Arroyo Formation
Stratigraphic range:Kungurian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Formation
Unit of	Clear Fork Group
Underlies	Vale Formation
Overlies	Waggoner Ranch Formation
Thickness	250 ft (76 m)
Lithology
Primary	Limestone,mudstone
Other	Gypsum
Location
Coordinates	33°48′N99°12′W / 33.8°N 99.2°W /33.8; -99.2
Approximate paleocoordinates	0°24′N29°36′W / 0.4°N 29.6°W /0.4; -29.6
Region	Texas
Country	United States
Type section
Named for	Los Arroyos,Runnels County, Texas
Named by	Beede & Waithe
Year defined	1918
Arroyo Formation (the United States) Show map of the United States Arroyo Formation (Texas) Show map of Texas

TheArroyo Formation, sometimes termed theLower Clear Fork Formation, is ageologic formation inTexas.^[1] It preservesfossils dating back to theKungurian stage of thePermianperiod.^[2] It is the lower-most portion of theClear Fork Group, part of a series of fossiliferous Permian strata in the south-central United States known as thered beds.

The Arroyo Formation is the oldest and most eastern component of the Clear Fork Group. It extends in a Northeasterly direction from Concho County up as far north as Wilbarger County. North of the Red River in Oklahoma, the equivalent formation is the upperGarber Formation and cave deposits ofRichards Spur (formerly Fort Sill).^[3][4]

The Arroyo Formation was first named by Beede and Waite (1918).^[5] Thetype locality was a series of marinelimestone,shale, andgypsum deposits cropping out at Los Arroyos (formerly Los Arroyo), a specificarroyo inRunnels County a few miles west ofBallinger. Wrather (1917) observed the same geological sequence inTaylor County, although he named it theAbilene Formation, which was a preoccupied name rejected by Beede and Waite (1918). At first, Beede and Waite (1918) tentatively placed the Arroyo Formation in theWichita (or Albany) Stage/Group, rather than the overlying Clear Fork Stage/Group.^[5] Subsequent authors, starting with Sellars (1932), have considered the Arroyo Formation the basal part of the Clear Fork Group.^[6] Olson (1989) called the portion of the Arroyo Formation below theSalt Fork of the Brazos River the "Southern area", contrasting it with the extensively studied "Classic area" further north.^[7]

"Southern area" deposits found north of Runnels County preserve a series of terrestrialmudstone layers interspersed with several distinct intervals of marinelimestone, four of which having been named. These four limestone intervals are, from stratigraphically lowest to highest, the Rainey, Lytle, Kirby Lake, and Standpipe Limestones. The composition and appearance of these limestones can be variable, with some localities having fine-grained layers and others being practically marine conglomerate. These four limestone layers do not persist in the entirety of the "Southern Area", with the Standpipe Limestone terminating just north ofAbilene. This makes it difficult to distinguish the boundary between the Arroyo andVale Formation north of Taylor County, where it occurs shortly after the top of the Standpipe Limestone.^[7]

The limestone layers are often fossiliferous, preserving fossils from both marine organisms and reworked inland fauna. A diverse assortment ofpalaeoniscoid fish teeth and scales andbivalve shells are by far the most abundant fossils. Worm burrows and ostracods are also very common. Tetrapod remains includeDimetrodon teeth, unusually small "Lysorophus" (Brachydectes) vertebrae, skull fragments from smallDiplocaulus andTrimerorhachis, a fewEryops components, and fragments from variousmicrosaurs, possibly referable toPantylus,Microbrachis, and/orPelodosotis. Most of the bones belong to small animals, likely due to taphonomic bias due to ease of transportation. This is also a possible reason as for why found teeth from the freshwaterlungfishGnathorhiza, are much smaller and rarer than those from terrestrial deposits.Captorhinus, a common small animal in the "Classic area", is curiously absent. Shark remains include teeth fromOrthacanthus andhybodonts. Marineinvertebrates mostly belonged to shallow water or estuary niches, apart from a single poorly preservednautiloid which likely drifted in from more open waters.^[7]

Fossils from terrestrial deposits (like streamconglomerate or red mudstone) are rare in the southern area, found at only a handful of sites inHaskell County and southwestern Baylor County. These fossils are probably from a later interval of the Arroyo Formation, a segment which would lie between the Kirby Lake and Standpipe Limestone layers further south. The fauna is similar to that of the "Classic area", withDiplocaulus,Dimetrodon, andOrthacanthus fossils being the most common andEryops andDiadectes also known, albeit from much more fragmentary remains.^[7]

Between the Salt Fork Brazos andRed River, the Arroyo Formation preserves a terrestrial floodplain ecosystem found in outcrops in Baylor, Wilbarger, and Wichita counties. This area, which Olson (1989) termed the "Classic area", has been extensively studied for its Permian plant and animal fossils.^[7] The portions of the Arroyo Formation exposed in this area are among the oxidized Permian sediments collectively termed theRed Beds of Texas and Oklahoma. They are represented by Redclays,shales, andmudstones, with occasional layers ofsandstone orconglomerate,^[8] as well as scatteredcarbonate nodules which become increasingly more common in younger levels. Some of the sandstone layers have been given informal names, such as the Red Tank and Brushy Creek Sandstone Members.^[9][10] Limestone layers are rare relative to the "southern area", and the diagnostic Taylor County layers are seemingly completely absent north of the Salt Fork,^[7] although a dolomite layer possibly equivalent to the Rainey or Lytle limestones has been reported.^[9][10] This dolomite layer, informally named the Craddock dolomite, has been known to preserve plant impressions as well asarthropod andtetrapod footprints.^[11]

With the absence of the characteristic Standpipe Limestone of the "southern area", Arroyo red beds in the "classic area" are difficult to differentiate from the overlying Vale Formation on a purely geological basis. A similar issue obscures the boundary between the Vale and Choza Formations, with the absence of the BullwagonDolomite which separates the two further south. As a result,sedimentologists generally do not distinguish the component formations of the Clear Fork Group, and simply prefer to call these northern red beds theClear Fork Formation.^[9] Under this system, the Arroyo Formation would be roughly equivalent to the informal"Lower Clear Fork Formation".^[10]

Olson (1958) attempted to provide an informal geological boundary between the Arroyo and Vale based on the presence of an even red shale or mudstone layer. This layer is found along a northeastern-oriented line that bisected the Clear Fork area in the western part of Baylor County. This even layer, he argued, was formed by slow, brackish streams in a lowland delta close to sea level. Therefore, their presence may have been a result of the same marine transgression responsible for the Standpipe Limestone further south. Under this hypothesis, the Vale Formation could be found in stratigraphically higher areas west of the line, and the Arroyo formation would be east of the line. Nevertheless, Olson admitted that this boundary was imprecise due to the variable depth of the shales and the varying topography of the surrounding landscape.^[12]

The "classic area" of the Arroyo Formation is one of the most fossiliferous parts of the Texas Red Beds, and it is typically differentiated from surrounding formations bypaleontologists on the basis of faunal differences.^[8][12][13] A large number of sites are known bearing either abundant plant or animal remains. The animal-bearing sites are among the most diverseEarly Permian tetrapod assemblages in the world, with numerous remains ofamphibians,pelycosaurs (mammal relatives),chondrichthyans (sharks), andeureptiles. The last and largest known species ofedaphosaurid,Edaphosaurus pogonias, is known from the Arroyo, with the family going extinct soon afterwards. Other pelycosaurs, includingSecodontosaurus,Varanosaurus, and several species ofDimetrodon, were abundant, though the abundance ofDimetrodon was retained in the Vale and Choza Formations.Captorhinids were the most common eureptiles in the Arroyo, represented by basal taxa such asCaptorhinus andLabidosaurus.^[13][7] Captorhinids experienced a taxonomic turnover at the beginning of the Vale Formation, as advanced taxa likeLabidosaurikos andCaptorhinikos replace or evolve from the more primitive captorhinids soon after the red shale boundary between the two formations.^[12] The first specimens of the gracile eureptileAraeoscelis were discovered at the Craddock Bonebed, one of the most productive Arroyo Formation sites in Baylor County. Aquatic amphibians likeDiplocaulus,Trimerorhachis, andEryops are common. Terrestrial amphibians likeSeymouria,Diadectes,microsaurs, and variousdissorophoids (Acheloma,Broiliellus,Aspidosaurus, etc.) were present as well. Many of these terrestrial amphibians did not survive into the Vale Formation.^[8][7] Burrows containingaestivatingBrachydectes,Gnathorhiza, andDiplocaulus are common in the middle part of the Arroyo Formation, likely indicating a period with a drier climate than the early or late Arroyo. The most common shark remains belong toOrthacanthus platypternus, although teeth fromXenacanthus luederensis are also known from some early Arroyo sites.^[13]

Genus	Species	Notes	Images
Brachydectes	B. elongatus	Alysorophian common in some sites. Historically referred to Lysorophus tricarinatus.[14]
Cardiocephalus	C. sternbergi	Agymnarthridmicrosaur[15]
Crossotelos	C. annulatus	Aurocordylidnectridean.[16]
Diplocaulus	D. brevirostris	Rare[17]
	D. copei	Indeterminate, likely a junior synonym ofD. magnicornis[17]
	D. limbatus	Junior synonym ofD. magnicornis[17]
	D. magnicornis	An abundantdiplocaulidnectridean[17]
	D. primigenius	May be synonymous withD. magnicornis[17]
Euryodus	E. primus	Agymnarthridmicrosaur[15]
Goniocara	G. willistoni	An indeterminategymnarthridmicrosaur known from skull fragments. Initially namedGoniocephalus.[15]
Gymnarthrus	G. willoughbyi	Junior synonym ofCardiocephalus sternbergi.[15]
Micraroter	M. erythrogeois	Anostodolepidmicrosaur which may have been present based on BPI 3839, a well-preserved skull and skeleton potentially referable to the genus.[18]
Ostodolepis	O. brevispinatus	Anostodolepidmicrosaur[15][18]
Pantylus	P. coicodus	A fragmentarymicrosaur. May be synonymous withPantylus cordatus, or completely unrelated.[15]
Pelodosotis	P. elongatum	Anostodolepidmicrosaur[18]
Permoplatyops	P. parvus	Also known asDiplocaulus pusillus andPlatyops parvus. Junior synonym ofDiplocaulus magnicornis.[17]
Quasicaecilia	Q. texana	Abrachystelechidmicrosaur. Type locality unknown, may be from the Arroyo Formation.[19]

Genus	Species	Notes	Images
Acheloma	A. cumminsi	Atrematopid
	A. willistoni
	A. casei	Junior synonym ofIsodectes obtusus[20]
Alegeinosaurus	A. aphthitos	Junior synonym ofAspidosaurus chiton[21]
Aspidosaurus	A. chiton	Adissorophid
	A. peltatus	Junior synonym ofDissorophus multicinctus
Broiliellus	B. arroyoensis	Adissorophid[22]
Cacops	C. aspidephorus	Adissorophid
Dasyceps	D. micropthalmus	Azatrachydid[23]
Dissorophus	D. multicinctus	Adissorophid
Eobrachyops	E. townendae	Junior synonym ofIsodectes obtusus[20]
Eryops	E. megacephalus	A commoneryopid
Isodectes	I. obtusus	Advinosaur[20]
	I. megalops	Junior synonym ofIsodectes obtusus[20]
Kermitops[24]	K. gratus[24]	Anamphibamiform[24]
Nanobamus	N. macrorhinus	Anamphibamiform[25]
Trimerorhachis	T. insignis	A commondvinosaur[26]
	T. mesops	Rare[26]
	T. conangulus	Junior synonym ofIsodectes obtusus[20]
	T. greggi	May be descended fromT. insignis[26]

Genus	Species	Notes	Images
Desmospondylus	D. anomalus	Junior synonym ofSeymouria baylorensis
Seymouria	S. baylorensis

Genus	Species	Notes	Images
Diadectes	D. tenuitectens	Adiadectid

Genus	Species	Notes	Images
Casea	C. broilii	Acaseid
Dimetrodon	D. giganhomogenes	Asphenacodontid
	D. grandis
	D. kempae	Dubious
	D. loomisi	Asphenacodontid
Edaphosaurus	E. pogonias	Anedaphosaurid
Secodontosaurus	S. obtusidens	Asphenacodontid
Tetraceratops	T. insignis	An advancedpelycosaur or earlytherapsid
Trichasaurus	T. texensis	Acaseid
Varanops	V. brevirostris	Avaranopid
Varanosaurus	V. acutirostris	Anophiacodontid

Genus	Species	Notes	Images
Araeoscelis	A. grandis	Anaraeoscelidiandiapsid
Captorhinus	C. aguti	Acaptorhinid
Ectocynodon	E. incisivus	Junior synonym ofCaptorhinus aguti. Has also been referred to the microsaurPariotichus and the captorhinidLabidosaurus.
Labidosaurus	A. hamatus	Acaptorhinid

Apart from sharks and lungfish, a large and well-preservedactinopterygian fish,Brachydegma caelatum, is known from the lower Clear Fork Formation.^[27]

• List of fossiliferous stratigraphic units in Texas
• Paleontology in Texas

• Permian System of North America
• Permian geology of Texas
• Kungurian
• Sandstone formations of the United States
• Shale formations of the United States
• Fluvial deposits
• Lacustrine deposits
• Permian northern paleotropical deposits
• Paleontology in Texas

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