New Mexico Geological Society Annual Spring Meeting, Apr 21, 2006
A Pleistocene site containing skeletons of large mammals preserved in fissure-fill deposits was d... more A Pleistocene site containing skeletons of large mammals preserved in fissure-fill deposits was discovered in 2005 at the White Mesa Mine near San Ysidro in Sandoval County, New Mexico. The fissures occur in gypsum of the Middle Jurassic Todilto Formation. Structural analysis suggests the fissures opened to the surface during the Pleistocene in response to extension associated with the Rio Grande rift. Bones were found 12 m below the land surface in four different fissures within a 30 m radius. The most fossiliferous fissure, containing skeletons of a bison and two camels, was 20 cm wide and contained bones over a length of about 3.5 m and a depth of about 30 cm. There are four species of large mammals in these fissure deposits, each known from at least one partial to nearly complete articulated skeleton: giant llama Camelops hesternus, extinct bison Bison antiquus, mule deer Odocoileus hemionus, and the rare stilt-legged horse Equus francisci. Camelops is represented by two partial skeletons, a juvenile with unworn teeth and an old individual with heavily worn teeth. There are nearly complete skeletons of a juvenile B. antiquus and a female O. hemionus, and a front limb of E. francisci. This site differs from Pleistocene cave faunas in the predominance of articulated skeletons and the lack of small vertebrates. An AMS (accelerator mass spectrometer) radiocarbon date on a Camelops bone is pending; however, the presence of Bison antiquus establishes a late Rancholabrean age (late Pleistocene, <100 ka).
A small assemblage of invertebrate fossils from the Cretaceous Dakota Sandstone at Arroyo del Yes... more A small assemblage of invertebrate fossils from the Cretaceous Dakota Sandstone at Arroyo del Yeso near Ghost Ranch, Rio Arriba County, New Mexico, is the first age-diagnostic fossil assemblage documented from the intertongued Dakota-Mancos succession in the Chama Basin. The fossils are assigned to the bivalves Legumen sp., Exogyra sp., Inoceramus arvanus Stephenson, and I. prefragilis Stephenson and the ammonite cf. Acanthoceras amphibolum Morrow. They indicate the Acanthoceras amphibolum Zone of middle Cenomanian age and support lithostratigraphic identification of the fossil-bearing strata as the Paguate Sandstone Tongue of the Dakota Sandstone.
The Welles Quarry (NMMNH locality 4825) is one of several pond/crevasse splay deposits in the Cut... more The Welles Quarry (NMMNH locality 4825) is one of several pond/crevasse splay deposits in the Cutler Formation exposures near Loma Salazar in the Arroyo del Agua collecting area, Rio Arriba County, New Mexico. The quarry yields fish, amphibian, reptile, invertebrate, and plant fossils of Wolfcampian age. We describe a rich deposit of thin-shelled, freshwater bivalves from the quarry. The clams are preserved as external and (rarely) internal casts with a few shell fragments in laminar gray and dark red, micaceous muddy shale. Typical ...
New Mexico Geological Society Annual Spring Meeting, Apr 27, 2012
The dermal skull bones of temnospondyl amphibians are textured with patterns of raised, honeycomb... more The dermal skull bones of temnospondyl amphibians are textured with patterns of raised, honeycomb-like and/or ridge-and-groove patterns. The function of this texture has been the subject of inquiry in several previous studies. Competing hypotheses propose that the texture: (1) increases the surface area of the skin in support of cutaneous respiration and/or thermal regulation, (2) strengthens the bone against stresses incurred during feeding, and/or (3) protects blood vessels supplying the skin and results from the growth of nutrient channels through the bone.
New Mexico Geological Society Annual Spring Meeting, Apr 7, 2017
Metoposaurs (Temnospondyli: Metoposauridae) were large, ubiquitous amphibians of the Late Triassi... more Metoposaurs (Temnospondyli: Metoposauridae) were large, ubiquitous amphibians of the Late Triassic. They were important members of the Upper Triassic Chinle Group fauna of New Mexico and are found in these strata across the state. One of the most important metoposaur bonebeds in the world occurs in the Adamanian (upper Carnian) Garita Creek Formation near Lamy, NM, where the fossils of hundreds of individuals were hydraulically concentrated on a Late Triassic floodplain. In nearly all metoposaur populations, worldwide, many skull features show shape change throughout ontogenetic growth (allometry). Some workers have believed these allometric growth trajectories to be of taxonomic value. Here, using the largest populations available (for maximum statistical sample size), we investigate the taxonomic utility of these shape changes. Skull allometry in three populations of the metoposaur Koskinonodon perfectum (Lamy, NM; Rotten Hill, TX; Popo Agie Formation of WY) was compared to establish intra-specific variation. Then, three populations representing different genera (Dutuitosaurus ouazzoui from northern Africa; Koskinonodon perfectum from western North America; Metoposaurus diagnosticus from eastern Europe) were compared to show variation at the generic level. Anteroposterior movement of the orbits and relative skull width at three positions (snout tip at the anterior nares, basal snout at the anterior orbits, greatest width across the quadratojugals) with respect to midline length throughout ontogeny were determined. Relative orbit position moved anteriorly in D. ouazzoui and K. perfectum (Rotten Hill), posteriorly in K. perfectum (Lamy) and K. perfectum (WY), and remained approximately fixed in M. diagnosticus. Skull width allometries were of mixed values, but with rare exceptions, the snout grew wider with respect to length throughout ontogeny, whereas the posterior skull became relatively narrower. Thus, the skulls of juveniles were somewhat triangular, and they grew to be more parallel-sided with a wider, blunter snout in adults. Comparison of the calculated allometric constants from the various populations yielded a surprising result: intra-specific variation in growth patterns was as great as or greater than inter-generic variation. The three K. perfectum populations, including the Lamy metoposaurs, were not significantly more similar to each other than they were to different genera. The salient point of this preliminary work is that allometric skull growth trajectories in metoposaurs probably have little or no taxonomic value, and metoposaur taxa should probably not be erected based solely on variations in growth patterns.
There is presently considerable interest being shown in high pressure, high speed plasma closing ... more There is presently considerable interest being shown in high pressure, high speed plasma closing switches. These switches are useful in many applications, for example providing fast risetime pulses to drive lasers and ultra wide band (UWB) radar equipment. One of the advantages of such switches is that they are capable of exhibiting rapid voltage collapse (sub-nanosecond) over a range of
ABSTRACT An experimental investigation into the rate of voltage collapse in plasma closing switch... more ABSTRACT An experimental investigation into the rate of voltage collapse in plasma closing switches has been undertaken. A transmission line test switch, incorporating a radial/conical topology to minimize wave reflections, has been used for this investigation. The results described were obtained from experiments using hydrogen, helium, nitrogen, and SF 6. The results show that there is a distinct change in breakdown behavior as the pressure increases. At the lowest tested pressures, there is a rapid voltage collapse phase. This phase reduces at intermediate pressures before recovering at higher pressures. This transition from low-pressure behavior to high-pressure behavior occurred in all the tested gases, although the transition pressure values were dependent on the gas type
New Mexico Geological Society Annual Spring Meeting, Apr 16, 2004
We report the first complete sclerotic ring of the Triassic dinosaur Coelophysis bauri from the N... more We report the first complete sclerotic ring of the Triassic dinosaur Coelophysis bauri from the New Mexico Museum of Natural History's Whitaker Quarry block, C-8-82. The block is from the Apachean Rock Point Formation of the Chinle Group in north-central New Mexico. Specimen P-42200 is a gracile morph of Coelophysis with a skull length of 123 mm, which places it in the large juvenile or small gracile adult range. The anterior orbit and sclerotic ring show no deformation, but the posterior portions show slight elongation in the caudal direction. The orbit has a mean diameter of 31 mm and contains ~20 articulated ossicles preserved in a ring measuring ~21 mm outside diameter, and ~12 mm inside diameter. As preserved, the ring occludes 63% of the orbit. Individual ossicles are sub-trapezoidal to subtriangular, except for the nasal and temporal ossicles, which are more elongate oval in shape. The ossicles show a sigmoid radial cross section. The ossicles typically measure ~7 mm wide (circumferentially) and ~5 mm high (radially), with the wider bases of the elements bounding the corneal aperture. The elongate nasal and temporal ossicles arẽ 10 to 12 mm long and ~5 mm high.
New Mexico Geological Society, 2004 Annual Spring Meeting, Proceedings Volume
New Mexico Museum of Natural History's Apachean age Whitaker Quarry block (C-8-82) has been under... more New Mexico Museum of Natural History's Apachean age Whitaker Quarry block (C-8-82) has been under preparation since September, 2003. The volume of the 2-m-long, 1.4-m-wide, and 1.1-m-thick block is ~2 m3. Microstratigraphic study of the block shows two fining upward sequences above a basal sandy siltstone. The upper 5 cm of this bed contains the ostracode, Darwinula, and the conchostracan, Lioestheria, that likely indicate a topographic low containing a (probably ephemeral) pond. This is the first report of calcareous microfossils from the Whitaker Quarry. The basal coarse silt and very fine sand of sequence one fines up to very fine silt with clay lenses. Sequence two has a basal matrix-supported conglomerate that fines up to very fine silt. All beds above the invertebrates contain elongate rip-up mud clasts that are closely aligned and trend WNW-ESE. These flow indicators agree well with the established scheme of a northwest-draining Chinle basin. The block stratigraphy indicates two flood events of increasing energy. To date, the fauna from the block comprises the aforementioned invertebrates; the redfieldiid fish Synorichthyes, the coelacanth fish Chinlea, the archosauromorph Vancleavea, a phytosaur, probably pertaining to Redondasaurus gregorii, and the theropod dinosaur Coelophysis bauri. Additionally, an indeterminate redfieldiid-like fish, several isolated enigmatic scutes, and teeth of a possible sphenosuchian or ornithischian dinosaur are present. In general, the fossil material coarsens up throughout the block. Sequence one contains (ascending order) the invertebrates, fish scales and bones, whole fish, non-dinosaurian tetrapods and very small juvenile dinosaurs. Sequence two contains the larger juvenile and adult dinosaurs. The invertebrates constitute the only previously undocumented fauna from the Whitaker quarry.
We introduce probability plotting for general use in the analysis of biological and palaeontologi... more We introduce probability plotting for general use in the analysis of biological and palaeontological morphometric data. When applied to morphometric data, this analytical tool can yield palaeoecological, ontogenetic and demographic information. We have adapted and expanded a statistical tool used in reliability engineering, mechanical or electrical lifetime analysis, geochemistry and ore mineral analysis to analyse fossil populations. The technique is explained stepwise in a ‘user's manual’ format. Its power is to characterize statistical distribution types, to determine whether these distributions are skewed or truncated, and to resolve and quantify the components of mixed distributions, which in palaeontological data potentially represent morphological groupings such as cohorts or sexual dimorphs. The ease of application and simplicity are surprising. A probability plot of growth series data (e.g. skull lengths, femoral lengths, snout‐to‐vent lengths or other morphometrics) ty...
New Mexico Geological Society, 2004 Annual Spring Meeting, Proceedings Volume
Two species of the unusual archosauromorph Trilophosaurus, T. buettneri Case and T. jacobsi Murry... more Two species of the unusual archosauromorph Trilophosaurus, T. buettneri Case and T. jacobsi Murry, are known from diverse localities in the Upper Triassic Chinle Group of Texas, New Mexico and Arizona. The tricuspid teeth from which Trilophosaurus derives its name are identifiable to the species level, so fragmentary tooth-bearing fossils, and even isolated teeth, have biostratigraphic utility. T. buettneri is the better-known taxon, but is restricted to a handful of localities stratigraphically low in the Chinle Group of Texas and Arizona. Until recently, T. jacobsi was poorly known, leading some workers to speculate that it was actually a procolophonid ("Chinleogomphius"). However, we have recovered fossils from an extremely rich bonebed that demonstrate that T. jacobsi is congeneric with T. buettneri. Teeth of T. jacobsi are readily distinguished from those of T. buettneri by their tall, faceted central cusp, low, transversely elongate lingual cusp, and the presence of cingulae along their anterior and posterior margins. T. jacobsi occurs in lower Chinle Group strata in Texas, New Mexico, and Arizona, and we document the first occurrence of T. jacobsi in New Mexico here, as isolated tooth fragments are readily identified from the Sixmile Spring locality (NMMNH locality 2739) in the Bluewater Creek Formation in west-central New Mexico. The two taxa are only known to co-occur at the upper Kalgary locality (NMMNH locality 1430) in the lower Tecovas Formation in West Texas-otherwise T. buettneri always occurs in stratigraphically lower strata than T. jacobsi. The first occurrence datum (FAD) of T. buettneri in Texas corresponds to that of Otischalkian index taxa such as the phytosaur Paleorhinus and the aetosaur Longosuchus, and implies an Otischalkian age for most T. buettneri occurrences. The FAD of T. jacobsi is often at or near the FAD of Adamanian index taxa such as the phytosaur Rutiodon and the aetosaur Stagonolepis, indicating that T. jacobsi is a possible index fossil of the Adamanian. T. jacobsi is probably an anagenetic descendant of T. buettneri, as it comes from generally younger strata and appears to be more derived. Trilophosaurus is endemic to southwestern North America, and may represent a relatively rare occurrence of an arboreal herbivore.
New Mexico Geological Society Annual Spring Meeting, Apr 21, 2006
A Pleistocene site containing skeletons of large mammals preserved in fissure-fill deposits was d... more A Pleistocene site containing skeletons of large mammals preserved in fissure-fill deposits was discovered in 2005 at the White Mesa Mine near San Ysidro in Sandoval County, New Mexico. The fissures occur in gypsum of the Middle Jurassic Todilto Formation. Structural analysis suggests the fissures opened to the surface during the Pleistocene in response to extension associated with the Rio Grande rift. Bones were found 12 m below the land surface in four different fissures within a 30 m radius. The most fossiliferous fissure, containing skeletons of a bison and two camels, was 20 cm wide and contained bones over a length of about 3.5 m and a depth of about 30 cm. There are four species of large mammals in these fissure deposits, each known from at least one partial to nearly complete articulated skeleton: giant llama Camelops hesternus, extinct bison Bison antiquus, mule deer Odocoileus hemionus, and the rare stilt-legged horse Equus francisci. Camelops is represented by two partial skeletons, a juvenile with unworn teeth and an old individual with heavily worn teeth. There are nearly complete skeletons of a juvenile B. antiquus and a female O. hemionus, and a front limb of E. francisci. This site differs from Pleistocene cave faunas in the predominance of articulated skeletons and the lack of small vertebrates. An AMS (accelerator mass spectrometer) radiocarbon date on a Camelops bone is pending; however, the presence of Bison antiquus establishes a late Rancholabrean age (late Pleistocene, <100 ka).
A small assemblage of invertebrate fossils from the Cretaceous Dakota Sandstone at Arroyo del Yes... more A small assemblage of invertebrate fossils from the Cretaceous Dakota Sandstone at Arroyo del Yeso near Ghost Ranch, Rio Arriba County, New Mexico, is the first age-diagnostic fossil assemblage documented from the intertongued Dakota-Mancos succession in the Chama Basin. The fossils are assigned to the bivalves Legumen sp., Exogyra sp., Inoceramus arvanus Stephenson, and I. prefragilis Stephenson and the ammonite cf. Acanthoceras amphibolum Morrow. They indicate the Acanthoceras amphibolum Zone of middle Cenomanian age and support lithostratigraphic identification of the fossil-bearing strata as the Paguate Sandstone Tongue of the Dakota Sandstone.
The Welles Quarry (NMMNH locality 4825) is one of several pond/crevasse splay deposits in the Cut... more The Welles Quarry (NMMNH locality 4825) is one of several pond/crevasse splay deposits in the Cutler Formation exposures near Loma Salazar in the Arroyo del Agua collecting area, Rio Arriba County, New Mexico. The quarry yields fish, amphibian, reptile, invertebrate, and plant fossils of Wolfcampian age. We describe a rich deposit of thin-shelled, freshwater bivalves from the quarry. The clams are preserved as external and (rarely) internal casts with a few shell fragments in laminar gray and dark red, micaceous muddy shale. Typical ...
New Mexico Geological Society Annual Spring Meeting, Apr 27, 2012
The dermal skull bones of temnospondyl amphibians are textured with patterns of raised, honeycomb... more The dermal skull bones of temnospondyl amphibians are textured with patterns of raised, honeycomb-like and/or ridge-and-groove patterns. The function of this texture has been the subject of inquiry in several previous studies. Competing hypotheses propose that the texture: (1) increases the surface area of the skin in support of cutaneous respiration and/or thermal regulation, (2) strengthens the bone against stresses incurred during feeding, and/or (3) protects blood vessels supplying the skin and results from the growth of nutrient channels through the bone.
New Mexico Geological Society Annual Spring Meeting, Apr 7, 2017
Metoposaurs (Temnospondyli: Metoposauridae) were large, ubiquitous amphibians of the Late Triassi... more Metoposaurs (Temnospondyli: Metoposauridae) were large, ubiquitous amphibians of the Late Triassic. They were important members of the Upper Triassic Chinle Group fauna of New Mexico and are found in these strata across the state. One of the most important metoposaur bonebeds in the world occurs in the Adamanian (upper Carnian) Garita Creek Formation near Lamy, NM, where the fossils of hundreds of individuals were hydraulically concentrated on a Late Triassic floodplain. In nearly all metoposaur populations, worldwide, many skull features show shape change throughout ontogenetic growth (allometry). Some workers have believed these allometric growth trajectories to be of taxonomic value. Here, using the largest populations available (for maximum statistical sample size), we investigate the taxonomic utility of these shape changes. Skull allometry in three populations of the metoposaur Koskinonodon perfectum (Lamy, NM; Rotten Hill, TX; Popo Agie Formation of WY) was compared to establish intra-specific variation. Then, three populations representing different genera (Dutuitosaurus ouazzoui from northern Africa; Koskinonodon perfectum from western North America; Metoposaurus diagnosticus from eastern Europe) were compared to show variation at the generic level. Anteroposterior movement of the orbits and relative skull width at three positions (snout tip at the anterior nares, basal snout at the anterior orbits, greatest width across the quadratojugals) with respect to midline length throughout ontogeny were determined. Relative orbit position moved anteriorly in D. ouazzoui and K. perfectum (Rotten Hill), posteriorly in K. perfectum (Lamy) and K. perfectum (WY), and remained approximately fixed in M. diagnosticus. Skull width allometries were of mixed values, but with rare exceptions, the snout grew wider with respect to length throughout ontogeny, whereas the posterior skull became relatively narrower. Thus, the skulls of juveniles were somewhat triangular, and they grew to be more parallel-sided with a wider, blunter snout in adults. Comparison of the calculated allometric constants from the various populations yielded a surprising result: intra-specific variation in growth patterns was as great as or greater than inter-generic variation. The three K. perfectum populations, including the Lamy metoposaurs, were not significantly more similar to each other than they were to different genera. The salient point of this preliminary work is that allometric skull growth trajectories in metoposaurs probably have little or no taxonomic value, and metoposaur taxa should probably not be erected based solely on variations in growth patterns.
There is presently considerable interest being shown in high pressure, high speed plasma closing ... more There is presently considerable interest being shown in high pressure, high speed plasma closing switches. These switches are useful in many applications, for example providing fast risetime pulses to drive lasers and ultra wide band (UWB) radar equipment. One of the advantages of such switches is that they are capable of exhibiting rapid voltage collapse (sub-nanosecond) over a range of
ABSTRACT An experimental investigation into the rate of voltage collapse in plasma closing switch... more ABSTRACT An experimental investigation into the rate of voltage collapse in plasma closing switches has been undertaken. A transmission line test switch, incorporating a radial/conical topology to minimize wave reflections, has been used for this investigation. The results described were obtained from experiments using hydrogen, helium, nitrogen, and SF 6. The results show that there is a distinct change in breakdown behavior as the pressure increases. At the lowest tested pressures, there is a rapid voltage collapse phase. This phase reduces at intermediate pressures before recovering at higher pressures. This transition from low-pressure behavior to high-pressure behavior occurred in all the tested gases, although the transition pressure values were dependent on the gas type
New Mexico Geological Society Annual Spring Meeting, Apr 16, 2004
We report the first complete sclerotic ring of the Triassic dinosaur Coelophysis bauri from the N... more We report the first complete sclerotic ring of the Triassic dinosaur Coelophysis bauri from the New Mexico Museum of Natural History's Whitaker Quarry block, C-8-82. The block is from the Apachean Rock Point Formation of the Chinle Group in north-central New Mexico. Specimen P-42200 is a gracile morph of Coelophysis with a skull length of 123 mm, which places it in the large juvenile or small gracile adult range. The anterior orbit and sclerotic ring show no deformation, but the posterior portions show slight elongation in the caudal direction. The orbit has a mean diameter of 31 mm and contains ~20 articulated ossicles preserved in a ring measuring ~21 mm outside diameter, and ~12 mm inside diameter. As preserved, the ring occludes 63% of the orbit. Individual ossicles are sub-trapezoidal to subtriangular, except for the nasal and temporal ossicles, which are more elongate oval in shape. The ossicles show a sigmoid radial cross section. The ossicles typically measure ~7 mm wide (circumferentially) and ~5 mm high (radially), with the wider bases of the elements bounding the corneal aperture. The elongate nasal and temporal ossicles arẽ 10 to 12 mm long and ~5 mm high.
New Mexico Geological Society, 2004 Annual Spring Meeting, Proceedings Volume
New Mexico Museum of Natural History's Apachean age Whitaker Quarry block (C-8-82) has been under... more New Mexico Museum of Natural History's Apachean age Whitaker Quarry block (C-8-82) has been under preparation since September, 2003. The volume of the 2-m-long, 1.4-m-wide, and 1.1-m-thick block is ~2 m3. Microstratigraphic study of the block shows two fining upward sequences above a basal sandy siltstone. The upper 5 cm of this bed contains the ostracode, Darwinula, and the conchostracan, Lioestheria, that likely indicate a topographic low containing a (probably ephemeral) pond. This is the first report of calcareous microfossils from the Whitaker Quarry. The basal coarse silt and very fine sand of sequence one fines up to very fine silt with clay lenses. Sequence two has a basal matrix-supported conglomerate that fines up to very fine silt. All beds above the invertebrates contain elongate rip-up mud clasts that are closely aligned and trend WNW-ESE. These flow indicators agree well with the established scheme of a northwest-draining Chinle basin. The block stratigraphy indicates two flood events of increasing energy. To date, the fauna from the block comprises the aforementioned invertebrates; the redfieldiid fish Synorichthyes, the coelacanth fish Chinlea, the archosauromorph Vancleavea, a phytosaur, probably pertaining to Redondasaurus gregorii, and the theropod dinosaur Coelophysis bauri. Additionally, an indeterminate redfieldiid-like fish, several isolated enigmatic scutes, and teeth of a possible sphenosuchian or ornithischian dinosaur are present. In general, the fossil material coarsens up throughout the block. Sequence one contains (ascending order) the invertebrates, fish scales and bones, whole fish, non-dinosaurian tetrapods and very small juvenile dinosaurs. Sequence two contains the larger juvenile and adult dinosaurs. The invertebrates constitute the only previously undocumented fauna from the Whitaker quarry.
We introduce probability plotting for general use in the analysis of biological and palaeontologi... more We introduce probability plotting for general use in the analysis of biological and palaeontological morphometric data. When applied to morphometric data, this analytical tool can yield palaeoecological, ontogenetic and demographic information. We have adapted and expanded a statistical tool used in reliability engineering, mechanical or electrical lifetime analysis, geochemistry and ore mineral analysis to analyse fossil populations. The technique is explained stepwise in a ‘user's manual’ format. Its power is to characterize statistical distribution types, to determine whether these distributions are skewed or truncated, and to resolve and quantify the components of mixed distributions, which in palaeontological data potentially represent morphological groupings such as cohorts or sexual dimorphs. The ease of application and simplicity are surprising. A probability plot of growth series data (e.g. skull lengths, femoral lengths, snout‐to‐vent lengths or other morphometrics) ty...
New Mexico Geological Society, 2004 Annual Spring Meeting, Proceedings Volume
Two species of the unusual archosauromorph Trilophosaurus, T. buettneri Case and T. jacobsi Murry... more Two species of the unusual archosauromorph Trilophosaurus, T. buettneri Case and T. jacobsi Murry, are known from diverse localities in the Upper Triassic Chinle Group of Texas, New Mexico and Arizona. The tricuspid teeth from which Trilophosaurus derives its name are identifiable to the species level, so fragmentary tooth-bearing fossils, and even isolated teeth, have biostratigraphic utility. T. buettneri is the better-known taxon, but is restricted to a handful of localities stratigraphically low in the Chinle Group of Texas and Arizona. Until recently, T. jacobsi was poorly known, leading some workers to speculate that it was actually a procolophonid ("Chinleogomphius"). However, we have recovered fossils from an extremely rich bonebed that demonstrate that T. jacobsi is congeneric with T. buettneri. Teeth of T. jacobsi are readily distinguished from those of T. buettneri by their tall, faceted central cusp, low, transversely elongate lingual cusp, and the presence of cingulae along their anterior and posterior margins. T. jacobsi occurs in lower Chinle Group strata in Texas, New Mexico, and Arizona, and we document the first occurrence of T. jacobsi in New Mexico here, as isolated tooth fragments are readily identified from the Sixmile Spring locality (NMMNH locality 2739) in the Bluewater Creek Formation in west-central New Mexico. The two taxa are only known to co-occur at the upper Kalgary locality (NMMNH locality 1430) in the lower Tecovas Formation in West Texas-otherwise T. buettneri always occurs in stratigraphically lower strata than T. jacobsi. The first occurrence datum (FAD) of T. buettneri in Texas corresponds to that of Otischalkian index taxa such as the phytosaur Paleorhinus and the aetosaur Longosuchus, and implies an Otischalkian age for most T. buettneri occurrences. The FAD of T. jacobsi is often at or near the FAD of Adamanian index taxa such as the phytosaur Rutiodon and the aetosaur Stagonolepis, indicating that T. jacobsi is a possible index fossil of the Adamanian. T. jacobsi is probably an anagenetic descendant of T. buettneri, as it comes from generally younger strata and appears to be more derived. Trilophosaurus is endemic to southwestern North America, and may represent a relatively rare occurrence of an arboreal herbivore.
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