Colorado City Formation
Geologic formation in Texas, United States
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The Colorado City Formation is a Late Triassic geologic formation in the Dockum Group of Texas, United States. It has previously been known as the Iatan Member,[1] Colorado City Member[2][3] or 'Pre-Tecovas Horizon'[4] (as it is assumed to be older than the Tecovas Formation).
| Colorado City Formation | |
|---|---|
| Stratigraphic range: Late Triassic | |
| Type | Formation |
| Unit of | Dockum Group |
| Underlies | possibly Cooper Canyon Formation |
| Overlies | Camp Springs Formation |
| Lithology | |
| Primary | siltstone, sandstone |
| Location | |
| Region | Texas |
| Country | United States |
The Colorado City Formation is mostly restricted to Howard and Borden counties. The formation hosts the Otis Chalk fossil sites, named after a ghost town in Howard County. Despite their importance, the Otis Chalk localities have been difficult to resolve in the stratigraphy of Triassic Texas.[5] They occupy a narrow band of sediments between the slightly older Camp Springs Formation and much younger Cretaceous deposits.
The first major excavations near Otis Chalk were led by UMMP paleontologists starting in 1927. Several new phytosaur species were discovered during these digs. University of Oklahoma paleontologists followed with their own expedition in 1931. The vast majority of fossils collected from the formation were recovered during a 1939–1941 state-sponsored Works Progress Administration paleontological survey. Several sites southeast of Big Spring were particularly productive. Fossils collected by these efforts were stored at the newly opened Texas Memorial Museum in Austin. Since the 1940s, collection from the Otis Chalk area has been more limited. One notable find is a pond deposit, the Schaeffer Fish Quarry, discovered in 1967 by AMNH paleontologist Bobb Schaeffer.[4]
Biochronological significance
The Otis Chalk localities that are situated in the Colorado City Formation form the basis of the Otischalkian Land Vertebrate Faunachron (LVF), which is defined by the first appearance of Parasuchus.[6]
Paleobiota
Color key
|
Notes Uncertain or tentative taxa are in small text; |
Reptiles
Archosaurs
Other archosaur fossils include remains of an unnamed silesaurid[7] and a partial femur of a theropod or herrerasaurian dinosaur referable to the Chindesaurus + Tawa clade.[8]
| Archosaurs of the Colorado City Formation | ||||||
|---|---|---|---|---|---|---|
| Genus / Taxon | Species | Material | Location | Notes | Images | |
| "Chatterjeea"[4] | C. elegans[4] | Vertebrae, hip bones, and femora[4] |
|
Postcranial bones of an unusual bipedal archosaur related to poposaurids.[4] Later determined to be synonymous with Shuvosaurus, a shuvosaurid poposauroid initially misidentified as a theropod dinosaur.[9] |  | |
| Coahomasuchus | C. kahleorum[3] | Two skeletons, one of which is nearly complete[3] while the other shows adaptations for a carnivorous diet.[10] | An aetosaurine aetosaur | |||
| Crocodylomorpha[11] | indet.[11] | Isolated ilium[11] |
|
An unnamed basal crocodylomorph, one of the oldest from North America.[11] | ||
| Desmatosuchus? | D. spurensis? | Osteoderms (scutes) and vertebrae[4] | Most Otis Chalk desmatosuchin aetosaur fossils have been referred to Longosuchus or Lucasuchus, but a few have been referred to Desmatosuchus (Episcoposaurus) haplocerus.[4] The modern valid combination for this species is Desmatosuchus spurensis.[12] |  | ||
| Dromomeron | D. gregorii[13] | Hindlimb bones (femora and tibiae)[13] |
|
A lagerpetid avemetatarsalian |  | |
| Lepidus[14] | L. praecisio[14] | Partial hindlimb (tibia, fibula, astragalus)[14] |
|
A possible coelophysid theropod, potentially the oldest dinosaur from North America.[14] |  | |
| Longosuchus | L. meadei | Skulls, osteoderms, vertebrae, other postcrania[4][10] |
|
A common desmatosuchin aetosaur previously considered a species of Typothorax. |  | |
| Lucasuchus[4] | L. hunti[4] | Osteoderms and other referred fossils (including a possible skull)[4][10] |
|
A desmatosuchin aetosaur similar to Longosuchus[3] but most likely a distinct valid taxon.[15] | ||
| Poposaurus | P. gracilis | Hip and ankle bones[4] |
|
A poposaurid poposauroid | _flipped.jpg) | |
| P. langstoni[4] | Hip bones and vertebrae[4] |
|
A poposaurid poposauroid previously given its own genus, Lythrosuchus[4] | |||
| Postosuchus | P. kirkpatricki | Vertebrae and hindlimb bones[4] |
|
A rausuchid loricatan |  | |
Phytosaurs
| Phytosaurs of the Colorado City Formation | ||||||
|---|---|---|---|---|---|---|
| Genus / Taxon | Species | Material | Location | Notes | Images | |
| Angistorhinus | A. alticephalus | Partial skeleton with skull[4] |
|
Probable junior synonym of A. grandis. | ||
| A. grandis | Several complete skulls and associated cranial and postcranial fragments[1][4][16] | A common mystriosuchine phytosaur. Some sources refer all Otis Chalk Angistorhinus material to "Angistorhinus megalodon",[4] while most other sources support affinities with the type species Angistorhinus grandis from Wyoming.[16] |  | |||
| Brachysuchus | B. megalodon | Two nearly complete skulls and postcranial fragments |
|
A mystriosuchine phytosaur closely related to Angistorhinus, and sometimes regarded as a species within that genus.[4] |  | |
| Paleorhinus | "P." sawini[4] | A skull[4] |
|
A basal (non-mystriosuchine) phytosaur from Borden County. Probably does not form a clade with other species referred to Paleorhinus or Parasuchus. | ||
| P. bransoni | Several skulls[4] |
|
A common basal (non-mystriosuchine) phytosaur, sometimes considered a species of Parasuchus. |  | ||
Other reptiles
| Other reptiles of the Colorado City Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Material | Location | Notes | Images | |
| Ankylosuchus[17] | A. chinlegroupensis[17] | Partial skeleton with cranial and pelvic fragments, osteoderms, vertebrae, and an indeterminate limb bone[17] |
|
A doswelliid archosauriform. | ||
| Doswellia | D. kaltenbachi | Vertebrae and osteoderms[4] |
|
A doswelliid archosauriform.[4] |  | |
| Malerisaurus | M. langstoni[18] | Disarticulated skeleton[18][19] |
|
A carnivorous azendohsaurid allokotosaurian in the subfamily Malerisaurinae. Regarded as a chimera by Spielmann et al. (2006),[20] but treated as valid by other sources.[18][19] Numerous disarticulated Malerisaurus-like fossils are also known from Quarry 1 (TMM 31025).[19] |  | |
| Otischalkia | O. elderae | Humeri, femora[4] |
|
A dubious putative rhynchosaur, likely synonymous with Malerisaurus langstoni.[19] Supposed premaxilla fossils have been reinterpreted as metoposaurid cleithra.[19] Femora and humeri previously referred to this species likely belong to Trilophosaurus[4] or Malerisaurus[19] instead. | ||
| Protecovasaurus[21] | P. lucasi[21] | Teeth[21] |
|
A reptile of uncertain affinities known from serrated teeth similar to ornithischian dinosaurs.[21] | ||
| Trilophosaurus | T. buettneri | Numerous skulls, braincases, and postcranial material[4][22] | A common trilophosaurid allokotosaurian.[22] | .jpg) | ||
| Triopticus[23] | T. primus[23] | Partial skull[23] |
|
A protopyknosian archosauriform with a reinforced skull similar to pachycephalosaurid dinosaurs.[23] | ||
Amphibians
| Amphibians of the Colorado City Formation | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Material | Location | Notes | Images | |
| Anaschisma | A. browni | Numerous skulls, interclavicles, clavicles, vertebrae, and other postcranial bones[4] |
|
A common metoposaurid temnospondyl, previously named under the genus Koskinonodon or Buettneria[4] |  | |
| Latiscopus[24] | L. disjunctus[24] | A skull[24] |
|
A latiscopid temnospondyl[24] | ||
Fish
Fish fossils from Quarry 1 (NMMNH 860 / TMM 31025) include lungfish teeth (Arganodus?[1]), coelacanth scales, "palaeoniscid" scales (aff. Turseodus[1]), and hybodont shark teeth and spines (Lissodus?[1] or Lonchidion?).[21]
| Fish of the Colorado City Formation | ||||||
|---|---|---|---|---|---|---|
| Genus / Taxon | Species | Material | Location | Notes | Images | |
| Cionichthys | C. greeni[25] | Articulated skulls and postcrania[25] |
|
A redfieldiiform actinopterygian (ray-finned fish) | ||
| Lasalichthys | L. hillsi | Skull, scale, and postcranial fragments[25][26] | A redfieldiiform actinopterygian | |||
| L. otischalkensis[26] | Numerous articulated skulls and postcrania[26] |
|
A redfieldiiform actinopterygian.[26] Some specimens were previously referred to Lasalichthys (= Synorichthys) stewarti.[26] | |||
