly aggraded with alluvium and colluvium transported in streams and deposited as alluvial fans along the flanks of the nearby eroding Chisos Mountains. These coarsely-laminated sands and gravels formed today's Delaho and Banta Shut-In formations. On the Maxwell et. al, (1967) geologic map of the park, these bolson-fill deposits were mapped collectively as Quaternary/Tertiary "old gravels" (abbreviated thereon as QTog). Portions of these alluvial fans supported intermittent faunal communities comprised of mammals such as early camels, skunks, and carnivores, as well as turtles, lizards, and amphibians.
Eventually, similar bolsons throughout west Texas were infilled and subsequently linked by the Rio Grande (achieving through-flow to the Gulf of Mexico only within the last 2 million years or so). Once established, the downcutting Rio Grande and its tributaries (forerunners of today's Terlingua and Tornillo creeks within BIBE) largely gutted the infilled bolsons during Pleistocene time leaving only remnants of them today. The Rio Grande is the youngest major river system in the United States and continues to serve as the principal erosional conduit in the region.
Geologic formations within Big Bend National Park: a primer The geologic formations within BIBE vary widely regarding composition, thickness, depositional environments, and fossil content. However, many are fossiliferous. Many formations exposed within the park also crop out on private lands just outside of its boundaries and so some fossils from just outside the park are also included here as well. In general, older, Late Cretaceous open marine carbonate strata are separated by unconformities representing relatively brief geologic intervals. Younger, Late Cretaceous (marine shelf) strata generally grade conformably into, and sometimes inter-tongue with, broadly coeval terrestrial rocks. These deposits then grade conformably into overlying Paleocene strata. Some localized unconformities are present in some strata (e.g., the Aguja/Javelina formations contact) as the result of penecontemporaneous erosion (i.e., stream downcutting which occurred simultaneously with overbank deposition in some areas), but these minor depositional gaps generally do not represent geologically significant intervals. Significant erosion of Cretaceous and Paleocene strata did occur as the result of Laramide uplifting in Eocene time however these eroded deposits were then covered by even younger fluvial deposits, volcaniclastics, and extrusive rocks. Basin and range development along with continued erosion of the Chisos Mountains volcanic complex initiated yet another period of deposition which resulted in infilling of the surrounding bolson. Despite the presence of unconformities, many of the strata within the park and immediate surrounds preserve a relatively continuous, 135 million-yearlong depositional sequence. The following formations are arranged in stratigraphic succession (low to high) ( Fig. 1).

Paleozoic Era
Maravillas Chert (Baker and Bowman 1917) Ordovician, marine, around 50 m thick. The Maravillas was deposited in a deep-water, basinal environment (Turner et al. 2011). The formation is exposed along the northern margins of the park northward (Persimmon Gap and Dog Canyon areas within BIBE) and is convolutedly deformed in some areas by pre-and post-Cretaceous thrusting. The formation contains dark brown/blackish cherts and thin conglomerate lenses, and a few limestone beds Fossils from BIBE include graptolites, brachiopods, bryozoans, and conodonts. Extensive deformation and poor exposures make sectional thickness measurements and definition of individual members within BIBE difficult.
Caballos Novaculite (Udden et al. 1916) Silurian-Devonian, marine, only 20 m thick. The origin of both the novaculite and chert members leads to contrasting interpretations of water depth during deposition (e.g., Folk and McBride 1978). This formation contains chert and silicious shale with thin but conspicuous, white novaculite beds. The unit is modestly exposed near Persimmon Gap near the entrance of the park however no fossils have been reported from BIBE.
Tesnus Formation (Udden et al. 1916) Mississippian -Pennsylvanian, marine, variably thick from 15-200 m. Deep-water sediments, thin to thickly bedded sandstone and dark gray, brown, and black shale. Several small outcrops are situated in the northernmost part of BIBE. No fossils have been collected from the park however nearby areas have produced conodonts, foraminifera, and a few Pennsylvanian Period plant fossils (King 1937).

Mesozoic Era -Lower Cretaceous (Comanchian Series)
Glen Rose Limestone (Hill 1891) Marine, massive, about 100-150 m thick. Primarily a massive limestone but contains clay, minor sandstone, marl, and conglomerate deposited in near-shore tidal and sub-tid-al marine environs (Maxwell 1967;Busbey 1989). This unit is exposed in several areas of BIBE including Persimmon Gap and Dog Canyon in the north, Marufo Vega trail in the southeast and Santa Elena Canyon in the southwest (Turner et al. 2011). These outcrops are generally exposed in areas which have been subjected to Cretaceous Laramide folding and/or the development of horst and graben structures emplaced during Miocene time. Invertebrate fossils include ammonites, oysters, gastropods, and echinoids. Rarely, dinosaur fossils have been found elsewhere in Texas from this formation (Upchurch et al. 2004). Although dinosaur trackways are somewhat common in the Glen Rose of Texas (e.g., Bird 1985) none have been reported in BIBE. However, several theropod dinosaur tracks are preserved along the Rio Grande in the Glen Rose Formation of Mexico just east of the park within the lower canyons (photos shown to the author by D. Corrick, BIBE Geologist).
Telephone Canyon Formation (Maxwell et al. 1967) Marine, generally 20-45 m thick. Lagoonal sediments (Busbey 1989) containing thin nodular limestone with marl beds This formation can be seen in several areas of BIBE including Heath Creek, along the Marufo Vega Trail in the east, and Santa Elena Canyon in the southwest where folding and faulting have exposed it (Turner et al. 2011). Common invertebrate fossils in this formation include gastropods, oysters, and echinoids. Ammonites have also been reported.
Del Carmen Limestone (Maxwell et al. 1967) Marine, massive, from 100-150 m thick. Open lagoon, tidal flat, and rudistid biostromal facies (Busbey 1989). Generally, a massive, dense limestone with abundant rudistids. This karstic formation also contains lenticular cherts and minor marl beds. Within the park, it is exposed in areas of tectonic folding and faulting such as Santa Elena Canyon in the southeast and Marufo Vega Trail, and Sierra del Caballo Muerto in the east (Turner et al. 2011). Typical invertebrate fossils include bivalves and gastropods although recovery of them from the hard matrix is difficult which makes their identification problematic. (Maxwell et al. 1967) Marine, around 25-30 m thick. Transgressional marine sediments containing shale, marl, thin nodular limestone ledges (Maxwell 1967;Busbey 1989). The formation is exposed in eastern and southwestern areas of the park including portions of the Sierra del Carmen, as well as Santa Elena Canyon where faults and folding have exposed it. Common invertebrate fossils include oysters, echinoids, gastropods, and numerous types of ammonites. Santa Elena Limestone (Maxwell et al. 1967) Marine, massive, up to 225 m thick. Open shelf carbonate environments (Busbey 1989). The Santa Elena is a massive, karstic limestone, hard, with some finely crystalline bedding along with nodular chert masses. Upper portions of this formation contain massive limestones with interbedded marls that weather to form a terrace-like topography. The formation can be found in eastern and southwestern parts of the park (and surrounds) such as the Sierra del Carmen, Santa Elena Canyon, and Sierra Ponce where faulting and folding have exposed it (Maxwell et al. 1967;Turner et al. 2011). Common invertebrate fossils include rudists with other pelycopods and gastropods being uncommon.

Sue Peaks Formation
Del Rio Clay (Hill and Vaughn 1898) Marine, fissile, around 1-35 m thick. A regressive marine environment facilitated development of this shaly, shallow-water facies (Busbey 1989). This formation consists mostly of claystone with interbeds of limestone and friable sandstone. It is exposed in the eastern and southwestern portions of the park including Mesa de Anguila, Dog Canyon, Alto Relex, and Sierra del Caballo Muerto (Turner et al. 2011). Invertebrate fossils include oysters, echinoids, and gastropods.
Buda Limestone (Vaughan 1900) Marine, 20-30 m thick. Shallow, inner-shelf environment. This formation primarily crops out in eastern, southern. and southwestern areas of the park such as Dog Canyon, Dagger Mountain, Mariscal Mountain, and Mesa de Anguila (Turner et al. 2011). Invertebrate fossils are rare in finegrained limestones and more common in marls including echinoids, gastropods, and bivalves. West of the park along route 170, the Buda/Boquillas limestone contact interval harbors the typical reddish tint of cinnabar.

Mesozoic Era -Upper Cretaceous (Gulfian Series)
Boquillas Formation (Udden 1907) Marine, massive to shaley, from 220-245 m thick. Foraminiferal limestone and shale deposited in relatively shallow, open marine (platform) conditions (Lehman 1989b;Cooper et al. 2017). This formation contains two members including the lower Ernst Member and upper San Vicente Member (Maxwell et al. 1967). The Ernst Member contains silty limestone flags, siltstone, and calcarious clay while the San Vicente Member contains chalk, marly clay, and shale. It is exposed widely in the park in areas such as San Vicente, Hot Springs, Mariscal Mountain, McKinney Hills and Mesa de Anguila (Turner et al. 2011). The Boquillas Formation is very fossiliferous. Fossils include invertebrates such as cephalopods, bivalves, and echinoids as well as a few vertebrate fossils from mosasaurs, fish, and sharks. Even soft-bodied organisms (squids) have been discovered in the Boquillas.
Pen Formation (Maxwell et al. 1967) Marine shelf, 70-200 m thick. Calcareous clay shale and chalky limestone with concretionary intervals. The Pen Formation was deposited upon a shallow marine shelf. This unit also includes a westerly-thinning wedge of dark gray marine shale within the overlying Aguja Formation (e.g., Lehman 1985). This formation is widely exposed in the park in areas such as San Vicente, Mariscal Mountain, Maverick Mountain and the McKinney Hills (Turner et al. 2011). Invertebrate fossils include echinoids, bivalves, gastropods, and ammonites. Vertebrate fossils are uncommon but include fragmentary sharks, fish, and mosasaurs. However, shed shark teeth and fish vertebrae are common throughout the formation. Rarely, reworked dinosaur bones (resulting from floods washing carcasses seaward) are also encountered (pers obs. by the author).

Aguja Formation (Adkins 1933)
Originally named "Rattlesnake Beds" by Udden (1907), these strata were later re-named the Aguja Formation as the previous name was already in use elsewhere. Nearshore marine, deltaic, and continental facies including paralic, estuarial, and coastal marsh and swamp deposits (Maxwell et al. 1967;Lehman 1985), 120-280 m thick. The coastal Aguja Formation records fluctuating periods of marine transgression and shoreline progradation. Transgressive and regressive marine Aguja facies include thicker, well-indurated marine sandstones, poorly developed coals, lignitic shales, and thin cross-bedded fluvial channel sandstones. The upper part of the formation contains coastal floodplain mudstones; some with incipient paleosol development. The Aguja Formation is widely exposed in BIBE in areas such as Dawson Creek, Rattlesnake Mountain, San Vicente, and McKinney Springs (Turner et al. 2011).
Some facies within these units are very fossiliferous while others are not. Plant fossils are locally abundant in the Aguja Formation. These usually include fossilized woods from conifers, palms (monocots), and flowering plants (dicots). Rarely, tree stumps are found upright, situated in their original growing positions. Fossil leaves have been found in a couple areas preserved as carbonate films within mudstone horizons or, in one area, as impressions within reworked volcanic ash. This ash bed and its fossils are currently under study by the author (S.W.). Aguja invertebrate fossils include bivalves, gastropods, cephalopods and rarely, crustaceans. Trace fossils from some of these taxa are also relatively common (e.g., Ophiomorpha burrows).
Occasionally, vertebrate fossils (and microfossils) are also found at various stratigraphic intervals in strata representing numerous environs. Taxa include sharks, fish, turtles, crocodilians, as well as dinosaurs among other reptiles. Very rarely, small fossil mammals are encountered (mostly teeth) as are dinosaur eggshell fragments. The vertebrate fossil assemblage of the Aguja Formation is the most inclusive of its kind reported from southernmost North America.
Javelina Formation (Maxwell et al. 1967) Continental, 100-190 m thick. The formation can be found along the flanks of the Chisos Mountains and is well exposed along the drainages of Tornillo, Terlingua, and Dawson creeks, as well as Rough Run (Turner et al. 2011). This formation contains facies from inland floodplain environs. Sedimentary strata include well-cemented fluvial sandstones, rhythmically-bedded lacustrine deposits, and floodplain mudstones -some containing fairly well-developed paleosol and paleocaliche horizons (Lehman et al. 2018). Generally, fossils are uncommon throughout this formation; however, several discreet areas (and representative habitats) are quite fossiliferous (e.g., Lehman and Langston 1996). Fossil wood is common in the Javelina Formation and includes fossils from fan palms as well as conifers and flowering plants. Abundant prone fossil logs can be found along a few stratigraphic horizons while others harbor stumps in their original growing positions. Invertebrate fossils are very rare but include fresh-water gastropods and crustacean burrows.
Isolated, broken vertebrate fossils are somewhat common within scree along deflated surfaces atop fluvial sandstone hogbacks but are also found in-situ at local intervals within overbank mudstones. Vertebrate fossils include those from fish, turtles, pterosaurs, dinosaurs, and small mammals (represented mostly by teeth). Vertebrate fossils usually occur as isolated, fragmentary bones. However, a few dinosaur skeletons have been found partially articulated or with bones in close association. Although typically well-preserved, Javelina Formation fossils are seemingly not as numerous as those of the underlying Aguja Formation. As such, I surmise that the paralic Aguja environment favored a greater variety (and populations) of vertebrate species and/or the paralic environment was more conducive the burial and preservation of remains. Lehman et al. (2006) obtained a radiometric date of around 69 Ma. for the middle of the formation Black Peaks Formation (Maxwell et al. 1967) -

Cretaceous interval
Continental, around 40 m thick (widely variable) (e.g., Lehman et al. 2018). The Black Peaks Formation con-tains inland flood plain deposits with interstitial fluvial sandstones. The formation is exposed widely in BIBE especially near Grapevine Hills, Dogie Mountain, and Tornillo Flat. Paleosols are sometimes well developed, appearing as somber red and black bands which are, in places, interrupted stratigraphically by fluvial sandstones. The bottom third of the formation is Cretaceous in age. Plant fossils present in the lower Black Peaks Formation including conifers and flowering plants. Invertebrate fossils are virtually unknown; however, freshwater ?crustacean burrow structures have been observed. Vertebrate fossils are uncommon in this portion of the formation but include those of fish, reptiles, as well as dinosaurs (especially those of the huge titanosaur Alamosaurus). Usually, vertebrate fossils are found isolated, weathering out of fluvial channel sandstones. Rarely, associated dinosaur bones have been located eroding from overbank mudstones.
The Cretaceous-Paleogene (K-Pg) boundary is situated in the lower third of the Black Peaks Formation although its exact stratigraphic position remains obscure. It has been defined within a two-meter section near the Grapevine Hills (Lehman and Coulson 2002). However, it has not been this well-defined elsewhere in BIBE (see discussion in Lehman et al. 2018Lehman et al. , p. 2225. It is possible that there was a depositional hiatus during the K/ Pg time interval and that the K/Pg boundary is only preserved in very localized lenses of deposition (if at all) within the park.

Cenozoic Era -Paleogene (Paleocene Series)
Black Peaks Formation (Maxwell et al. 1967) -Paleogene interval Continental, up to 400 m thick (widely variable). The Black Peaks Formation straddles the K-Pg boundary. The Paleogene portion of the formation contains inland floodplain deposits with thick, fluvial sandstones. It is exposed near Dogie Mountain, Grapevine Hills, and Tornillo Flat (Turner et al. 2011). Paleosol horizons are often striking, appearing maroon, black or somber gray sometimes with interstitial, tan fluvial channel sandstones. Paleosols within the Cretaceous, Aguja and Javelina formations are often poorly developed. However, they become increasingly better developed higher in section with the Black Peaks having the most conspicuous forms. Typical vertebrate fossils include garfish, turtles, and mammals.
Plant fossils (mostly conifers) are rarely found in the lower part of the formation but are more common higher in section. Two, closely-space stratigraphic intervals of very large fossil dicot logs (Paraphylanthoxylon) in the middle portion (Torrejonian-Tiffinian) of the Black Peaks section (informally called the "log jam sandstone") suggest the post K-Pg resurgence of trees during this time.
This fossil log horizon is conspicuous in many areas of the park and is a useful stratigraphic marker (Lehman et al. 2018).
Cenozoic Era -Paleogene (Eocene Series) Hannold Hill Formation (Maxwell et al. 1967) Continental, varies from around 30 to 70 m in thickness (e.g., Lehman et al. 2018). This relatively thin formation is very limited in area with all known outcrops in the Tornillo Flat region of BIBE and represents the final infilling of the Tornillo Basin (Turner et al. 2011). The inland floodplain formation contains variegated mudstone-dominated facies along with coarse fluvial sandstones and conglomerates. Vertebrate fossils include those from several mammalian taxa. The fossil bone exhibit in BIBE is situated atop fluvial channel sandstones of the Hannold Hill Formation (Exhibit Ridge Sandstone Member) where numerous specimens of Coryphodon were excavated and displayed as part of the park's original Fossil Bone Exhibit.
Canoe Formation (Maxwell et al. 1967) Continental (upland), up to 350 m thick. This formation is exposed in the north-central portion of BIBE especially on Tornillo Flat (Turner et al. 2011). It contains rocks from a sandy, braided fluvial system with associated flood plain deposits (e.g., Rigsby 1986;Runkel 1988) which rest unconformably on the Hannold Hill Formation. Thick sandstones and conglomerates comprising the conspicuous Big Yellow Sandstone Member are present in the lowest part of the Canoe Formation with gray and variegated mudstones situated a bit higher in section. These paleosol horizons (along with interstitial sandstones and tuffaceous mudstones) make up a large portion of the Canoe Formation section above the Big Yellow Sandstone.
Vertebrate fossils are widespread within the formation in BIBE as well as areas northwest of the park in the Devil's Graveyard Formation which is temporally coeval with the Canoe Formation (e.g., Runkel 1988). The reader is cautioned that the Devil's Graveyard Fm. is not exposed within the park so its reported taxa are not included herein. Vertebrate fossils in the Canoe include those from mammals, turtles, and crocodilians. Fossilized wood is also common in the Big Yellow Sandstone including not only Eocene conifers and dicots but reworked and abraded, fossilized Cretaceous wood fragments exhumed during entrenchment of the younger, Eocene fluvial system. A striking example of its fossil ensemble includes a dense 'forest' of at least 92 fossil tree stumps in their original growing position observed by the author near the McKinney Hills. Whether these represent conifers or dicot trees is not yet known. However, these stumps (~10 to 15 cm in diameter) are the remains of smaller trees that apparently grew on islets within the confines of the braided fluvial corridor.

Cenozoic Era -Paleogene Period (Late Eocene and Oligocene Series)
Chisos Formation (Udden 1907) Continental (upland), from 500-700 m thick. The Chisos Formation is exposed in many areas of BIBE along the flanks of the Chisos Mountains (Turner et al. 2011). This widely variable formation contains lavas, tuff, tuffaceous sandstone, clay, and conglomerates. Vertebrate fossils include turtles and large mammals while invertebrates include fresh-water gastropods and snails). Fossil wood is present but not common.
South Rim Formation and "Burro Mesa" Formation (Maxwell et al. 1967;Turner et al. 2011, respectively) Please note that the Burro Mesa Formation is not considered valid by all researchers and so both are included together here. Continental (volcanic), from 300-500 m thick. These typically massive, volcanically-derived strata are exposed in the central and southwest portions of BIBE in the Chisos Mountains and near Burro Mesa. They contain lavas, flow breccias, conglomerates, tuff, and tuffaceous sediments from various localized eruptive events and are apparently non-fossiliferous.

Neogene (Miocene Series)
Delaho Formation (Stevens et al. 1969) Continental (bolson deposits), up to 300 m thick. The formation is exposed on the west side of BIBE near Castolon (Lehman and Busbey 2007;Turner et al. 2011). Originally identified by Maxwell et al. (1967) as 'older gravels', the Delaho has two members including the lower member and Smokey Creek Member. These contain pink friable sandstone and gray conglomerate representing mid and distal alluvial fan deposits that accumulated in a fault bounded basin in the western half of BIBE (the Delaho Bolson). Vertebrate fossils include those from small and large mammals as well as from several reptiles including a unique Gila monster. Banta Shut-In formation (informally proposed by Stevens and Stevens 1985) Continental (bolson deposits), up to 150 m thick. This formation is exposed in the east-central portion of BIBE near Banta Shut-In. These include pink fine-grained sandstone, siltstone and red mudstone which represent distal alluvial fan facies in the eastern half of BIBE (Estufa Bolson). Vertebrate fossils include amphibians, reptiles, and mammals (including those from canids, camels, and primitive horses). This formation is exposed in areas along Tornillo Creek that are not easy to reach and it is likely that its fossiliferous nature has yet to be fully realized.

Neogene (Pliocene -Pleistocene series)
Fingers and Estufa Canyon formations (informally named by Stevens and Stevens 1989) Continental (bolson deposits), variably thick up to 300 m. These formations are exposed in the western portion of BIBE near Sotol Vista and along the flanks of Tornillo Creek east of Dugout Wells and consist mostly of bolson deposits. They were originally identified as 'older gravels' by Maxwell et al. (1967) and consist of proximal alluvial fan facies which overlie the Delaho and Banta Shut-In formations (Turner et al.2011). Primarily these contain larger sand and gravel clasts eroded relatively recently from the volcanic and plutonic rocks of the Chisos Mountains. However, they also contain scree from Paleozoic and Late Cretaceous strata exposed along the margins of the ancient bolson. The fingers and Estufa Canyon formations represent the youngest deposits within the Delaho and Estufa bolsons and have yet to produce fossils.

Pleistocene terrace deposits and grottos
Thin alluvial gravels, sands, silts, caliche-cemented silts, small dune fields harboring a variety of localized cut-andfill structures and small head-cutting drainages harboring a variety of finely to poorly sorted rock types. These thin deposits form desert pavement atop alluvial terrace remnants where aeolian erosion and sheet-wash have often removed finer sediments (Turner et al. 2011). Fossils from the Pleistocene of BIBE are almost unknown at present however mammoth teeth have been found within a caliche deposit in BIBE near Grapevine Spring which may represent the former location of a Pleistocene ciénega during the most recent glacial age (see Maxwell et al. 1967, p. 154 for a photo of the in-situ teeth).
Numerous cliffside grottos can be also found throughout BIBE. Of interest is the discovery within one of these near Mule Ears Peaks of remains pertaining to California condors which no longer live in the Big Bend region. Whether these remains are truly fossils or not is debatable. However, they are estimated to be thousands of years old (Wetmore and Friedmann 1933).

Fossil taxonomic lists: methods
'Taxonomy' is the scientific study of naming, defining, and classifying groups of biological organisms based on shared or differing morphological characteristics. The following taxonomic lists were compiled from hundreds of reliable sources. These included peer-reviewed scientific reports, graduate-level academic studies (e.g., Ph.D. dissertations and Master's Theses), field trip guidebooks, scientific abstracts, as well as verifiable first-hand accounts (current research) reported to the author by qualified researchers. In the interest of compiling a comprehensive taxonomic catalog of fossils from Big Bend National Park (and immediate surrounds), all reported taxa are included. This distinction is important because, in some cases, a species reported decades ago may have more recently been taxonomically re-classified differently as something else. As a result, some older taxa may no longer be valid and/or a few may be recorded twice as the result of different taxonomic interpretations. In other cases, taxa may be listed multiple times with varying degrees of certainty (e.g., sometimes with a question mark or designated as a possible new species -see below). These are all included in the present report as they may represent more than one species. This circumstance serves to illustrate our constantly changing understanding of how species relate to one another.
The taxonomic lists presented here are organized alphabetically within classes of the Linnaean taxonomic classification system. Their common names are also provided as well. This serves to simplify the identification and listing of each species (from the perspective of interpretation) and allows for the convenient addition of future data within each table. This simplified method was chosen because taxonomic groupings at family-level (and below) often complicate matters to the point of utter confusion for non-scientists -especially as classification systems and taxonomic relationships are revised when new information comes to light.
Furthermore, additional taxa have been added to the original catalog produced by Wick and Corrick (2015) given that new discoveries have occurred since that time. For example, new taxonomic information was included by the author as late as September 2021 as the result of his ongoing (preliminary) research involving boney fishes from the Aguja Formation. However, although the present catalog is an exhaustive listing of taxa, it likely does not include absolutely every fossil species known from BIBE. Certainly, some discoveries have yet to be formally recorded (for example, the author and his colleagues have several works in progress), or some species may have been presented in older, more obscure, and/or unpublished contexts such as field trip guides and/or scientific abstracts and academic poster sessions. As such, some species have likely been missed during the literature survey. However, there are around 1400 different fossil species listed in this catalog alone.
These lists also embrace the 'morphotype concept' of taxonomy and is used so that scientists can communicate with each other more effectively. For example, different types of plant fossils from a single taxon are often named differently because that plant species may be expressed in the paleontological record by multiple fossil morphotypes (such as fossil wood, leaf impressions, and/or pollen). From this example, unless all three types of plant fossils are found in close association, each type of fossil cannot be conclusively determined to pertain to the same plant species. Hence, each form is given its own name until a direct association can be confirmed. As such, a single plant species may unknowingly be represented here by more than one morphotype (and scientific name). Also included in these lists are non-body fossils (such as crustacean burrows and dinosaur eggshell fragments) produced by a living organism. These are also classified and named using the morphotype concept since they do not represent the actual fossilized remains of a particular animal, but only the preserved evidence of its lifeway.
Also included are the formations in which the fossils occur as well as the original (or significant) publications in which they were reported. Because commonly encountered species (e.g., various sharks among others) are mentioned in numerous reports, it is simply impractical to include every reference for many of these commonly reported species. It is, therefore, up to the reader to use the listed sources as springboards for further research. Problematic taxa and /or references indicated by an asterisk are discussed at the bottom of each list.
Finally, the reader needs to be aware that the author of the present work did not make any of the taxonomic interpretations for a particular species listed herein unless (as in a few cases) he actually authored one of the referenced papers. Among the names of the species listed herein, the reader will sometimes see various abbreviations associated with them. The applications of abbreviations such as these are standard practice among taxonomists (e.g., see Bengston 1988) and were assigned by the various authors of the referenced works and serve to indicate that they had some doubt regarding their taxonomic assignment of a particular species. This doubt may have resulted from a specimen being broken or incomplete, being obscured by rock, or the fact that it exhibits some morphological variation compared to others of its kind. For example, the use of "cf." before a species name indicates that a particular author felt that a particular specimen "compared favorably" enough to the listed species to suggests that it likely pertains to it. On the other hand, the term "aff." suggests that although a specimen has "affinities" to particular taxon, it is different enough that it may, in fact, represent a different, closely related species. Question marks are also sometimes used immediately before a specie's name to indicate even more doubt. In any case, a number of specimens listed here represent new genera and/or species that were deemed by the various authors of the referenced works as potentially being new (or potentially new) to science (e.g., those designated with n. gen and/or n. sp. in the taxonomic tables). These species are indicated immediately after their listed names in the following manner: 1) formally published new species (scientifically valid holotypes) are designated by a black dot; 2) specimens that are likely new to science (but have yet to be formally named) are designated by a cross; and 3) specimens that have been named but not published in a formal context (e.g., an unpublished Ph.D. dissertation) are designated by an open triangle. A legend to this effect is present at the bottom of each table. It is worth noting that among the many species new and potentially new to science listed here, only 44 are presently considered to be scientifically valid species (black dots). The remainder (open triangles and crosses) are not considered scientifically valid at the present time. Their inclusion in this publication was done out of thoroughness and their listings herein are not an attempt to formally validate them.

Discussion
Fossil plants (Table 1) Since 1907, when Johan Udden first reported the occurrence of fossil wood in what would become Big Bend National Park, over 300 fossil plant taxa have been described including flowering plants (dicots), palms (monocots), conifers, tree ferns, leaf impressions, algae, palynomorphs and tree resin (amber). Because of the changing environment over time, fossil plant remains range from marine, coastal, and inland varieties spanning a diverse range of paleohabitats. Numerous type specimens (nine) have been formally described with several others having been recognized but not yet reported. Two-thirds of the fossil plant species reported from BIBE pertain to palynomorphs (e.g., pollen, spores, fungi, etc.).
Although fragmentary fossil wood specimens are observed within many continental strata in BIBE, they are uncommon or absent in most locations. However, a few horizons produce spectacular fossil logs, sometimes by the dozens (Lehman et al. 2018). The fossils within these assemblages normally occur as prone trunk segments up to several meters in length and up to three meters in diameter. In some areas, dozens of fossil trunks can be observed holding up small ridges within mudstone-dominated flats or protruding from fluvial sandstone horizons. In rare occurrences, stumps are preserved intact in their original positions of growth with root buttresses splayed from their bases. Several sites of this type have multiple individuals of the same species or a combination of species forming true fossilized paleo-forests (e.g., Lehman and Wheeler 2001;Lehman and Shiller 2020).
The degree of preservation involving fossil woods from BIBE ranges from those having experienced near-complete permineralization (i.e., exhibiting few visible diagnostic attributes) to those that preserve very detailed morphological features such as growth rings and cellular structure such as compression wood, parenchyma, and cross-field pitting (e.g., Wheeler andLehman 2000, 2005). It is the latter type which is most useful from a diagnostic standpoint. This has resulted in the diagnosis of several new fossil species and provided insights into tree growth rates, sizes, and their preferred environments. Other specimens of fossilized wood are interesting from additional perspectives. In some cases, fossil woods are almost completely carbonized suggesting the occurrence of an-  Lawson (1972), the "Tornillo Formation" section containing botanicals and palynomorphs is now recognized as the lower Canoe Formation (Turner et al. 2011;T. Lehman, pers. comm.). * M.L. Abbot passed away prior to formal submission of her unpublished (1985) manuscript (now accessioned at BIBE). Oddly, Abbott's report contains a number of new taxa (14), all with the specific epithet maxwellii. It is not known if all were to be named in honor of Ross A. Maxwell or if the epithet was simply a placekeeper for different specific names to be added later. Additionally, a later review of Abbott's work was conducted by E. Wheeler and T. Lehman during the course of their research on fossil woods from the park. Their findings suggest that Abbott's specimens are, in fact, different morphotypes of the same wood taxon (Paraphyllanthoxylon) which casts doubt on the validity of the "holotypes" presented by Abbott. However, Wheeler recognized the contributions made by Abbot and named Paraphylanthoxylon abbotti in her honor (Wheeler 1991). A posthumous synthesis of Abbot's work (Abbot 1986) was presented by D. Rohr (Ed.). cient forest fires. In others, insect (?termite) borings and frass have also been preserved (Rohr et al. 1986).
Fossil wood is widespread in BIBE and many specimens are situated near roads, trails or camping areas. From the public's perspective, they are also some of the most recognizable types of fossils in BIBE and as a result, are often reported to park management by visitors. Because they are somewhat obvious and popular, "petrified" woods also remain one of the most easily vandalized fossil types in many NPS fossil-parks which has led to the loss of valuable scientific information (Wick and Corrick 2015).
Invertebrate fossils (Table 2) Over 500 fossil invertebrate taxa have been reported from BIBE including sponges, corals, bivalves, gastropods, ammonites, nautiloids and crustaceans, as well as a host of foraminifera. Invertebrates have been observed in many formations within BIBE from marine, brackish and freshwater facies. Five, scientifically valid type specimens have been described from BIBE and several 'new' taxa have yet to be formally reported.
Invertebrate fossils are regularly discovered in marine and brackish water facies within BIBE. Because of their abundance, form, and common occurrence along the modern shores of North America, invertebrate fossils are very popular as they are easily recognizable to park visitors of all ages. Fresh water taxa are much less common than their saltwater counterparts however they are occasionally discovered in lacustrine and fluvial deposits within some continental strata in the park. Many invertebrate fossils are preserved as steinkerns which represent the fossilized fill of a hollow organic structure (such as a mollusk shell) that formed when mud or sediment consolidated within the structure and the structure itself disintegrated or dissolved. Many invertebrate fossils are found in Lower Cretaceous, marine carbonate rocks along the fault-scarps which flank the northeastern and southwestern margins of the park. Upper Cretaceous forms are commonly preserved along with the remains of vertebrate taxa (such as sharks and mosasaurs) in near-shore marine mudstones and marls of the Boquillas and Pen formations surrounding the Chisos Mountains and exposed just west of the park. Some invertebrates from BIBE are particularly useful as stratigraphic index fossils. These include the ammonite Allocrioceras hazzardi (Young) and the bivalve Inoceramus undulatoplicatus (Roemer) both from the shallow marine, Boquillas Formation (e.g., Maxwell et al. 1967).
The preservation of invertebrate fossils varies by formation and facies. Lower Cretaceous, marine carbonate rocks often preserve invertebrate fossils such as bivalves and gastropods, but these are often entombed in dense carbonate matrix and are very difficult to extract without damage. Microinvertebrates are also difficult to separate from these rocks and require laboratory preparation (thin section samples) to study the fossils within. Upper Cretaceous strata have produced well preserved, intact invertebrate specimens (e.g., ammonites and bivalves) which occur as steinkerns in carbonate facies or within concretionary horizons. Some are difficult to remove from bedrock while others can be quarried easily. Other bottom-dwelling invertebrates often occur in marine mudstones and shales as isolated individuals or in loose, congregated groups such as the oyster Flemingostrea pratti (Stephenson) and sea urchin Hemisaster (Desor).
Occurrences of this type are often observed in horizons within marine or brackish water facies which may contain dozens of individuals which inhabited muddy estuarial bottoms. Some fossils, such as the oyster Crassostrea cusetta (Sohl and Kauffman) are often found in dense groups (formerly bioherms). Many individuals exhibit obvious warping of their shells as the result of a congested colonial lifeway. Trace fossils (e.g., burrow structures) are also routinely observed in various marine strata.
Vertebrate fossils (Table 3) Over 250 vertebrate fossil taxa have been reported from strata within BIBE with 30 type specimens (holotypes) having been so far described. Numerous other specimens have been identified as pertaining to unique species but have not yet been formally described or named. The fossil taxa recovered from BIBE involve a variety of animals from marine, brackish, and freshwater habitats as well as many others from inland terrestrial environs. Although the park has good exposures of marine strata representing open marine environs, marine vertebrates are not well represented in the park. For example, marine rocks of the Boquillas Formation have been more productive just outside of BIBE where this formation is better exposed and more accessible; local private collectors have discovered some outstanding vertebrate specimens from these strata (e.g., Bell et al. 2013). Correspondingly well-preserved specimens pertaining to these same marine species are likely present within the park as well but have yet to be found.
Vertebrate fossils are more numerous (but still uncommon) throughout the Late Cretaceous, paralic and terrestrial strata within the park with some Late Cretaceous formations being more productive than others. However, sharks, fish, amphibians, reptiles, and mammals are well-represented in the BIBE fossil record. Most of these fossils are commonly observed as isolated fragmentary bones, many of which show some degree of damage or reworking as the result of pre-burial transport. Furthermore, those formations that more frequently produce vertebrates (e.g., the Aguja and Javelina Formations) are apparently devoid of them in many stratigraphic exposures and horizons while other outcrops are locally productive. In uncommon cases, numerous bones pertaining to a single individual have been found in close association or (more rarely) in articulation (e.g., Lehman and Wick 2010;Tykoski and Fiorillo 2016). However, complete skeletons are unheard of in BIBE. This circumstance has vexed many of us who have spent decades searching for good specimens in the park. However, it is very likely that deposition rates did not favor the rapid burial of carcasses here.
Although complete fossilized bones are infrequently encountered in Late Cretaceous strata, conspicuous vertebrate fossils are less common in Tertiary strata of BIBE. Although some larger, associated specimens have been discovered (e.g., Wilson 1967) most Tertiary fossil taxa have been diagnosed from small bone fragments or isolated teeth (e.g., Stevens et al. 1969;Schiebout 1974;Standhardt 1986). Vertebrate microfossils are also common within both Cretaceous and Tertiary strata of BIBE although finding especially productive sites is remarkably challenging. Furthermore, although some microvertebrate specimens can be surface picked in the field, much of the microfossil material so far reported from the park has been collected via screen-washing or acidization of bulk matrix and collected microscopically (sometimes over years) in the laboratorya laborious process. In any case, the critical importance of vertebrate microfossil sites cannot be overstated. Microfossils representing multiple, coexisting species from a single locality almost always tell scientists much more about an ancient ecosystem than do large, isolated bones or partial skeletons of a single animal. For example, just a handful of highly productive sites within the Aguja Formation have produced thousands of microvertebrate fossil specimens Abbot ML (1986)