Richards Spur is aPermian fossil locality located at the Dolese Brothers Limestone Quarry north ofLawton, Oklahoma. The locality preserves clay and mudstone fissure fills of akarst system eroded out of Ordovicianlimestone anddolomite, with the infilling dating to theArtinskian stage of the early Permian (Cisuralian), around 289 to 286 million years ago. Fossils of terrestrial animals are abundant and well-preserved, representing one of the most diversePaleozoictetrapod communities known.^[1][2] A common historical name for the site is Fort Sill, in reference to thenearby military base.^[3] Fossils were first reported at the quarry by workers in 1932, spurring a wave of collecting by local and international geologists. Early taxa of interest included the abundant reptileCaptorhinus^[4][5] andmicrosaurs such asCardiocephalus andEuryodus.^[3] Later notable discoveries includeDoleserpeton (one of the mostlissamphibian-like Paleozoic tetrapods),^[6] the most diverse assortment ofparareptiles in the Early Permian,^[7] and the rare earlydiapsidOrovenator.^[8]

The caves of Richards Spur formed in theOrdovician-ageArbuckle Limestone, which was uplifted, exposed, and tilted into a vertical orientation within thePennsylvanian andPermian. In the early Permian, akarst system formed within the limestone, complete with caves containingspeleotherms (stalagmites,stalactites,cave popcorn, etc.) made ofcalcite. Most of the karsts are narrow, 40–60 cm (16-24 inches) in width, and vertically oriented. Due to active mining at the site constantly destroying and exposing new layers, the layout of the system has not been recorded. Most of the Permian infill is discarded in the quarry's waste dumps without sedimentological and stratigraphic data, hampering studies into those aspects of the locality. However, it is known that the lower sections of the system (25 meters or 82 feet below the surface) lacks fossil material.^[1]

Many of the fossils of Richards Spur were found in softcalcareousclaystone orconglomerate. They likely ended up in the caves as a result of water runoff from the surface, as indicated by the presence of surface minerals such asquartz,kaolinite, andsulfides among the fossils. Individual organisms may have been already disarticulated by scavenging or decomposition on the surface, decomposed within the caves after the fresh corpse had been washed in, or even died within the caves after becoming trapped. Organisms which became disarticulated on the surface experienced more wear and erosion on their fossils, induced by exposure to the elements and transportation by water within and/or outside the karst system. On the other hand, recently deceased or living organism would have been more articulated due to their decomposition occurring in the more stable cave environment, with their tendons keeping their individual bones in place prior to fossilization. The most complete fossils were encased in a residue which was almost completelycalcite, indicating that the cave structures precipitated around their skeletons. The caves likely had to have been submerged in water (or at least persistently humid) for active speleotherm formation, and therefore this mode of spectacular preservation, to have been possible. Some fossils are encrusted bypyrite, indicating the presence ofanoxic fluids ordiagenesis in the systems at some point. Most (but not all) fossils are stained a dark color by seepage ofhydrocarbons into the deposits. These assorted biochemical conditions are the likely cause of unusually variableCarbon isotope values found within different preserved speleotherms.^[1]

Isotope analysis of preservedspeleotherms shows several regular fluctuations inδ¹⁸O levels within a time span of 1-20 thousand years. Similar fluctuations in modern low-latitude environments are considered to be indicative of strong variation in precipitation between wet and arid periods on the scales ofcenturies ormillennia. Sometrace elements agree with this data, asBarium andPhosphorus concentrations increase with higherδ¹⁸O (drier periods); this is explained by increased incorporation of dust and seafoam in drier, windier periods, as demonstrated by climatological analyses in a modern cave system inIsrael.^[9][1]

Other than exceedingly rare fragments ofxenacanthids anderyopoids, aquatic animals are practically absent from Richards Spur. Although amphibians are common at the site, most of them are terrestrially-adapted taxa such asdissorophoids,microsaurs, andseymouriamorphs. This is in strong contrast to contemporary floodplain environments in Oklahoma and Texas, which have abundant fossils of aquatic animals likeEryops andDiplocaulus, along with large lowland amniotes likeEdaphosaurus. As a result, the site is considered to represent animals living in a drier environment upland from the humid floodplains which preserve most of the Permianred beds. The only other productive Early Permian geological locale commonly considered to preserve an upland community is theTambach Formation ofGermany.^[10][11][2]

The unique preservational environment of Richards Spur precludes geological stratigraphy. Based on the faunal composition (particularly the abundance ofCaptorhinus aguti,Cardiocephalus, andEuryodus), Richards Spur has been considered roughly equivalent in age to theArroyo Formation (Lower Clear Fork) ofTexas.^[3][10][12] In Oklahoma, the equivalent may be the upperGarber Formation or lowerHennessey Formation.^[13] The South Grandfield site of the Hennessey Formation is an example of a more typical Oklahoman fossil locale which has similar captorhinid and microsaur taxa to Richards Spur.^[2] To determine the absolute age of the Richards Spur deposits, the speleotherm studied for the Oxygen isotope and trace element analyses was also sampled forUranium-Lead dating. It was determined that the speleotherm was formed between 289.68 and 288.32 million years ago. This time period was originally stated to beSakmarian in age,^[9] but after a later refinement to theICS timescale, it was specified as belonging to the earlyArtinskian. Two more speleotherms studied later gave date ranges of 283.8 to 289.6 Ma, and 286.0 to 286.4 Ma, indicating that the locality was deposited over several million years.^[1]

Acheloma dunni, atrematopidtemnospondyl^[14]

Aspidosaurus sp., adissorophidtemnospondyl^[15]

Cacops morrisi, adissorophidtemnospondyl^[16]

Cacops woehri, adissorophidtemnospondyl^[17]

Cardiocephalus peabodyi, agymnarthridmicrosaur^[3]

Dissorophus multicinctus, adissorophidtemnospondyl^[15]

Doleserpeton annectens, anamphibamidtemnospondyl^[6]

Euryodus primus, agymnarthridmicrosaur^[3]

Llistrofus pricei, ahapsidopareiidmicrosaur^[18]

Nannaroter mckinziei, anostodolepidmicrosaur^[19]

Pasawioops mayi, amicropholidtemnospondyl^[20]

Seymouria sp., aseymouriamorph^[10]

Sillerpeton permianum, anaistopod^[21]

Tersomius dolesensis, amicropholidtemnospondyl^[22]

Arisierpeton simplex, acaseid^[23]

Dimetrodon sp., asphenacodontid^[24]

Mesenosaurus efremovi, avaranopid^[25]

Mycterosaurus longiceps, avaranopid^[12]

Oromycter dolesorum, acaseid^[26]

Varanops brevirostris, avaranopid^[12]

Abyssomedon williamsi, anyctiphruretid^[27]

Bolosaurus grandis, abolosaurid^[28]

Colobomycter pholeter, alanthanosuchoid^[7]

Colobomycter vaughni, alanthanosuchoid^[29]

Delorhynchus cifellii, alanthanosuchoid^[30]

Delorhynchus multidentatus, alanthanosuchoid^[31]

Delorhynchus priscus, alanthanosuchoid^[30]

Feeserpeton oklahomensis, alanthanosuchoid^[32]

Microleter mckinzieorum, a basal parareptile^[33]

Baeotherates fortsillensis, acaptorhinid^[34]

Captorhinus aguti, acaptorhinid^[5]

Captorhinus kierani, acaptorhinid^[35]

Captorhinus magnus, acaptorhinid^[36]

Labidosauriscus richardi, acaptorhinid^[34]

Maiothisavros dianeae, a basalneodiapsid^[37]

Opisthodontosaurus carrolli, acaptorhinid^[38]

Orovenator mayorum, a basalneodiapsid^[8]

Dolesea subtila, an indeterminatemillipede^[39]

Karstiulus fortsillensis, axyloiuloidmillipede^[39]

Oklahomasoma richardsspurense, ajuliformmillipede^[39]

• Permian paleontological sites
• Paleontology in Oklahoma

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