The Nacimiento Formation is a sedimentary rock formation in the San Juan Basin of western New Mexico (United States). It is dated to approximately 61.0–65.7 million years ago, corresponding to the early and middle Paleocene. The formation has yielded numerous fossil assemblages from the interval following the Cretaceous–Paleogene extinction event, which have been used to study biotic recovery and the early diversification of mammals after the extinction.
Description The Nacimiento Formation is a heterogeneous nonmarine formation composed of shale, siltstone, and sandstone, deposited in floodplain, fluvial and lacustrine settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~65.7 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot and stable, but with a distinct dry season. This unit interbeds with the underlying Ojo Alamo Formation but is separated by an unconformity from the overlying San Jose Formation.
The Nacimiento Formation is divided into several subunits known as members. In outcrops in southern areas of the formation, the Puercan fauna is found in the Arroyo Chijuillita Member, the Torrejonian fauna is found in the Ojo Encino Member, and the uppermost Escavada Member lacks age-diagnostic fossils. In northern outcrops, the two lower members are indistinguishable, and are called the "main body". Above them are two more informal members. These preserve a younger, Tiffanian fauna. The Puercan and Torrejonian faunas are further subdivided into several biostratigraphic zones.
Fossils Many fossils are known from the Nacimiento Formation, although bone is often altered into phosphatic concretions. Fossils belonging to a number of different organisms have been found here, including: plants (mostly dicotyledonous angiosperms), gastropods, freshwater bivalves, cartilaginous fish and bony fish, salamanders, turtles, champsosaurs, amphisbaenians, lizards, snakes, crocodilians, birds, and a variety of archaic mammals. Mammalian groups represented include multituberculates, plesiadapiforms, didelphid marsupials, insectivorans, carnivorans, taeniodonts, mesonychians, condylarths, and cimolestans. Fossil remains found in the formation support the validity of the genus Thylacodon and the species T. montanensis.
These fossils provide important clues to the impact of the Cretaceous–Paleogene extinction event on mammals and to the recovery, evolution, and turnover of mammals shortly after the event. The formation and its fossils provide a particularly clear record of the To2-To3 turnover event, allowing the timing of the event to be constrained to between 62.59 and 62.47 million years ago. The event may have been associated with climate change or with the rapid development of a river system across the San Juan basin, which caused a temporary pause in sediment deposition that separates the Nacimiento Formation from the San Jose Formation.
Fossil content Based on the Paleobiology Database:
Actinopterygii Boney Fish of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesLepisosteidae indet.TorrejonianA gar of uncertain affinities.
Amphibians Amphibians of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesHabrosaurusH. dilatusTorrejonianA sirenid salamander.
Reptiles
Crocodyliformes Crocodyliformes of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesAkanthosuchusA. langstoniPuercan & TorrejonianAn alligatorid.ListrognathosuchusL. multidentatusTorrejonianAn alligatoroid.NavajosuchusL. mookiPuercan & TorrejonianAn alligatorid.
Turtles Turtles of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesAdocusA. hesperiusPuercanAn adocid turtle.A. onerosusTorrejonianAxestemysA. montinsanaPuercanA softshell turtle.CompsemysC. victaPuercan/TorrejonianA compsemydid turtle."Emys""E." sp.Puercan/TorrejonianA pond turtle.HutchemysH. acupictusPuercanA plastomenid stem-softshell turtle.HoplochelysH. crassaPuercan/TorrejonianA kinosternoid turtle. Type locality of species.NeurankylusN. cf. eximiusPuercanA baenid turtle. Type locality of N. torrejonensis.N. torrejonensisTorrejonian, potentially PuercanPlastomenusP. vegetusPuercanA plastomenid stem-softshell turtle."Platypeltis""P." antiquaTorrejonianA stem-softshell turtle. Nomen dubium."Trionyx" (=Paleotrionyx)"T." eloisaePuercanA softshell turtle. Type locality of species."T." reesidei"T." singularis
Squamates Squamates of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesGlyptosaurinae indet.Puercan & TorrejonianA glyptosaurine anguid of uncertain affinities.HelagrasH. prisciformisPuercanAn afrophidian snake, the largest known Early Paleocene snake.MachaerosaurusM. torrejonensisTorrejonianA glyptosaurine anguid.OdaxosaurusO. pigerPuercanA glyptosaurine anguid.Parodaxosaurus P. sanjuanensisTorrejonianA glyptosaurine anguid.PlesiorhineuraP. tsentasiTorrejonianA rhineurid worm lizard.ProxestopsP. silberlingiiPuercan & TorrejonianA glyptosaurine anguid.
Birds Birds of the Nacimiento Formation Genus Species Location Stratigraphic position Material Notes ImagesTsidiiyazhiT. abiniTorrejonianA relative of the mousebirds.
Mammals Primary based on Kondrashov and Lucas (2015), with changes where necessary:
Archaic Ungulates 'Condylarths' of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesAnisonchusA. sectoriusTorrejonian 1–3A periptychidCarsioptychus C. coarctatusPuercan 2–3A periptychid originally described as a species of the genus Periptychus.ConacodonC. cophaterPuercan 2–3A periptychidC. entoconusPuercan 2C. kohlbergeriPuercan 2EctoconusE. ditrigonusPuercan 2–3A periptychidHaploconusH. augustusTorrejonian 1–2A periptychidHemithlaeusH. kowalewskianusPuercan 2A periptychidMithrandirM. gillianusPuercan 2–3A periptychid originally described as a species of the genus Anisochus.OxyacodonO. agapetillusPuercan 2A periptychidO. apiculatusPuercan 2–3O. priscillaPuercan 2PeriptychusP. carinidensTorrejonian 1–3A periptychid
Arctocyonia Arctocyonians of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesArctocyonA. corrugatusTorrejonian 2An arctocyonidA. feroxTorrejonian 2–3ChriacusC. baldwiniTorrejonian 2–3An arctocyonidC. orthogoniusTorrejonianC. pelvidensTorrejonian 2–3ColpoclaenusC. procyonoidesTorrejonian 3An arctocyonidDesmatoclaenusD. dianaePuercan 2–3An arctocyonidD. protogonoidesPuercan 2–3DeuterogonodonD. montanusTorrejonian 1–2An arctocyonidLoxolophusL. hyattianusPuercan 2–3An arctocyonidL. pentacusPuercan 2–3L. priscusPuercan 3ProthryptacodonP. ambiguusTorrejonian 3An arctocyonidTricentesT. subtrigonusTorrejonian 1–3An arctocyonid
Carnivorans Carnivorans of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesBryanictisB. paulusTorrejonian 2A viverravid carnivoranIntyrictisI. vanvaleniTorrejonian 2A viverravid carnivoranProtictisP. haydenianusTorrejonian 2–3A viverravid carnivoranP. minorTorrejonian 2P. simpsoniTorrejonian 2
Cimolesta Cimolestans of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesAcemeodonA. secansTorrejonian 2–3A cimolestid cimolestanBetonniaB. tsosiaPuercan 2–3A cimolestid cimolestanChacopterygusC. minutusPuercan 2–3A cimolestid cimolestanPalaeoryctesP. puercensisTorrejonian 2–3A palaeoryctid cimolestanPuercolestesP. simpsoniPuercan 2–3A cimolestid cimolestan
Euarchonta Euarchontans of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesAnasaziaA. williamsoniTorrejonian 2A palaechthonid plesiadapiformMixodectesM. malarisTorrejonian 2–3A mixodectidM. pungensTorrejonian 3PalaechthonP. woodiTiffanian 1A palaechthonid plesiadapiformParomomysP. libedianusTorrejonian 2A paromomyid plesiadapiformPicrodusP. calgariensisTorrejonian 2A picrodontid plesiadapiformPlesiolestesP. nacimientiTorrejonian 2–3A palaechthonid plesiadapiformP. torrejoniusTorrejonian 2TorrejoniaT. wilsoniTorrejonian 3A palaechthonid plesiadapiform
Eutheria Incertae sedis Other Eutherians of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesDeltatheriumD. dandreaeTorrejonian 2–3A eutherian mammal of unknown placementD. fundaminisTorrejonian 4–5
Leptictida Leptictidans of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesProdiacodonP. puercensisTorrejonian 1–3A leptictiid leptictidan
Mesonychians Mesonychians of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesAnkalagonA. saurognathusTorrejonian 2–3A mesonychid mesonychianCarcinodonC. antiquusPuercan 3A triisodontid mesonychianC. olearyiPuercan 2–3C. simplexPuercan 2DissacusD. navajoviusTorrejonian 2–3A mesonychid mesonychianEoconodonE. coryphaeusPuercan 2–3A triisodontid mesonychianE. gaudrianusPuercan 2E. ginibitohiaPuercan 2GoniacodonG. levisanusTorrejonian 2–3A triisodontid mesonychianOxyclaenusO. cuspidatusTorrejonian 2–3A triisodontid mesonychianTriisodonT. crassicuspisTorrejonian 2A triisodontid mesonychianT. quivirensisTorrejonian 2
Metatheria Metatherians of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesPeradectesP. coprexechesTorrejonian 2–3A peradectid metatherianP. minorPuercan 2–3SwaindelphysS. encinensisTorrejonian 2A herpetotheriid metatherianS. johnsoniTorrejonian 3ThylacodonT. pusillusPuercan 2–3A herpetotheriid metatherian
Multituberculata Multituberculates of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesAnconodonA. gidleyiTorrejonian 2A neoplagiaulacid multituberculateCatopsalisC. fissidensTorrejonian 2A taeinolabid multituberculateC. foliatusPuercan 2EctypodusE. szalayiTorrejonian 2A neoplagiaulacid multituberculateEucosmodonE. americanusPuercan 2An eucosmodontid multituberculateE. molestusTorrejonian 2E. primusPuercan 2KimbetohiaK. campiPuercan 2A ptilodontid multituberculateKimbetopsalisK. simmonsaePuercan 2A taeinolabid multituberculateKrauseiaK. clemensiTorrejonian 2A neoplagiaulacid multituberculateLiotomusL. vanvaleniPuercan 2A cimolodontid multituberculateMesodmaM. formosaPuercan 2A neoplagiaulacid multituberculateM. thompsoniPuercan 2MimetodonM. krauseiTorrejonian 2A neoplagiaulacid multituberculateNeoplagiaulaxN. macintyreiPuercan 2A neoplagiaulacid multituberculateN. macrotomeusTorrejonian 2ParectypodusP. trovessartianusTorrejonian 2A neoplagiaulacid multituberculatePtilodusP. mediaevusTorrejonian 2–3A ptilodontid multituberculateP. tsosiensisPuercan 2StygimysS. teilhardiTorrejonian 2An eucosmodontid multituberculateTaeniolabisT. simmonsaePuercan 3A taeinolabid multituberculateT. taoensisPuercan 3
Palaeanodonta Palaeanodonta of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesEscavadodonE. zygusTorrejonian 3An escavadodontid palaeanodont
Panperissodactyla Panperissodactyls of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesBomburodonB. priscusPuercan 2–3A hyopsodontidChoeroclaenusC. turgidunculusPuercan 2–3A hyopsodontidChacomylusC. sladeiPuercan 3A hyopsodontidLitomylusL. osceolaeTorrejonian 2–3A hyopsodontidMioclaenusM. turgidusTorrejonian 1–3A hyopsodontidPromioclaenusP. wilsoniPuercan 2–3A hyopsodontidProtoseleneP. bombadiliPuercan 2–3A hyopsodontidP. opisthacusTorrejonian 2–3TetraclaenodonT. puercensisTorrejonian 1–3A phenacodontidTiznatziniaT. vanderhoofiPuercan 2–3A hyopsodontid
Pantodonts Pantodonts of the Nacimiento FormationGenusSpeciesStratigraphic positionNotesImagesPantolambdaP. bathmodonTorrejonian 3A pantolambdid pantodontP. cavirictumTorrejonian 2–3P. intermediumTorrejonian 2
Pantolesta