North America

Several previously mentioned occurrences of possible mammals in the Late Triassic of North America—specifi-cally North Carolina and Texas (Clemens et al., 1979)— are now thought more likely to represent therapsids, as noted earlier (Sues, 2001). In the meantime, however, more secure records of mammals from both the Late Triassic and Early Jurassic have been reported from the continent. Adelobasileus cromptoni (see Lucas and Hunt, 1990) is known by the posterior part of a skull from Home Creek, Crosby County, Texas (figure 2.8, table 2.4). The occurrence is in the Tecovas Member of the Dockum Formation. Various lines of evidence, summarized by Lucas and Luo (1993), suggest a late Carnian age; hence, Adelobasileus may be the oldest known mammal (but see earlier comments on Gondwanadon from the Late Triassic of

figure 2.7. Mesozoic mammal localities of South America. Asterisks, Late Triassic or Jurassic (localities 1-3); diamonds, Early (white; locality 4) and Late (gray; localities 5-12) Cretaceous. Localities: 1, Los Menucos (unnamed unit, Norian-Rhaetian, Río Negro Province, Argentina); 2, Piedra Pintada ("Lias de Piedra Pintada," Early or Middle Jurassic, Neuquén Province, Argentina); 3, Queso Rallado (Cañadón Asfalto Formation, Callovian-Oxfordian, Chubut Province, Argentina); 4, Zapala (La Amarga Formation, Hauterivian-Barremian, Neuquén Province, Argentina); 5, Paraná (Adamantina Formation, Late Cretaceous; Sao Paulo, Brazil); 6, Paso Córdoba (Río Colorado Formation, Campanian or Maastrichtian, Río Negro Province, Argentina); 7, Los Alamitos (Los Alamitos Formation, Campanian or Maastrichtian, Río Negro Province, Argentina); 8, Meseta de Somuncura (La Colonia Formation, Campanian-Maastrichtian, Chubut Province, Argentina); 9, Fundo el Triunfo (Fundo el Triunfo Formation, late Campanian-early Maastrichtian, Peru); 10, Paruro (unspecified unit, Late Cretaceous; Cuzco Department, Peru); 11, Laguna Umayo (Vilquechico Group, ?late Maastrichtian, Puno Department, Peru); 12, Pajcha Pata (El Molino Formation, middle Maastrichtian, Bolivia).

figure 2.8. Late Triassic to Early-Middle Jurassic (North America and Greenland) and Late Cretaceous (Alaska) mammal localities. Asterisks, Late Triassic-Early Jurassic (black; localities 1-3) and Early-Middle Jurassic (gray; locality 4); diamond, Late Cretaceous (locality 5). Localities or local faunas: 1, Jameson Land (Malmros Klint and 0rsted Dal members, Fleming Fjord Formation; ?middle No-rian, Greenland); 2, Home Creek (Tecovas Member, Dockum Formation; late Carnian, Texas); 3, Gold Springs (Kayenta Formation; Sinemurian or Pliensbachian, Arizona); 4, Huizachal Canyon (La Boca Formation; Early or Middle Jurassic, Tamaulipas, Mexico); 5, Colville River (Prince Creek Formation; ?late Maastrichtian: Lancian, Alaska).

India). The skull, found by bulk screenwashing of rock matrix, unfortunately lacks a dentition. Adelobasileus shares a number of cranial features in common with Lias-sic mammals; in other respects, it appears to be structurally intermediate between cynodont therapsids and mammals (Lucas and Luo, 1993, see chapter 4).

Early Jurassic mammals have been recovered from the Kayenta Formation at Gold Springs (figure 2.8), northern Arizona (Jenkins et al., 1983). Evidence constraining the age of the unit is sparse; it is currently considered to be of Liassic (Sinemurian to Pliensbachian) age, based on the presence of the dinosaur Scelidosaurus (see Padian, 1989).

table 2.4. Late Triassic and Early Jurassic Mammals of North America (see figure 2.8; locality numbers do not correspond between table and figure). Localities or local faunas: 1, Home Creek, Texas (Dockum Formation, Carnian); 2, Gold Springs, Arizona (Kayenta Formation, Sinemurian-Pliensbachian); 3, Huizichal Canyon, Mexico (La Boca Formation, ?Early or Middle Jurassic)

Mammalia incertae sedis

Adelobasileus cromptoni (1) Morganucodonta


Dinnetherium nezorum (2) cf. Dinnetherium sp. (3) Morganucodontidae Morganucodon sp. (2) Eutriconodonta ?"Amphilestidae"

Gen. et sp. indet. (3) ?Triconodontidae Gen. et sp. indet. (3)

Several elements of the fauna are known at the generic level from other parts of the world. The theropod Syntar-sus, for example, is also known from the Stormberg Group of southern Africa (Rowe, 1989); and the tritylodontid therapsid Oligokyphus is known from the Liassic of Britain (Sues, 1985a).

The best-represented mammal from the Kayenta Formation is the "triconodont" Dinnetherium nezorum, known by associated upper and lower jaws, which is similar in many respects to Morganucodon (Crompton and Luo, 1993); the latter is also present in the fauna. Available material, consisting of a crushed skull, several isolated teeth, and some postcranial elements, shows that the species is probably distinct from those represented in the Late Triassic-Early Jurassic of Western Europe and from the Liassic of China (Jenkins et al., 1983). The final mammal known from the Kayenta Formation is a possible haramiyid, represented by a single molariform tooth.

Several mammals are known from Huizachal Canyon, Tamaulipas, Mexico (figure 2.8). The occurrence is in the La Boca Formation; the fossiliferous part of the unit in Huizichal Canyon is not well constrained biochronologi-cally and may be of Early or Middle Jurassic age. Preliminary interpretation of radiometric dates from an underlying level suggests that the vertebrate fauna is at least as old as 186 Ma, or late Early Jurassic (Fastovsky et al., 1998). A relatively recent summary of the fossils and their occurrence was given by Clark et al. (1994). The rock consists of well-indurated siltstones and sandstones, which may represent debris flows (Fastovsky et al., 1987). Fossil preservation is evidently quite variable, but the available sample includes associated skeletal elements and, in a few instances, nearly complete skulls for some of the vertebrate fauna. The assemblage includes a lepidosaur, rhyncho-cephalians, several archosaurs (including a well-preserved pterosaur skeleton), and rather abundant remains of the highly derived tritylodontid Bocatherium (see Clark and Hopson, 1985). Interestingly, all the specimens represent rather small animals.

The mammals have not yet been fully described or named. Six specimens (dentulous jaw fragments: five den-taries and one maxilla) are known to date, all with molars of a "triconodont" pattern (Montellano et al., 1998). At least three taxa are represented. One, known from a den-tary fragment with several teeth, is broadly similar to Dinnetherium from the Early Jurassic of Arizona (Jenkins et al., 1983). It clearly has a "triconodont" molar pattern of rather primitive design, and the presence of a postdentary trough (a primitive feature, see chapter 4) suggests that postdentary elements may have been attached, as in mor-ganucodontids. A second "triconodont" is known by two specimens, a partial dentary and a partial skull. Dental features (Fastovsky et al., 1987) suggest reference to the Tri-conodontidae, a family not otherwise known prior to the Late Jurassic. Moreover, unlike Dinnetherium, the back of the jaw lacks a postdentary trough, and several other derived features of the jaw and ear region are present. Indeed, the advanced morphology of this mammal suggests that the fauna of Huizichal Canyon may be younger than Early Jurassic (Clark et al., 1994). The final mammal is a diminutive form, known by both maxillary and mandibular fragments. It is similar, insofar as is known, to "Am-philestidae."

0 0

Post a comment