Santiago Formation Research Articles

Paleoenvironmental evolution of a Carboniferous marine succession during active volcanism in southern Mexico and its implications in the western Pangea margin configuration

Carboniferous units in southern Mexico consist of mixed carbonate-volcaniclastic facies representing shallow marine environments, from the middle ramp to shallow subtidal environments in the inner ramp. The Tournaisian-Visean Santiago Formation is characterized by the dominance of mixed clastic and bioclastic sediments in a mid-ramp setting without a supply of volcanic material within the basin. Instead, the newly reported locality of the Mississippian-Pennsylvanian Ixtaltepec Formation is a mixed succession of volcaniclastic and carbonated sediments deposited under a tidal influence in the middle to the inner ramp. The age is determined to be late Visean and early Serpukhovian (∼333-325 Ma) based on the presence of Actinoconchus lamellosus, Syringothyris sp., and Punctospirifer sp. and corroborated by geochronological data. This study provides new evidence of Carboniferous volcanism in the sediment-sources area in southern Mexico. Volcanic activity in the area is linked to the subduction of the oceanic crust of the Rheic Ocean, which resulted in the formation of magmatic arcs on the Gondwanan realm and the assembly of Pangea. During the Early Mississippian to early Permian, extensive areas of the northwest of South America and southwest of North America were covered by epicontinental seas. Sedimentation in the area was mainly characterized by shallow clastic-carbonate-dominated environments and deep marine settings, where basins within the peri-Gondwana Terranes received volcanic detritus from the North Gondwana and western Pangea arcs.

Provenance of Carboniferous-Permian sedimentary units in southern Mexico: evidence for peri-arc basin evolution during the Pangea assembly

ABSTRACT The Oaxacan Complex Carboniferous-Permian sedimentary cover in southern México records provenance shifts through time, reflecting the collision between Gondwana and Laurentia to amalgamate Pangea. The integration of petrological analysis and LA-ICP-MS U-Pb zircon geochronology from Santiago, Ixtaltepec and Yododeñe formations compared with adjacent terranes suggests that: (1) during the Early Mississippian, the Santiago Formation received sediments mainly from local sources such as the Oaxacan Complex and Tiñu Formation, with minor contributions from adjacent peri-Gondwana sources. The magmatic activity may have started during this time (~359–346 Ma) (2) during the Late Mississippian (Ixtaltepec Formation), detrital zircon grains of Ediacaran-Cambrian age are dominant, derived from sediments either related to the Pan-African/Brasiliano orogeny or the opening of the Iapetus Ocean; (3) during the Late Mississippian-Middle Pennsylvanian, intercalated marine volcaniclastic sandstone (Ixtaltepec Formation) provides the first record of Carboniferous arc-related volcanism reported in southern Mexico, dated between 330 and 308 Ma; (4) the early Permian Yododeñe Formation records the exhumation and erosion of the sedimentary cover during the final stage of Pangea assembly. Rhyodacitic subvolcanic sills and lavas dated at ~282–270 Ma are present throughout the succession. Volcanism and ca. 360–308 Ma detrital zircon grains could be associated with a Carboniferous magmatic arc formed by subduction of the Rheic oceanic plate beneath Gondwana. Slightly younger detrital zircon and subvolcanic rocks dated at ~300–270 Ma are linked to a western Pangea arc developed in response to the subduction of the Paleo-Pacific Ocean following Pangea assembly. Our results suggest that the Carboniferous-Permian units were deposited in a peri-arc basin, sharing sediment provenance with the Maya and Coahuila blocks, the Sierra de Juárez Complex, and northwestern South America.

Microstructural effects on the unconfined mechanical behavior of a tectonically deformed calcareous shale, a study case in the Santiago Formation, México

Microstructural properties play a crucial role in the mechanical response of shale rocks. This work examines the impact of fabric anisotropy on the strength and elastic behavior of a highly deformed Jurassic calcareous shale from the Santiago Formation in eastern México by means of unconfined compression strength tests (UCS). The Santiago Formation is attracting considerable interest for the development of unconventional resources of shale gas and shale oil in México. However, the effect of tectonic deformation on the mechanical properties of these Mexican Shales has not been addressed in detail. The anisotropy of the shale was evaluated by means of rock samples cored in the direction normal to layers and two horizontal directions defined along the foliation planes. Petrography of oriented thin sections, X-ray diffraction, scanning electron microscopy (SEM), and petrophysical properties (porosity and permeability), were used to assess the rock composition and microstructural properties. UCS tests results revealed different mechanical responses between the normal direction (V) and the direction parallel to layers, but also differences between the two mutually orthogonal horizontal directions (H1, H2). Tangent Young's modulus (E) showed higher values in the H2 direction (13–22 GPa) and lower values in the H1 and V directions (8–10 GPa). All the specimens yielded very low Poisson's ratios (<0.11). H2 samples displayed the highest strength (48–84 MPa) and reached higher axial strains at failure (0.4–0.7%). Specimens in the H1 direction presented the lowest strength and axial strains (15–29 MPa and 0.22–0.35%, respectively). Compared to H1 samples, vertical samples yielded slightly higher strengths (26–33 MPa) and axial strains (0.31–0.40%). The results reported here confirm that microstructure strongly influenced the mechanical anisotropy and fracture mechanisms of the Santiago Shale. A direct relation was documented between fabric anisotropy, elastic response, UCS, and brittle failure modes validating the importance of fabric characterization for a complete description of anisotropy in tectonically deformed shale rocks.

Diegoaelurus, a new machaeroidine (Oxyaenidae) from the Santiago Formation (late Uintan) of southern California and the relationships of Machaeroidinae, the oldest group of sabertooth mammals.

Machaeroidinae is a taxonomically small clade of early and middle Eocene carnivorous mammals that includes the earliest known saber-toothed mammalian carnivores. Machaeroidine diversity is low, with only a handful of species described from North America and Asia. Here we report a new genus and species of machaeroidine, Diegoaelurus vanvalkenburghae, established on the basis of a nearly complete dentary with most of the dentition from the late Uintan (middle Eocene) portion of the Santiago Formation of southern California. The new taxon is the youngest known machaeroidine and provides the first evidence for the presence of multiple machaeroidine lineages, as it differs substantially from Apataelurus kayi, the only near-contemporaneous member of the group. Phylogenetic analysis indicates that Diegoaelurus is the sister taxon of Apataelurus, while older species are recovered as a monophyletic Machaeroides. Both phylogenetic results are relatively weakly supported. The new taxon extends the record of machaeroidines to the end of the Uintan, potentially tying machaeroidine extinction to the faunal turnover spanning the middle to late Eocene transition in North America.

Microstructural Effects on the Unconfined Mechanical Behavior of a Tectonically Deformed Calcareous Shale, a Study Case in the Santiago Formation, México

Microstructural Effects on the Unconfined Mechanical Behavior of a Tectonically Deformed Calcareous Shale, a Study Case in the Santiago Formation, México

Jurassic to Early Paleogene sedimentation in the Amazon region of Ecuador: Implications for the paleogeographic evolution of northwestern South America

The retroarc Amazon region of Ecuador recorded distinct events that are preserved in the Mesozoic-Cenozoic sedimentary record of the Subandean Zone and Oriente Basin. In this contribution, we review the Jurassic to Paleocene geologic and paleogeographic evolution of the Ecuadorian foreland using stratigraphic, sedimentologic, provenance, and geochronological data. Our results provide new insights into the sedimentary systems, stratigraphic framework, depositional ages, and sediment dispersal patterns of retroarc basin fill. During the Early Jurassic, marine deposits of the Santiago Formation formed a carbonate platform in eastern Ecuador. At ~180 Ma, an eastward-prograding deltaic system transported volcaniclastic material from a western volcanic arc. Nonmarine conditions prevailed during most of the Middle to Late Jurassic, characterized by deposition of red beds and intercalated volcanic and volcaniclastic deposits of the Chapiza Formation. Clastic and volcanic deposition dated from ~160 to 130 Ma was coeval with regional extension, as suggested by sedimentologic and seismic data from the Oriente Basin. Provenance results show that a ~ 120 Ma fluvial system at the base of the Hollín Formation was initially sourced from the west and progressively evolved to a mixed source with contributions from the Amazonian Craton and incipient Andean topography. Deposition of the Hollín Formation occurred in a northwest-flowing fluvial system that evolved into an estuary system confined by the Andes to the west and the Amazonian Craton to the east. Coastal to shallow marine deposits of the Napo Formation accumulated during regional sea-level variations throughout the Late Cretaceous. The geometry of coastal deposits and supporting provenance data suggest a drainage system characterized by west-flowing rivers derived chiefly from the Amazonian Craton. The latest Cretaceous (~70 Ma) onset of shortening in the Northern Andes coincided with collision of the Caribbean Plateau. This tectonic episode induced a continental-scale drainage reorganization documented in provenance data showing an Andean origin of the east-directed fluvial deposits of the Maastrichtian–Paleocene Tena Formation.

Evidence of large Anacardiaceae trees from the Oligocene–early Miocene Santiago Formation, Azuero, Panama

Evidence of large Anacardiaceae trees from the Oligocene–early Miocene Santiago Formation, Azuero, Panama

Discovery, Geology, and Origin of the Fruta del Norte Epithermal Gold-Silver Deposit, Southeastern Ecuador

Fruta del Norte is a completely concealed and extraordinarily well preserved epithermal gold-silver deposit located in the remote Cordillera del Condor mountain ranges of southeastern Ecuador. Currently defined resources in a single, steep, relatively small body, displaying exceptional grade continuity, contain 9.81 million ounces (Moz) of gold and 15.0 Moz of silver, with an indicated resource grade of 9.59 g/t Au and 12.9 g/t Ag. The deposit was discovered in early 2006 during a greenfields program conducted by Aurelian Resources, a Canadian junior explorer. Discovery resulted from systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence. The rib is highly anomalous in arsenic, antimony, and mercury but contains low-order gold values. The second drill test—the discovery hole—intersected >230 m of ore-grade gold-silver mineralization beneath ~200 m of the barren conglomerate cover. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccias are assigned to the Piuntza unit of the Santiago Formation, which, along with the Zamora batholith and a series of porphyry copper stocks, was generated in a continental margin magmatic arc during the Middle to Late Jurassic. The deposit is located near the northeastern extremity of the ~10-km-long, Suarez pull-apart basin where it is linked to the steep West and Central faults, part of the regionally extensive Las Penas strike-slip fault zone. The pull-apart basin was progressively filled by fluviatile conglomerate, dacitic ignimbrite flows, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows, all assigned to the Chapiza Formation. The Fruta del Norte deposit consists of a 1.3-km-long and up to >300-m-wide vein-stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz and calcite. Adularia is a minor main-stage gangue mineral in each. Both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (±pyrite). The uppermost part of this silicic zone is sulfide deficient, probably the result of a short-lived supergene oxidation event prior to initial conglomerate deposition. The deposit is notable for the widespread occurrence of fine to coarse visible gold, which gives rise to bonanza grades and is closely associated with quartz, chalcedony, carbonate, and sulfide gangue. The sulfide-free, silicic zone is overlain by an extensive silica sinter horizon, which may either directly overlie the Piuntza volcanic rocks and/or occur as interbeds in the basal 20 m of the conglomerate, which are invariably silicified and marcasite bearing. Otherwise, the conglomerate above the orebody lacks silicification, with one critical exception: the steeply inclined zone that is exposed as the silicified rib that led to discovery. The sinter horizon, containing localized mud-pool deposits and hydrothermal eruption breccias, is in unusual proximity to the underlying gold-silver orebody. The northern and southern parts of the Fruta del Norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits, respectively; they may have required two discrete mineralizing fluids, both of which are suspected to have ascended via the West and Central faults. The upward change to the silicified zone is attributed to cooling of the ascendant fluid on approach to the paleosurface sinter. Progressive burial of the deposit beneath the Suarez basin fill may have contributed to the suppression and eventual extinction of the epithermal system, with gold mineralization of the conglomerate being inhibited by rheologic and chemical factors. Formation of the Fruta del Norte deposit and complete filling of the Suarez basin took place in >3 to 6 m.y. during the Late Jurassic ( 157–154 Ma).

Conuláridos misisípicos de Nochixtlán, Oaxaca, México: consideraciones sistemáticas, estratigráficas y paleobiogeográficas

MISSISSIPPIAN CONULARIIDS FROM NOCHIXTLAN, OAXACA, MEXICO: SYSTEMATICS, STRATIGRAPHIC AND PALEOBIOGEOGRAPHIC CONSIDERATIONS. This work represents the fi rst systematic study of conulariids of Mexico. Three species are described and illustrated: Conularia cf. C. subcarbonaria, Paraconularia chesterensis and P. cf. P. subulata, all of which come from four outcrops of the Santiago formation (informal unit), in the Santiago Ixtaltepec region, Nochixtlan municipality, Oaxaca. The stratigraphical distribution of these species corroborates the early Mississippian age (Osagean = late Tournaisian-early Visean) of the calcareous sandy lower member of the Santiago formation, and also supports the idea that at least part of the upper member belongs to the middle-late Mississippian (Meramecian-Chesterian = middle-late Visean). The studied conulariids reaffi rm the similarity between Carboniferous faunas of southwestern Mexico and those of the Mid-Continental United States, and hence the continuity of the epicontinental sea that covered both regions.

Placas columnares de crinoides del Carbonífero de la región de Santiago Ixtaltepec, Municipio de Nochixtlán, Oaxaca; consideraciones estratigráficas y paleobiogeográficas

Eleven crinoid columnal morphotypes of Mississippian and Pennsylvanian ages found in different stratigraphic levels of the Santiago (informal unit) and Ixtaltepec formations in outcrops in the Santiago Ixtaltepec region, municipality of Nochixtlan, Oaxaca, are reported and described. Axilinucrinus angustus is proposed as a new morphogenus and new morphospecies. Cyclocaudex insaturatus and Floricyclus welleri have been found in middle Mississippian (Meramecian-Chesterian?) strata from the upper member of the Santiago formation. Lomalegnum cf. L. hormidium, Axilinucrinus angustus and one indeterminate genus are present in middle-late Mississippian (Meramecian-Chesterian) strata of the Ixtaltepec Formation. Pentagonomischus cf. P. plebeius and Cyclocion distinctus occur in earlymiddle Pennsylvanian (Morrowan-Desmoinesian) strata of the Ixtaltepec Formation. Cylindrocauliscus fiski and Cyclocaudex typicus are morphotypes with a broad distribution throughout the carboniferous section of Nochixtlan. A strong similarity exists between all of the parataxa from Oaxaca and those reported from lower Mississippian-middle Pennsylvanian faunas of the Mid-Continent region of the central eastern United States.

Hydrocarbon source potential of the Santiago Formation, Oriente Basin, SE of Ecuador

The Santiago Formation (Late Hettangian–Sinemurian), described in the area of Santiago in the Oriente Basin of eastern Ecuador, consists of three distinct sedimentary members. The Santiago River Member is composed of limestones and calcareous sandstones. The Yuquianza Member is a monotonous sequence of black shales. The Patuca Member consists of a sequence of sandstones, greywackes, and shales, intercalated with lava flows and dikes. The fine-grained sediments of the three members are characterized by a high content of particulate organic matter (POM). Palynofacies and rock-eval analyses indicate the predominance of kerogen types II and III, with HI values that indicate a moderate to low source potential. At the type locality, the organic matter is thermally mature and locally overmatures.

LOWER MISSISSIPPIAN (OSAGEAN) SPIRE-BEARING BRACHIOPODS FROM CAÑÓN DE LA PEREGRINA, NORTH OF CIUDAD VICTORIA, TAMAULIPAS, NORTHEASTERN MÉXICO

Abstract Brachiopods are common in the lower and middle parts of the early Mississippian (Osagean) Vicente Guerrero Formation that is mainly a fine-grained quartz arenite found in the Ciudad Victoria area, Tamaulipas, northeastern Mexico. This shallow marine brachiopod fauna includes several spire-bearers: Lamellosathyris lamellosa, Cleiothyridina cf. tenuilineata, Camarophorella sp., Alispirifer tamaulipensis n. sp., Tylothyris? sp., Torynifer pseudolineatus, Syringothyris cf. typa, Syringothyris? sp., and Punctospirifer sp. Similar assemblages have been found in Osagean rocks of the Santiago Formation, Oaxaca, southeastern Mexico, and in several coeval formations in the USA: Arizona, Arkansas, Illinois, Ohio, and Texas. North American biogeographic affinities are suggested for these Mississippian faunas of northeastern Mexico. These contrast markedly with the Late Silurian situation, where the Tamaulipas material has Old World Realm, rather than North American, biogeographic affinities. This supports th...

Lower Mississippian (Osagean) brachiopods from the Santiago Formation, Oaxaca, Mexico: stratigraphic and tectonic implications

Four species of spiriferid brachiopods from the Santiago Formation of Oaxaca are described. The species described belong to the orders Athyridida ( Actinoconchus lamellosus), Spiriferida ( Torynifer pseudolineatus), and Spiriferinida (cf. Syringothyris sp. and Punctospirifer gigas new species). The association between A. lamellosus and T. pseudolineatus indicates an Early Mississippian (Osagean) age for the Santiago Formation. The Carboniferous succession in the area was originally divided into the Santiago Formation (formed by lower and upper members) and the Ixtaltepec Formation. The studied fauna belongs to the lower member of the Santiago Formation. Because Pennsylvanian brachiopods were found in strata of the upper member, it is now considered to have more affinity with the Ixtaltepec Formation. Therefore, we propose a change in the stratigraphy of the area to include the upper member of the Santiago Formation in the Ixtaltepec Formation. This change in the stratigraphy is supported because a major low-angle normal fault places the former upper member of the Santiago Formation on the lower member and indicates a tectonic contact between both members of the original Santiago Formation. Brachiopods from the Santiago Formation are similar to those in the Midcontinent province located in the eastern and central regions of the United States. The affinity of these Mississippian faunas has important tectonic implications, because Ordovician and Silurian faunas of Oaxaquia have strong biogeographic affinities with Gondwanan–European provinces. Therefore, there may have been a geographic connection between southeastern Mexico and North America during Lower Carboniferous times. This could change previous interpretations, which suggest that the collision of Oaxaquia with North America occurred during Permian time.

Pseudomulceodens: A Mississippian rostroconch from Mexico

This finding of Pseudomulceodens in Mississippian-age rocks of the Santiago Formation provides the first evidence of the molluscan class Rostroconchia in Mexico. Elsewhere in North American Mississippian rocks the class occurs in Arkansas (Hoare et al., 1982, 1988); Illinois (Weller, 1916); Indiana (Beede, 1906); Iowa (White and Whitfield, 1862); Michigan (Winchell, 1870); Montana and Nevada (Pojeta and Runnegar, 1976); Ohio (Hyde, 1953; Hoare, 1990); and Oklahoma (Branson, 1958).

PSEUDOMULCEODENS: A MISSISSIPPIAN ROSTROCONCH FROM MEXICO

This finding of Pseudomulceodens in Mississippian-age rocks of the Santiago Formation provides the first evidence of the molluscan class Rostroconchia in Mexico. Elsewhere in North American Mississippian rocks the class occurs in Arkansas (Hoare et al., 1982, 1988); Illinois (Weller, 1916); Indiana (Beede, 1906); Iowa (White and Whitfield, 1862); Michigan (Winchell, 1870); Montana and Nevada (Pojeta and Runnegar, 1976); Ohio (Hyde, 1953; Hoare, 1990); and Oklahoma (Branson, 1958).

Protoreodon walshi, a new species of agriochoerid (Oreodonta, Artiodactyla, Mammalia) from the late Uintan of San Diego County, California

A new agriochoerid, Protoreodon walshi new species, from the middle Eocene Santiago Formation, San Diego County, California, is described from the Jeff's Discovery and Rancho del Oro local faunas, considered to be late Uintan in age. It differs from most other species of Protoreodon in having an undivided P4 para-metacone and a P3 protocone connected to the paracone by a buccolingual crest. Associated postcranial material is abundant and well preserved. The postcranial skeleton shows several features which probably represent the primitive oreodont condition, including closely appressed metapodials, hoofed ungual phalanges, the lunar in broad contact with both the magnum and the unciform in anterior view, and having the magnum larger than the trapezoid.Two specimens of a larger morph, Protoreodon cf. walshi, show a different P3 morphology and might represent another new species. Diversity of agriochoerid oreodonts in California is at least as high as has been reported for the Vieja Group of Texas, although not as high as in the Uinta Basin of Utah, where the genus Diplobunops is also present.

Latest Paleozoic–early Mesozoic structures in the central Oaxaca Terrane of southern Mexico: deformation near a triple junction

Paleozoic rocks in the Oaxaca Terrane of southern Mexico occur as two outliers (Rio Salinas and Santiago Ixtaltepec) unconformably overlying the 1-Ga Oaxaca Complex. They consist of the Upper Cambrian–Lower Ordovician Tiñu Formation, Mississippian Santiago Formation, lower–middle Pennsylvanian Ixtaltepec Formation and the unfossiliferous Yododeñe Formation of presumed Permian or younger age. These Paleozoic rocks have been deformed by several sets of structures. Three moderately westerly-dipping, bedding-parallel shear zones displaying dextral kinematic indicators (C–S fabrics, curvilinear isoclinal folds, en-echelon boudinage, en-echelon veins, and strained amygdales) occur along the boundaries between the Tiñu and Santiago formations, between the Santiago and Ixtaltepec formations, and within the Ixtaltepec Formation in the Santiago Ixtaltepec outlier. The bedding and these bedding-parallel shear zones are deformed by N–S, upright-asymmetric, subhorizontal folds accompanied by a slaty or spaced cleavage, that are in turn deformed by several sets of kink bands: subhorizontal and steeply dipping E–W, NW–SE and NE–SW. In the northern part of the Santiago Ixtaltepec outlier, the stratigraphy, bedding-parallel shear zones and slaty cleavage are displaced by NW–SE normal faults, all of which are truncated by the angular unconformity at the base of the Cretaceous, which brackets their age as post-Early Permian and pre-Cretaceous. Geometric correlation of the major N–S folds with E-vergent thrusting of the Oaxacan Complex over the Juarez Terrane suggests that they are older than Middle Jurassic. No age constraints are available for the kink and chevron folds; however, most may be related to Laramide structures in the overlying Cretaceous rocks. Unfolding the major structures in the Paleozoic rocks reorients the bedding-parallel shear zones to subhorizontal detachment shear zones or faults with a top-to-the-north sense of displacement. They may be related to either gravity sliding following amalgamation of Pangea, extensional rifting of the Yucatán Peninsula from southern Laurentia or intra-arc rifting associated with a Permo–Triassic arc within southern Mexico. The major N–S fold structures and E-vergent thrusting of the Oaxacan Complex over the Juárez Terrane may represent a response to (1) convergent subduction of the Kula and/or Farallon plates beneath western Mexico, (2) migration of a Gulf of Mexico–Yucatan–South America triple junction.

Comment on possible cut marks and taphonomic history of Senga 5A in the Western Rift Valley, Zaire

A new specimen of Dyseolemur pacificus Stock, 1934, represents the most complete dentition known for any North American omomyid primate. Crowns are preserved from each tooth position except I2 on either the right or left side, or both. The specimen comes from the upper part of the Santiago Formation near Oceanside, San Diego County, California, and is of middle Eocene (late Uintan) age. The crowns of the anterior teeth are tarsiiform in shape (they are high and pointed, tapering from base to tip). The size relationships among the anterior teeth differ from those of many omomyids in having relatively large canines and small incisors, a pattern that is similar to anthropoids and adapids. Despite this similarity, the crowns of these teeth in Dyseolemur are unmistakably tarsiiform in structure. The implications of these results are discussed in the light of recently published hypothetical phylogenies concerning the origin of anthropoids and the origin of modern Tarsius .

The dentition of Dyseolemur, and comments on the use of the anterior teeth in primate systematics

Abstract A new specimen of Dyseolemur pacificus Stock, 1934, represents the most complete dentition known for any North American omomyid primate. Crowns are preserved from each tooth position except I 2 on either the right or left side, or both. The specimen comes from the upper part of the Santiago Formation near Oceanside, San Diego County, California, and is of middle Eocene (late Uintan) age. The crowns of the anterior teeth are tarsiiform in shape (they are high and pointed, tapering from base to tip). The size relationships among the anterior teeth differ from those of many omomyids in having relatively large canines and small incisors, a pattern that is similar to anthropoids and adapids. Despite this similarity, the crowns of these teeth in Dyseolemur are unmistakably tarsiiform in structure. The implications of these results are discussed in the light of recently published hypothetical phylogenies concerning the origin of anthropoids and the origin of modern Tarsius .

Uintan land mammals (excluding rodents) from an estuarine facies of the Laredo Formation (Middle Eocene, Claiborne Group) of Webb County, Texas

A newly discovered vertebrate fossil assemblage, the Casa Blanca local fauna, comes from the Laredo Formation, Claiborne Group, of Webb County, Texas, and is the first reported Eocene land-mammal fauna from the coastal plain of North America. The mammalian fauna is correlated with the Serendipity and Candelaria local faunas of west Texas, the Uinta C faunas of the Rocky Mountains, the Santiago Formation local fauna of southern California, and the Swift Current Creek local fauna of Saskatchewan. The vertebrate-bearing deposit lies approximately 32 m above a horizon containing the marine gastropod Turritella cortezi, which ranges from east Texas to northeast Mexico in the lower half of the Cook Mountain and Laredo Formations and is a guide fossil for the Hurricane Lentil in the Cook Mountain Formation. Nannoplankton found in these middle Eocene formations belong to the upper half of Nannoplankton Zone 16 and allow correlation with European beds of late Lutetian to early Bartonian age.Over 700 specimens represent at least 30 species of 28 mammal genera. The Casa Blanca fauna is the southernmost and easternmost North American land-mammal fauna of definite Eocene age, and is the westernmost Paleogene vertebrate fauna from the Gulf Coastal Plain.