Texas Fossils: An Amateur Collector's Handbook

by William Henry Matthews

Geology

Geologic studies of the State of Texas have indicated the presence of rocks formed during every era and [period] of geologic time. These range from the [Precambrian] granites of the Llano uplift to the [Quaternary] gravels of the High Plains.

Plate 10
GENERALIZED [GEOLOGIC MAP] OF TEXAS
Modified from Geologic Map of Texas, 1933

[This map in a higher resolution]

One of the best ways to become acquainted with the geology of Texas is to study the [geologic map] of the State ([Pl. 10]). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On [Plate 10], colors are used to show the distribution and [geologic age] of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and [formation] contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings.

[PRECAMBRIAN] ROCKS

The [Precambrian] rocks of Texas are composed of igneous and metamorphic rocks and some sedimentary rocks. Most of the Precambrian outcrops are in the Llano uplift and El Paso and Van Horn regions.

Alterations produced by vast amounts of time, heat, and pressure have obliterated any trace of fossils that may have been present in these rocks. With the exception of some questionable primitive plants collected in the Van Horn region, no [Precambrian] fossils have been reported from Texas.

[PALEOZOIC] ROCKS

Rocks of [Paleozoic] age are widespread in Texas, and rocks of each [period] are well exposed. Outcrops are found in the Llano uplift, North-Central Plains, and Trans-Pecos region. The most extensive exposures are of [Pennsylvanian] and [Permian] age, and the former are highly [fossiliferous] in parts of the North-Central Plains.

[Cambrian]

Rocks of late [Cambrian] age are exposed in the Llano, Marathon, and Solitario uplifts, and the Franklin Mountains near El Paso. These are sedimentary rocks consisting of conglomerates, sandstones, shales, limestones, and some dolomites.

Some of these formations are relatively [fossiliferous], but the specimens are commonly fragmental and very poorly preserved. Fossils that are apt to be found in the [Cambrian] rocks of the Llano uplift include brachiopods, gastropods, trilobites, and small rounded objects believed to have been formed by algae (primitive one-celled plants). In other parts of the State, Cambrian rocks are sparsely fossiliferous and the fossils consist primarily of fragmental brachiopods, trilobites, and algae.

[Ordovician]

[Ordovician] outcrops are present in the Llano uplift of central Texas and in the Marathon, Solitario, El Paso, and Van Horn regions of Trans-Pecos Texas. These are sedimentary rocks and consist largely of sandstones, cherts, limestones, and dolomites.

Although some of the [Ordovician] formations are [fossiliferous], they are seldom collected by amateur paleontologists because they are exposed in relatively inaccessible places and the fossils are usually poorly preserved. Ordovician fossils reported from Texas include sponges, corals, brachiopods, gastropods, cephalopods, and trilobites. In addition, the Marathon [formation] of the Marathon uplift contains large numbers of well-preserved graptolites ([fig. 24], [p. 86]).

[Silurian]

The [Silurian] of Texas is poorly represented in surface exposures, and only one [formation], the Fusselman, has been described. The Fusselman crops out in the El Paso and Van Horn regions where it is a white dolomitic limestone. Fossils are not abundant in this formation, but brachiopods and corals have been collected at a few localities.

[Devonian]

[Devonian] rocks are best developed in Trans-Pecos Texas, especially in the Marathon, El Paso, and Van Horn regions. In addition to the Trans-Pecos exposures, there are minor outcrops of Devonian rocks in the Llano uplift of central Texas.

Fossils are rare and fragmental in the Trans-Pecos exposures and consist primarily of radiolarians and brachiopods. The [Devonian] rocks of central Texas are predominantly [calcareous] and, although the material is usually poorly preserved, many fossils have been collected from them. These include bryozoans, corals, brachiopods, gastropods, and trilobites. Conodonts and fragments of primitive armored fishes ([Pl. 37]) have also been reported.

[Mississippian]

[Mississippian] rocks are exposed in the Llano region and in the Hueco Mountains of the Trans-Pecos area. The Trans-Pecos rocks primarily contain brachiopods with some bryozoans and gastropods.

The central Texas [Mississippian] rocks are much more [fossiliferous] and some of the material is well preserved. Fossils reported from this area include brachiopods ([Pl. 17]), crinoids, gastropods, cephalopods, trilobites, and ostracodes.

[Pennsylvanian]

[Pennsylvanian] rocks are well represented in Texas and are exposed in the Llano uplift, north-central Texas, and Trans-Pecos Texas.

In Trans-Pecos Texas [fossiliferous] rocks crop out in the Hueco and Diablo Mountains. Fossils found in this area are algae, fusulinids, corals, brachiopods, pelecypods, gastropods, cephalopods, and crinoids. There is also a thick section of [Pennsylvanian] rocks in the Marathon uplift, but only one [formation], the Gaptank, is very fossiliferous. It contains many fossils including fusulinids, sponges, corals, bryozoans, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Certain [Pennsylvanian] strata in the Llano region are very [fossiliferous], and the material is well preserved. The more abundant forms are fusulinids, corals, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.

Probably the best [Pennsylvanian] collecting areas are to be found in north-central Texas. Here the thick marine limestones and shales contain large numbers of well-preserved invertebrate fossils, and the terrestrial or shallow marine strata have yielded an abundance of plant fossils. Invertebrate fossils are apt to be found along the banks of streams and gullies and in railroad and highway cuts. Many of the limestones bear large numbers of fusulinids or crinoid stems, and the shales may contain many corals, brachiopods, and mollusks. The best collecting will, of course, be found where the rocks have been sufficiently weathered.

Fig. 8. Sketch of typical crinoidal limestone from the [Pennsylvanian] of north Texas.

Typical invertebrate fossils are foraminifera (principally fusulinids), corals (especially the [solitary] or “horn” corals), brachiopods, bryozoans (the lacy and branching types are most common), pelecypods, gastropods (exhibiting a [variety] of coiling), cephalopods (nautiloids and goniatites predominate), and crinoids, which in many areas are found in thick crinoidal limestones ([fig. 8]). Some typical [Pennsylvanian] fossils are illustrated in Plates [14], [15], [17], [18], [19], [20], [21], [24], [32], and [35].

[Permian]

[Permian] rocks are found in widely separated areas in Texas. The best exposed section of marine Permian rocks is found in the Glass Mountains of Brewester County, and many of these rocks are very [fossiliferous]. The original shell material of some of the Permian fossils of this area has been replaced by [siliceous] material which is very well preserved. These silicified fossils are removed from the limestone by solution in acid, and some most remarkable specimens have been recovered in this manner ([Pl. 3]). Brachiopods are the most common fossils, but corals, bryozoans, and mollusks have also been recovered.

Extensive [Permian] exposures occur also in the central part of the North-Central Plains region. These rocks were formed from sediments of both marine and continental origin and some of them are [fossiliferous]. The marine rocks contain a [variety] of invertebrate fossils including brachiopods, pelecypods, gastropods, and ammonoids. Those rocks representing terrestrial deposits contain [vertebrate] remains at many localities, and numerous amphibians and primitive reptiles ([Pl. 40]) have been collected from them.

[MESOZOIC] ROCKS

[Mesozoic] rocks occur over a wide area of Texas and include exposures of [Triassic], [Jurassic], and [Cretaceous] age. Many of the Upper and Lower Cretaceous outcrops are quite [fossiliferous] and easily accessible and thus of considerable interest to many amateur collectors.

[Triassic]

[Triassic] rocks crop out in parts of the High Plains, the Glass Mountains of Trans-Pecos Texas, and parts of Pecos, Crockett, Upton, Reagan, and Glasscock and other west Texas counties. These are predominantly nonmarine rocks consisting of conglomerates, sandstones, shales, and some gypsum beds.

[Triassic] fossils are almost exclusively vertebrates, although some poorly preserved plant and invertebrate remains have been reported. [Fossil] vertebrates of the Texas Triassic include phytosaurs ([Pl. 42]), crocodiles, amphibians, and fish.

[Jurassic]

In Texas, surface exposures of [Jurassic] rocks are known only from Malone Mountain in southwestern Hudspeth County. The rocks there are limestones, shales, sandstones, and conglomerates. Fossils reported from that locality include marine and fresh-water pelecypods, fresh-water gastropods, and ammonites.

[Cretaceous]

Rocks of [Cretaceous] age are widely distributed in Texas and represent one of the more important [rock] systems of the State. Cretaceous outcrops occur in central Texas, north Texas, the Edwards Plateau, parts of the High Plains, the Gulf Coastal Plain, and Trans-Pecos Texas.

As mentioned [earlier], the Texas [Cretaceous] has been divided into the Lower Cretaceous (Comanche [series]) and Upper Cretaceous (Gulf series). These rocks consist primarily of marls (a type of [calcareous] clay), shales, chalks, and limestones, but sands and conglomerates also occur. Cretaceous rocks occur on the surface of about 28 percent of Texas, and many of the larger cities of the State are situated on Cretaceous strata.

Many of the Gulf and Comanche formations contain fossils which are of interest both to amateur and professional paleontologists. Because of their wide distribution in and near large population centers, [Cretaceous] outcrops can be conveniently visited by many amateur [fossil] collectors. The fossils are usually abundant and varied, and some are well preserved. Although numerous kinds of fossils may be collected, the more common forms are cephalopods, pelecypods, gastropods, and echinoids. Some of the more typical Cretaceous fossils are shown in Plates [16], [21], [25]-28, [32], [33], [35], and [36].

[Cretaceous] fossils are more commonly found in shales and chalky limestones. [Fossiliferous] outcrops of these rocks can be found along many streams, roads, and highways of central Texas, north Texas, and the Edwards Plateau. Outcrops which have been weathered are more likely to provide good collecting. In general, collecting is poor in areas covered with heavy vegetation or recent stream deposits. Good collecting localities are outcrops which have a fairly steep slope with a covering of weathered [rock] material and a minimum of vegetation. One should move slowly from the base of the slope upward while searching the ground for any evidence of fossils, and particular attention should be given to any small gullies since these often contain fossils that have been washed out of upper beds in the exposure.

[CENOZOIC] ROCKS

[Cenozoic] rocks are widespread in Texas but occur primarily in a broad belt along the Gulf Coastal Plain. In addition, there are exposures of nonmarine Cenozoic strata in the High Plains, North-Central Plains, and Trans-Pecos region. There are also many exposures of Cenozoic igneous rocks in Trans-Pecos Texas.

Rocks of [Cenozoic] age occur in more than one-third of Texas and consist of conglomerates, sands, clays, and some limestone and lignite beds.

[Tertiary]

Extensive exposures of [Tertiary] rocks trend northeast-southwest in a broad band across the Gulf Coastal Plain area. These strata, consisting of sands, clays, and poorly consolidated limestones, are underlain by [Cretaceous] rocks.

Invertebrate fossils are common in certain [Tertiary] formations and pelecypods, gastropods, and corals are the predominant forms. In general, however, [fossiliferous] exposures are of local occurrence and most of the Tertiary formations are unfossiliferous. Those Tertiary invertebrates that are present, however, are often well preserved and represent many interesting types (Pls. [16], [22], [23], [29], [30], [31]).

[Tertiary] invertebrate fossils are commonly found in sands, clays, and marls. Many of these sands and marls have a green color which is due to the presence of [glauconite] (a green mineral containing iron and closely related to the micas). At certain localities on the Gulf Coastal Plain the glauconite marls and sands of the Weches and Crockett formations contain large numbers of well-preserved clams, snails, and corals. [Fossiliferous] exposures of Tertiary rocks are sometimes found in road cuts, but better exposures may be found along the banks of rivers and creeks. Certain bluffs along the Brazos, Sabine, and Trinity rivers are well-known Tertiary [fossil] collecting localities. Many of these better localities are listed in some of the Bureau of Economic Geology bulletins included in the bibliography of this publication (pp. [109]-110).

[Quaternary]

[Quaternary] deposits of [Pleistocene] age (geologic time scale, [Pl. 1]) are found in many parts of Texas and consist of sands, clays, and gravels.

These rocks are distributed along the Gulf Coast in a belt from 50 to 100 miles wide. They occur also as stream terraces in the Edwards Plateau and North-Central Plains regions. In addition, [Quaternary] sands and gravels are widely distributed over the surface of much of Trans-Pecos Texas. There are also [fossiliferous] [Pleistocene] strata in the High Plains region.

Invertebrate fossils are rare in [Pleistocene] rocks, but some fresh-water and terrestrial mollusks occur. [Vertebrate] remains, however, are abundant in many localities, and large numbers of horses, camels, mammoths, and other mammals (Pls. [46]-49) have been collected. [Fossil] bones and teeth (figs. [25], [26], [p. 104]) are commonly found in the gravels and sands of many of the river terraces of the State.