| Xiaowa Formation | |
|---|---|
| Stratigraphic range:Carnian | |
| Type | Geological formation |
| Underlies | Laishike Formation |
| Overlies | Zhuganpo Formation |
| Lithology | |
| Primary | limestone,marl |
| Location | |
| Country | China |
| Extent | Yunnan–Guizhou Plateau |
TheXiaowa Formation is aCarnian-age geological formation found in southernChina. It is a sequence oflimestone andmarls from the Carnian stage of the Triassic. Its lower section was previously known as theWayao Formation orWayao Member of the Falang Formation (a nomenclature still used by some authors). In 2002, the Wayao Member was renamed and raised to the Xiaowa Formation to prevent confusion with anEocene unit of the same name.[1]Crinoids and marine reptiles are abundant in the Xiaowa Formation, forming alagerstätte known as theGuanling biota.Ammonoids andconodonts found in the formation constrain its age to the early Carnian.[2][1][3][4][5] Reptiles of the Guanling biota includeichthyosaurs,thalattosaurs,placodonts, andOdontochelys (an early relative of turtles).[4] Sedimentary events within this formation have been tied to theCarnian Pluvial Event.[6][7]
The Xiaowa Formation has three members. The lower member is relatively thin but is also very fossiliferous. It begins with thick-bedded greybiomicrite (fine-grained fossiliferous limestone)interbedded with greenishshale.Bivalves and crinoid fragments are the most common fossils in the biomicrite layers, which sometimes grade upwards to dark grey laminated marls. The lower section of the lower member represents a relatively well-oxygenated pelagic environment. The lower member then transitions to a section of darker and moreclastic layers indicative of anoxic conditions and reduced reef activity. Most of the articulated crinoids and vertebrate fossils of the Guanling biota hail from a dark grey micrite at the base of the lower member's upper section. This is followed by dark grey marls and black shale rich in bivalves,ammonoids, and slightly radioactiveclay minerals. The lower member concludes with a sequence of dark grey laminated marls incorporatingconodont fossils andsiltyquartz grains.[1][4]
The middle member of the Xiaowa Formation is by far the thickest unit and includes thick-bedded grey limestone and marl layers interbedding with each other. Sediment deformation is characteristic of layers in this member, while fossils are represented mainly by occasional bivalves and ammonoids. The middle member represents a deep-water environment influenced bytectonic events which disturb sediment layers and create distantturbidites that periodically supply increased clastic material. The upper member is mostly dominated by laminated limestone. Though marl interbedding and fossils are practically absent, silty to sandy quartz grains are common and dominate the last few meters of the formation. This member represents a shallower ocean environment (likely raised by tectonic uplift) supplied with dust from terrestrial areas[1][4]
The Xiaowa Formation encompasses severalbiostratigraphic zones. TheProtrachyceras costulatum ammonoid zone of the upperZhuganpo Formation continues into the first few meters of the Xiaowa Formation's lower member. However, the rest of the lower member (including the Guanling biota) belongs to theTrachyceras multituberculatum ammonoid zone. This unit has also been called theAustrotrachyceras triadicum zone, and is likely equivalent to theT. aon orT. aonoides zone of the westernTethys (Europe). The middle member of the Xiaowa Formation belongs to theSirenites cf.senticosus ammonoid zone.[2][1][3][5]
Color key
| Notes Uncertain or tentative taxa are insmall text; |
| Bivalves of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Angustella | A. sp. | A rarebakevelliidbivalve.[1] |
| Asoella | A. sp. | A rareasoelliid bivalve.[1] |
| Daonella | ||
| D. bifurcata | An abundanthalobiid bivalve, namesake of theH. subcomata-D.bifurcata bivalve zone.[1] | |
| D. bulogensis | A halobiid bivalve.[8] | |
| D. indica | A common halobiid bivalve.[1] | |
| Halobia | ||
| H. brachyotis | A common halobiid bivalve.[1] | |
| H. kui | A common halobiid bivalve.[1] | |
| H. planicosta | A common halobiid bivalve.[1] | |
| H. rugosoides | A common halobiid bivalve.[1] | |
| H. subcomata | An abundant halobiid bivalve, namesake of theH. subcomata-D.bifurcata bivalve zone.[1] | |
| Krumbeckiella | K. sp. | A rarepergamidiid bivalve.[1] |
| Plagiostoma | P. sp. | A rarelimid bivalve.[1] |
| Brachiopods of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Crania? | sp. | A possiblecraniidbrachiopod.[1] |
| Similingula | S. cf.lipoldi | A rare brachiopod.[1] |
| Cephalopods of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Arctosirenites | A. canadensis | Atrachyceratidammonoid which occurs alongsideT. multituberculatum.[2] |
| A. columbianus | A trachyceratid ammonoid which occurs alongsideT. multituberculatum.[2] | |
| Austrotrachyceras | A. triadicum | A trachyceratid ammonoid which occurs alongsideT. multituberculatum.[2][5] |
| Buchites | B cf.aldrovandii | Abuchitid ammonoid which occurs in theS. cf. senticosus zone.[1][4] |
| Clionites | C. cf. zeilleri | Aclionitid ammonoid which occurs in theP. costulatum zone.[1][4] |
| Enoploceras? | E. sp. | A raretainoceratidnautiloid tentatively referred toEnoploceras.[2] |
| Guanlingoceras | G. guanlingensis | A trachyceratid ammonoid[9] |
| Hauerites | H. cf. himalayanus | A rare ammonoid which occurs in theT. multituberculatum zone.[1][4] |
| Paratrachyceras | P. cf. hoffmani | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1][4] |
| Protrachyceras | P. costulatum | A trachyceratid ammonoid, namesake of theP. costulatum zone[2][1][4] |
| P. deprati | A trachyceratid ammonoid which occurs in the oldest part of the Lower Member.[1][4] | |
| P. douvillei | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1][4] | |
| P. cf. douvillei | A trachyceratid ammonoid which occurs in theP. costulatum zone.[1][4] | |
| P. ladinum | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1] | |
| P. longiangense | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1] | |
| P. sp | An unnamed species of trachyceratid ammonoid which occurs in theS. cf. senticosus zone.[2][1][4] | |
| Simonyceras | S. simonyi | Aussuritid ammonoid.[3] |
| Sirenites | S. cf. senticosus | A trachyceratid ammonoid, namesake of theS. cf. senticosus zone.[1][4] |
| Sibyllites | S. cf. tenuispinosus | Atropitid ammonoid which occurs in theS. cf. senticosus zone.[1] |
| S. sp. | An unnamed species of tropitid ammonoid which occurs in theS. cf. senticosus zone.[1] | |
| Trachyceras | T. aonoides | A trachyceratid ammonoid, namesake of theT. aonoides zone.[3] |
| T. cf. aon | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1][4] | |
| T. multituberculatum | An abundant trachyceratid ammonoid, namesake of theT. multituberculatum zone.[2][1][4][5] | |
| T. sinensis | A trachyceratid ammonoid which occurs in theT. multituberculatum zone.[1] | |
| T. uraniae | A trachyceratid ammonoid which occurs alongsideT. multituberculatum.[2][5] | |
| T. sp. A | An unnamed species of trachyceratid ammonoid similar toProtrachyceras douvillei.[2] | |
| T. sp. B | An unnamed species of trachyceratid ammonoid similar toProtrachyceras deprati.[2] | |
| T. sp. | An unnamed species of trachyceratid ammonoid which occurs in theS. cf. senticosus zone.[1][4] | |
| Conodonts of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | |
| Metapolygnathus / Paragondolella / Quadralella | auriformis[5] | |
| carpathica[10] | ||
| foliata foliata[11][5] | ||
| foliata inclinata[11][5] | ||
| jiangyouensis[11] | ||
| maantangensis[11][10] | ||
| navicula navicula[11] | ||
| nodosus[1][4] | ||
| polygnathiformis[1][4] | ||
| prelindae[12] | ||
| robusta[10] | ||
| tadpole[11][5] | ||
| wayaoensis[13][12][14] | ||
| xinpuensis[13] | ||
| Echinoderms of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Calclamnidae | indet. | Holothurian (sea cucumber) ossicles.[1] |
| Osteocrinus | O.cf. spinosus | A free-swimmingroveacrinidcrinoid.[1] |
| O.cf. virgatus | A free-swimmingroveacrinidcrinoid.[1] | |
| Traumatocrinus | T. cf.lipoldi | An abundanttraumatocrinid crinoid.Pseudoplanktonic, forming colonies on floating driftwood.[1] |
| Fish of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Annulicorona | A. pyramidalis | Elasmobranch denticles[1] |
| Asialepidotus | A. sp. | Ahalecomorphholostean[1] |
| Birgeria | ||
| B. guizhouensis | A large predatorybirgeriid[4] | |
| B. sp. | A large predatory birgeriid[4] | |
| Colobodus | C. sp. | Acolobodontidperleidiform[4] |
| Guizhoucoelacanthus | G. largus | Awhiteiidcoelacanth[4] |
| Guizhoueugnathus | G. largus | Aeugnathid holostean[4] |
| Parvicorona | P. dacrysulca | Elasmobranch denticles[1] |
| Peltopleurus | P. brachycephalus | Apeltopleurid holostean[1][4] |
| Pholidopleurus | P. xiaowaensis | Apholidopleurid holostean[4] |
| Saurichthys | S. taotie | Asaurichthyidsaurichthyiform[15] |
| Reptiles of the Xiaowa Formation | ||
|---|---|---|
| Taxon | Species | Notes |
| Anshunsaurus | A. huangguoshuensis | A largeaskeptosauroidthalattosaur[4] |
| "Callawayia" | "C." wolonggangensis | Ashastasauridichthyosaur. Originally referred toCallawayia, but likely unrelated.[16] |
| Concavispina | C. biseridens | A largethalattosauroid thalattosaur[17] |
| Cyamodus | C. orientalis | Acyamodontidplacodont[18] |
"Cymbospondylus" | "C." asiaticus | A junior synonym ofGuizhouichthyosaurus tangae, unrelated toCymbospondylus[4] |
| Eorhynchochelys | E. sinensis | A large aquaticpantestudine (early turtle relative)[19] |
| Guanlingsaurus | G. liangae | A giant shastasaurid ichthyosaur[4] |
| Guizhouichthyosaurus | G. tangae | A common shastasaurid ichthyosaur[4] |
| Miodentosaurus | M. brevis | A large askeptosauroid thalattosaur[4] |
| Neosinasaurus | N. hoangi | A poorly-known marine reptile. Originally described as apachypleurosaur, though may be a thalattosaur instead.[20] |
| Odontochelys | O. semitestacea | An aquatic pantestudine[21][19] |
Panjiangsaurus | P. epicharis | A junior synonym ofGuizhouichthyosaurus tangae[4] |
| Psephochelys | P. polyosteoderma | Aplacochelyid placodont[4] |
| Qianichthyosaurus | Q. zhoui | A common smalltoretocnemid ichthyosaur[4] |
| Sinocyamodus | S. xinpuensis | Acyamodontoid placodont[4] |
Typicusichthyosaurus | T. tsaihuae | Likely a junior synonym ofGuanlingsaurus lingae[4] |
| Wayaosaurus | W. bellus | A poorly-known marine reptile. Originally described as a pachypleurosaur, though may be a thalattosaur instead.[20] |
| W. geei | A poorly-known marine reptile. Originally described as a pachypleurosaur, though may be a thalattosaur instead.[20] | |
| Xinpusaurus | X. bamaolinensis | A thalattosauroid thalattosaur,[4] possibly an invalid synonym of anotherXinpusaurus species[22][23] |
| X. kohi | A thalattosauroid thalattosaur[23] | |
| X. suni | A thalattosauroid thalattosaur[4] | |
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