Trilobites are extremely abundant and diverse in west-central Utah, in the middle Cambrian rocks exposed in the House Range of Millard County (Figure 3.43). One of these trilobite species is especially familiar: Elrathia kingii is one of the most abundant trilobite species found in North America, and is commonly sold commercially. Utah’s middle Cambrian rocks (especially the Wheeler Formation) also contain fossils of soft-bodied organisms, including arthropods, sponges, brachiopods, and echinoderms, similar to those of the famous Burgess Shale in British Columbia, Canada (Figure 3.44). Such exceptional preservation was possible in part because these rocks formed from marine sediments deposited in anoxic (very low-oxygen) conditions. Cambrian and Ordovician fossils are also common in the rocks of southern Arizona (especially Cochise County) and in southwestern and south-central New Mexico. In New Mexico, the Bliss Formation contains at least 19 species of trilobites as well as numerous brachiopods and conodonts.
The early Ordovician El Paso Formation and overlying Montoya Formation extend from southern Arizona and New Mexico into western Texas. More than 400 species of marine invertebrates have been reported from these two units, including trilobites, corals, brachiopods, bryozoans, nautiloids, gastropods, graptolites, and the oldest known crinoids in the world (Figures 3.45 and 3.46). Ordovician rocks also occur in scattered outcrops across west-central and northwest Utah, and contain some of the most diverse and best preserved marine assemblages of this age anywhere in the world. Silurian-aged rocks are much less abundant in the Basin and Range than are older or younger rocks. In Millard County, Utah, Silurian rocks contain corals and brachiopods. In southern New Mexico and central Arizona, middle and late Devonian deposits contain abundant and diverse marine fossils, especially brachiopods, corals, and bryozoans (Figures 3.47–3.49).
Figure 3.43: Trilobites from the middle Cambrian of western Utah. A) Kootenia spencei, approximately 6 centimeters (2 inches) long. B) Itagnostus interstictus, approximately 0.6 centimeters (0.2 inches) long. C) Genevievella granulata, approximately 3 centimeters (1.3 inches) long. D) Chancia ebdome, approximately 4 centimeters (1.6 inches) long. E) Athabaskia bithus, approximately 5 centimeters (2 inches) long. F) Elrathia kingii, approximately 2.5 centimeters (1 inch) long. G) Asaphiscus wheeleri, approximately 6 centimeters (2.4 inches) long. H) Amecephalus idahoense, approximately 5 centimeters (2 inches) long.
Figure 3.44: Exceptionally preserved Cambrian fossils from western Utah Lagerstätten. A) Sponge, Valospongia, approximately 17 centimeters (7 inches) tall. B) Worm, Wronascolex ratcliffei, approximately 12.5 centimeters (5 inches) long. C) Wiwaxia, fossil and reconstruction, approximately 3.6 centimeters (1.4 inches) long. Wiwaxia's classification is subject to debate, and it is currently classified as either an annelid or mollusk. D) Arthropod, Nettapezoura basilica, approximately 13.5 centimeters (5 inches) long. E) Echinoderm, Gogia, fossil and reconstruction, approximately 5 centimeters (2 inches) long. Gogia belongs to a primitive group of echinoderms known as eocrinoids, which existed from the Cambrian to Silurian periods.
Trilobites
Trilobites are iconic Paleozoic fossils; they were more common in the Cambrian and Ordovician than in later periods, and became extinct at the end of the Permian. They were marine arthropods, and had well-defined head, tail, and thoracic (leg-bearing) segments. Most had large compound eyes, often with lenses that are visible to the naked eye. In life, they had antennae like many other arthropods, but since these were not mineralized, they only fossilize under exceptional circumstances. Many could roll up for protection, and several species also had large spines.
Sponges
Sponges (Phylum Porifera) are the simplest major group of animals; their earliest fossils appear in the late Precambrian. Most modern sponges live in the ocean and usually have basket-shaped bodies. They live by filtering food and oxygen out of water pumped in through openings in their body walls and out through a larger opening at the top. The familiar bath sponge has no mineralized skeleton, but many other kinds of sponges have skeletons composed of tiny structures called spicules, which are made of calcium carbonate (CaCO3) or silica (SiO2). It is these skeletonized sponges that have the greatest likelihood of becoming fossils. Over their long history, such sponges have frequently been important contributors to reefs and reef-like mounds.
Figure 3.45: Ordovician marine invertebrates from the Basin and Range region (Bliss, El Paso, Montoya formations). A) Trilobite, Presbynileus, approximately 3.7 centimeters (1.5 inches) long. B) Trilobite, Illaenurus, approximately 4 centimeters (1.6 inches) long. C) Trilobite, Hintzeia, approximately 2.5 centimeters (1 inch) long. D) Trilobite, Briscoia (head and tail), approximately 2 centimeters (0.75 inches) wide. E) Nautiloid, Bisonoceras, approximately 10 centimeters (4 inches) long. F) Restoration of the world's oldest known crinoid, Titanocrinus sumralli, from the Ordovician of Utah, approximately 20 centimeters (8 inches) tall. G) Edrioasteroid (extinct group of encrusting echinoderms), approximately 7 millimeters (0.3 inches) wide. H) Brachiopod, Shoshonorthis, approximately 2 centimeters (0.75 inches) long. I) Sea star, 4 centimeters (1.6 inches) wide.
Carboniferous rocks in the Basin and Range represent an archipelago of warm, shallow seaways and uplifted islands. The Mississippian Escabrosa Limestone in south-central Arizona is similar to the Grand Canyon’s Redwall Limestone. Although many fossils in the Escabrosa are not as well preserved as those in the Redwall, the unit still contains abundant crinoids, mollusks, bivalves, brachiopods, corals, bryozoans, and foraminifera (Figure 3.50). Mississippian marine invertebrates are also abundant in Utah, and there is a strong Pennsylvanian fossil record in New Mexico. Fusulinid foraminifera (Figure 3.51) are the most important group for biostratigraphy in the Carboniferous and Permian rocks of the Basin and Range. More than 200 species of these creatures have been described from Pennsylvanian and Permian rocks in the Big Hatchet Mountains of Hidalgo County in southwestern New Mexico. Other Pennsylvanian marine invertebrates found in the Basin and Range include corals, bryozoans, gastropods, bivalves, trilobites, and (as usual) brachiopods. The Pennsylvanian Naco Formation near Payson, Arizona (just south of the southern edge of the Colorado Plateau) also contains the teeth of many types of fossil sharks (Figure 3.52). Nonmarine Pennsylvanian fossils in central and southern New Mexico include freshwater fishes, plants, and insects, as well as terrestrial amphibians and reptiles.
Crinoids
Crinoids are echinoderms, related to sea urchins and sea stars. These invertebrate animals feed by using their arms to filter food out of the water. Most are attached to the sediment by a stalk that ends in a root-like structure called the holdfast—some forms, however, are free floating. Crinoid fossils are most commonly found as "columnals," pieces of the stalk that hold the head (calyx) above the surface. The calyx and the holdfast are only occasionally preserved as fossils. Crinoids are still around today; those in shallow water are mostly stalkless, while those with stalks are restricted to deep water.
A) Crown and stem, about 15 centimeters (6 inches) long. B) Stem fragments.
Figure 3.46: Graptolites from the Basin and Range region. A) Didymograptus, scale in centimeters. B) Phyllograptus, approximately 3.5 centimeters (1.4 inches) long. C) Clonograptus, scale in centimeters.
Figure 3.47: Thamnopora, a Devonian bryozoan from Utah.
Graptolites
Graptolites (meaning "rock writing") are an extinct group of colonial, free-floating organisms. They lived from the Cambrian to the Carboniferous, and were relatives of modern hemichordates such as acorn worms. Graptolites are frequently preserved as thin, black, sawblade-like streaks across black shale; tiny cups along these structures held individual animals. Graptolites are often useful as index fossils.
A) Specimen with many fragments of colonies of Climacograptus. Slab is 7.5 centimeters (3 inches) on each side. B) Restoration of what graptolite colonies may have looked like when they were alive, floating in the water.
As the ocean receded from the Southwest during the Permian, the Basin and Range transitioned to a mainly terrestrial environment. Marine sediments are exposed in a few places—red beds in the Los Pinos Mountains of Socorro County, New Mexico contain abundant fossil lungfish burrows, and Permian outcrops in western Utah contain fusulinids, conodonts, bivalves, and brachiopods (Figure 3.53). In New Mexico’s Permian rocks, the footprints of terrestrial vertebrates are widespread and abundant. Reptile and amphibian prints in the Abo and Robledo Mountain formations, for example, are the basis for the recent establishment of Prehistoric Trackways National Monument near Las Cruces in Doña Ana County (Figure 3.54). This area contains one of the most abundant assemblages of nonmarine Paleozoic trace fossils in the world, with more than 20 different kinds of traces, and it has been called an “ichnofossil Lagerstätte.”
Figure 3.48: Silurian and Devonian corals from the Basin and Range region. A) Colonial tabulate coral, Favosites, approximately 12 centimeters (5 inches) wide. B) Colonial rugose coral, Hexagonaria (polished section), individual corallites approximately 12 millimeters (0.4 inches) in diameter. C) Fossil assemblage consisting of the corals Diphyllum and Syringopora, and stromatoporoid sponge; approximately 20 centimeters (8 inches) wide. D) Branching tabulate coral, Cladopora.
Corals
Corals are sessile relatives of jellyfish and sea anemones. They possess stinging tentacles, which they use to feed on small planktonic prey. Each group of coral possesses distinctly shaped "cups" that hold individual animals, or polyps. Colonial corals live in colonies of hundreds or even thousands of individuals that are attached to one another. Solitary coral lives independently, as a single isolated polyp.
Rugose corals were both colonial and solitary. (Solitary forms are often called "horn corals.") Tabulate corals were exclusively colonial and produced a variety of shapes, including sheetlike and chainlike forms. These corals receive their name from the table-like horizontal partitions within their chambers. Both rugose and tabulate corals went extinct at the end of the Permian. Modern corals—scleractinians—first appeared in the Triassic, and include both solitary and colonial species. Many scleractinian corals have photosynthetic symbiotic algae in their tissues, called zooxanthellae. These algae provide nutrition to the coral polyps, helping them to grow more rapidly.
Figure 3.49: Devonian brachiopods from the Basin and Range region. A) Camarotechia, 3 centimeters (1.2 inches) wide. B) Allanaria, 3 centimeters (1.2 inches) wide. C) Rhipodomella, 3 centimeters (1.2 inches) wide. D) Cyrtiopsis, 3 centimeters (1.2 inches) wide. E) Ambothyris, 1 centimeter (0.4 inches) wide. F) Paurorhynchia, 3 centimeters (1.2 inches) wide. G) Atrypa, 1.5 centimeters (0.6 inches) wide. H) Stringocephalus, 3 centimeters (1.2 inches) wide. I) Cluster of Devonian brachiopods from Arizona (Schizophoria iowensis), approximately 20 centimeters (8 inches) wide. The original calcite shells have been replaced with silica.
Brachiopods
Brachiopods are filter-feeding animals that have two shells and are superficially similar to bivalves (such as clams). Instead of being mirror images between shells (symmetrical like your hands), brachiopod shells are mirror images across each shell (symmetrical like your face). There are two major types of brachiopod shells, distinguished by how the two valves connect to each other: articulate brachiopods have tooth-and-socket hinges that tightly interlock, whereas inarticulate brachiopod shells lack hinge structures entirely. Internally, brachiopods are substantially different from bivalves, with a lophophore (filter-feeding organ made of thousands of tiny tentacles), and a small and simple gut and other organs. Bivalves, in contrast, have a fleshier body and collect their food with large gills.
The difference between the shells of a typical brachiopod (left) and a typical bivalve mollusk (right). Most brachiopods have a plane of symmetry across the valves (shells), whereas most bivalves have a plane of symmetry between the valves.
Figure 3.50: Carboniferous marine invertebrates from the Basin and Range. A) Solitary rugose coral, Caninia, side and top view, approximately 2.75 centimeters (1.08 inches) tall. B) Mississippian brachiopod, Inflatia inflatus, approximately 3 centimeters (1.2 inches) wide. C) Mississippian brachiopod, Ovatia (Linoproductus) ovatus, approximately 1.6 centimeters (0.6 inches) wide. D) Pennsylvanian brachiopod, Composita trilobata, approximately 2 centimeters (0.8 inches) wide. E) Mississippian brachiopod, Spirifer centronatus, approximately 3.5 centimeters (1.4 inches) wide. F) Pennsylvanian brachiopod, Derbya crassa, approximately 2.5 centimeters (1 inch) wide. G) Pennsylvanian brachiopod, Schizopora texana, approximately 2.5 centimeters (1 inch) wide. H) Bryozoan, Anisotrypa, approximately 2 centimeters (0.8 inches) tall. I) Bryozoan, Fenestella, approximately 2 centimeters (0.8 inches) tall. J) Bryozoan, Archimedes, approximately 5 centimeters (2 inches) tall. K) Ammonoid, Cravenoceras hesperium, approximately 1.75 centimeters (0.7 inches) in diameter. L) Gastropod, Euomphalus utahensis, approximately 1.3 centimeters (0.5 inches) diameter. M) Crinoid stem columnals, approximately 1 centimeter (0.4 inches) in diameter.
Figure 3.51: Single-celled fusulinid foraminifera from the Pennsylvanian. A) A cluster of the shells, roughly the size and shape of large rice grains. B) Photograph of a cross-section through a single fusulinid, as seen through a microscope.
Figure 3.52: Pennsylvanian shark, Petalodus. A) Tooth. B) Restoration, approximately 1 meter (3 feet) long.
Figure 3.53: Permian marine fossils of Utah. A) Brachiopod, Bathymyonia (Pustula) nevadensis, approximately 3.5 centimeters (1.4 inches) wide. B) Bivalve, Acanthopecten, approximately 2.6 centimeters (1.1 inches) wide. C) Brachiopod, Dictyoclostus bassi, approximately 9 centimeters (3.5 inches) wide. D) Brachiopod, Spiriferina (Punctospirifer) pulcher, approximately 5.25 centimeters (2 inches) wide. E) Conodont elements, approximately 0.4 millimeters long. F) Brachiopod, Neospirifer triplicatus, approximately 4 centimeters (1.6 inches) wide.
Figure 3.54: Possible footprint of Dimetrodon (see also Figure 3.12), from the Abo Formation at Prehistoric Trackways National Monument.
Mesozoic marine rocks are uncommon in the Basin and Range. While marine environments spread over the area multiple times during the Mesozoic, many of the corresponding rocks have been destroyed by erosion or buried by other sediments. Outcrops containing marine fossils include the Triassic Moenkopi, Dinwoody, and Thaynes formations; the Jurassic Twin Creek, Carmel, and Curtis formations; and the Cretaceous Aspen-Mowry Shale and Dakota beds. Ammonoids, snails, fish scales, and bivalves are found in these rocks (Figures 3.55 and 3.56); rudistid bivalves are uncommon, but have been found at a number of Cretaceous localities in New Mexico. In Washington County, southwesternmost Utah, late Triassic-aged rocks exposed at the St. George Dinosaur Discovery Site preserve an extraordinary diversity and abundance of trackways, as well as the skeletal remains of dinosaurs and other tetrapods, fish, and terrestrial plants (see Figures 3.17–3.19 and 3.21).
Above the Cretaceous-Paleogene (K-Pg) boundary, the rocks of southeastern and west-central Arizona and central and southern New Mexico are rich in Neogene land mammals (Figures 3.57–3.59). These deposits include proboscideans such as Gomphotherium, Cuvieronius, and Stegomastodon, as well as the American mastodon (Mammut americanum) and at least two species of mammoth (Mammuthus columbi and M. meridionalis). Also present are camels (Stenomylus, Protolabis, Michenia, Megacamelus, Megatylopus, Hemiauchenia, Procamelus), rhinos (Diceratherium, Teleoceras), carnivores (Borophagus), horses (Dinohippus, Onohippidium, Nannippus, Equus giganteus, E. simplicidens, and E. occidentalis), ground sloths (Megalonyx leptostomus, Glossotherium chapadmalense), and the glyptodont Glyptotherium texanum, as well as many kinds of rodents (including the earliest known species of porcupine) and rabbits.
Figure 3.55: Mesozoic bivalves of the Basin and Range region. A) Claria, Triassic, 2.25 centimeters (0.9 inches) long. B) Unio, Triassic, 3.5 centimeters (1.4 inches) long. C) Camptonectes platessiformis, Jurassic, 3 centimeters (1.3 inches) long. D) Cardium curtum, Cretaceous, 1.5 centimeters (0.6 inches) across. E) Meleagrinella (Eumicrotis) curta, Jurassic, 2.5 centimeters (1 inch) across. F) Myophorella montanensis, Jurassic, 5 centimeters (2 inches) across. G) Gryphaea nebrascensis, Jurassic, 3 centimeters (1.3 inches) across. H) Exogyra columbella, Cretaceous, 3 centimeters (1.3 inches) across. I) Inoceramus labiatus, Cretaceous, 3.5 centimeters (1.4 inches) across. J) Reconstruction of a rudistid bivalve growing on a dead ammonoid shell on the sea floor.
Bivalves
Clams and their relatives, such as mussels, scallops, and oysters, are mollusks possessing a pair of typically symmetrical shells. Most are filter feeders, collecting food with their gills. Paleozoic bivalves typically lived on the surface of the sediment ("epifaunally"), but in the Mesozoic they evolved the ability to burrow more deeply into the sediment, becoming "infaunal." This innovation led to the rapid evolution of a large number of groups present in today’s oceans.
Figure 3.56: Mesozoic ammonoids of Utah. A) Columbites, Triassic, 4.75 centimeters (1.9 inches) in diameter. B) Anasibrites, Triassic, 2.5 centimeters (1 inch) in diameter. C) Flemingites, Triassic, 3 centimeters (1.2 inches) in diameter. D) Meekoceras, Triassic, 3 centimeters (1.2 inches) in diameter. E) Scaphites warreni, Cretaceous, 3 centimeters (1.2 inches) in diameter. F) Cadoceras, Jurassic, 5 centimeters (2 inches) in diameter. G) Scaphites ventricosus, Cretaceous, 4.75 centimeters (1.9 inches) in diameter. H) Clioscaphites vermiformis, Cretaceous, 4.75 centimeters (1.9 inches) in diameter. I) Colligniceras woolgari, Cretaceous, 3 centimeters (1.3 inches) in diameter.
Figure 3.57: Mammals of the Basin and Range region. A) Horse, Equus simplicidens, skull and reconstruction, height 110–145 centimeters (43–57 inches) at the shoulder. B) Camel, Procamelus, skull, approximately 28 centimeters (11 inches) long. C) Rhinoceros, Teleoceras, skull and restoration, body approximately 4 meters (13 feet) long.
Ammonoids
Ammonoids are a major group of cephalopods that lived from the Devonian to the end of the Cretaceous. Both nautiloids (the group that today contains the chambered nautilus) and ammonoids have chambered shells subdivided by walls, or septa (plural of septum). These shells are frequently, but not always, coiled. The term "ammonoid" refers to the larger group of these extinct cephalopods, distinguished by complex, folded septa. Within ammonoids, "ammonites" is a smaller sub-group, distinguished by the extremely complex form of their septa. Ammonites were restricted to the Jurassic and Cretaceous periods. The form of the septa in nautiloids and ammonoids is not visible in a complete shell; it is most often seen in the trace of the intersection between the septum and the external shell. This trace is called a suture. Sutures are usually visible in fossils when sediment has filled the chambers of a shell, and the external shell has been broken or eroded away.
Ammonite shell break-away cross-section; surface plane of a septum and sediment-filled chamber.
Figure 3.58: Glyptodont. A) Skeleton, with and without the external armor. B) Detail of the bony scutes that formed the solid outer armor. Glyptodonts reached lengths of up to 3 meters (10 feet).
Figure 3.59: Restorations of A) Gomphotherium, approximately 2.3 meters (7.5 feet) high at the shoulder. B) Columbian mammoth, Mammuthus columbi, approximately 4 meters (13 feet) high at the shoulder. C) American mastodon, Mammut americanum, approximately 2.3 meters (7.5 feet) high at the shoulder.
Mastodons and Mammoths
These two kinds of ancient elephants (or, more technically, proboscideans) are frequently confused. Both were common during the Pleistocene, but they had different ecological preferences and are usually found separately. Mammoths are close cousins of modern African and Asian elephants; mastodons are more distant relatives, from a separate line of proboscideans that branched off from the modern elephant line in the Miocene. Mastodons have a shorter, stockier build and longer body; mammoths are taller and thinner, with a rather high "domed" skull. In skeletal details, the quickest way to tell the difference is by the teeth: mastodons have teeth with conical ridges, a bit like the bottom of an egg carton; mammoths, in contrast, have teeth with numerous parallel rows of ridges. The teeth are indicative of the two species' ecological differences. Mastodons preferred to bite off twigs of brush and trees, while mammoths preferred tough siliceous grasses. Thus, mastodon teeth are more suitable for cutting, while mammoth teeth are more suitable for grinding. Both mammoths and mastodons became extinct around 10,000 years ago.
A mastodon tooth, suitable for chewing twigs and tree leaves. Approximately 20 centimeters (8–9 inches) long.
A mammoth tooth, suitable for grinding grass and softer vegetation. Approximately 25 centimeters (1 foot) long.
