UC Berkeley Paleobios

UC Berkeley PaleoBios

Title New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA

Permalink https://escholarship.org/uc/item/6dv597pp

Journal PaleoBios, 38(1)

ISSN 0031-0298

Authors Stoneburg, Brittney E. McDonald, Andrew T. Dooley, Jr., Alton C. et al.

Publication Date 2021

DOI 10.5070/P9381052265

Supplemental Material https://escholarship.org/uc/item/6dv597pp#supplemental

License https://creativecommons.org/licenses/by-nc-sa/4.0/ 4.0

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38:1–10, February 25, 2021 PaleoBios

OFFICIAL PUBLICATION OF THE UNIVERSITY OF CALIFORNIA MUSEUM OF PALEONTOLOGY

BRITTNEY E. STONEBURG, ANDREW T. MCDONALD, ALTON C. DOOLEY JR., ERIC SCOTT, & CHARLOTTE J. H. HOHMAN (2021). New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardi- no National Forest, San Bernardino County, California, USA.

Cover: A. Parahippus brevidens B. Ar- chaeohippus mourningi C. Scaphohippus sumani A selection of Miocene horse teeth figured in this paper. , WSC 8914 upper left M1. Citation: , WS 8826 upper right M3. , WSC 8922 partial right dentary, p4–m3. Scale bars=1 cm. Paleo- Bios Stoneburg, B.E., A.T. McDonald, A.C. Dooley Jr., E. Scott, and C.J.H. Hohman. 2021. New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA. , 38. ucmp_paleobios_52265. New remains of middle Miocene equids from the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA

BRITTNEY E. STONEBURG¹ , ANDREW T. MCDONALD¹, ALTON C. DOOLEY JR.¹, ERIC SCOTT², AND CHARLOTTE J.H. HOHMAN³ [email protected]; [email protected]; [email protected] ¹Western Science Center, 2345 Searl Parkway, Hemet, CA 92543, [email protected] ²Cogstone Resource Management, 1518 West Taft Avenue,[email protected] Orange, CA 92865 ³Montana State University, 100 Culbertson Hall, Bozeman, MT 59717

Archaeohippus Newmourningi material, Scaphohippus of three equids sumani is describedParahippus from the brevidens middle MioceneScaphohippus Cajon Valleyintermontanus Formation in San Ber- nardino National Forest, San BernardinoS. sumani County,Parahippus California. brevidens The material includes teeth of , and . Pa. brevidens is considered a junior subjective synonym of . is identified from an upperPa. molar brevidens that closely resembles the morphology of the holotype as well as referred specimens of from the Mascall Formation in Oregon and the Temblor Formation in California. The presence of in the Cajon Valley Formation represents a geographic range extension for the taxonHypohippus of over affinis 400 km. InterestingMegahippus ecological mckennai implications emerge for the Cajon Valley Formation when compared to the nearby Barstow Formation, including the presence of chalicotheres and apparent lack of and . Keywords: Parahippus brevidens, Scaphohippus sumani, Scaphohippus intermontanus, Archaeohippus mourningi

, Equidae, Cajon Pass

, INTRODUCTION North American Land Mammal Ages (NALMA) (Coombs The Cajon Valley Formation in the San Bernardino and Reynolds 2015 Loughney and Smiley 2019). Previ- National Forest has been excavated by multiple institu- ous investigations, of fossils from the Cajon Valley For- tions for over 40 years. Originally considered part of the mation reportedArchaeohippus multiple mourningi perissodactyls (Pagnac and Punchbowl Formation and located in the southwestern Reynolds1918 2010 LoughneyScaphohippus and sumaniSmiley 2019), including Mojave Desert province of southern California, the Ca- the equids (Merriam, 1913) Os- jon Valley Formation is subdivided into six units dating born ( ) and (Merriam, 1915) between approximately 18.0–12.7 Ma (Loughney and Pagnac (2006), as well as chalicotheres (Coombs and Smiley 2019, ). It is temporally overlapping, but distinct Reynolds 2015). Specimens collected by Western Science from the adjacent Crowder Formation (Woodburne and Center (WSC) personnel on six separate expeditions in Golz 1972 Morton and Miller 2003 Reynolds et al. 2008, 2018, 2019, and 2020, represent three morphologically Coombs and Reynolds 2015, Loughney and Smiley 2019) distinct middle Miocene equid species. The WSC mate- (Fig. 1). This paper follows Morton and Miller (2003) rial consists primarily of teeth with scattered postcranial and uses the name Cajon Valley Formation for these six fragments. TheseAr. mourningi teeth not onlyS. indicatesumani the presence of units that were previously referred to as the Punchbowl equids previouslyParahippus known brevidens from the Cajon Valley Forma- Formation. tion, such as and , but also the first Specimens found in the Cajon Valley Formation gener- description of (Marsh, 1874) Gidley *allyauthor span for the correspondence late Hemingfordian to middle Barstovian (1907) from the formation. These fossils provide further insight into the understudied Cajon Valley Formation, Citation: as well as opportunities for comparison to the fauna of PaleoBios, Stoneburg, B.E., A.T. McDonald, A.C. Dooley Jr., E. Scott, and C.J.H. Hohman. 2021. New remains of middle Miocene Permalinkequids from: the Cajon Valley Formation, San Bernardino National Forest, San Bernardino County, California, USA. Copyright:38. ucmp_paleobios_52265 . https://escholarship.org/uc/item/6dv597pp Published under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC-BY-NC-SA) license. 2 PALEOBIOS, VOLUME 38, FEBRUARY 2021

conglomeratic sandstone, siltstone, and minor occurrences of lignite, limestone, and carbonaceous shale (Woodburne and Goltz 1972). Digital 3D models of select specimens were created at the WSC through laser scanning with a NextEngine 3D scanner. Resultant scans were processed in Meshmixer. The 3D models are available for viewing and download on Sketchfab (https://sketchfab.com/WesternScience- Center/collections/miocene-horses1984 ). p P All equid dental nomenclature used dpin this paper DPfollows MacFadden ( ): lowercase /uppercasem =lower and M upper premolar; lowercase /uppercase =lower and upper deciduous premolar; lowercase / uppercase =lower and upper molar. SpecimensLACM and images of Miocene equid teeth from the following institutions were RAMused for comparative purposes: , Natural History Museum of Los AngelesSBCM, County, Los Angeles, California; , Raymond M. Alf UCMPMuseum of Paleontology, Claremont, California; San Bernardino County Museum,YPM Redlands, California; , University of California Museum of Paleontology, Berkeley,A California; and , Yale Peabody Museum, New Haven, Connecticut. Figure 1. A table of identifications, anatomical descriptions, and . Geology of the Cajon Pass area; image taken from measurements for all equid teeth in the WSC collections the United States Geological Survey geological map of the San from the Cajon Valley Formation can be found in Supple- Bernardino Quadrangle (MortonB. and Miller 2003). Divisions mentary Materials 1. of the Cajon Valley Formation listed as Tcv1 through Tcv6. WSC Locality No. 381 located in Tcv5. Photograph of the locality at which the Western Science Center collected the equid speci- SYSTEMATIC PALEONTOLOGY mens described herein. The outcrop is composed of conglomeratic sandstone. Places at which equid teeth or mammalian bone MAMMALIA Linnaeus, 1758 fragments were collected are highlighted by red circles. For EQUIDAE Gray, 1821 scale, two rock hammers (~ 33.0 cm long) are leaning upright Archaeohippus mourningi against the outcrop near the bottom of the photograph. ParahippusARCHAEOHIPPUSmourningi Gidley, 1906 Archaeohippus mourningi (Merriam, 1913) Osborn, 1918 (?) Merriam, 1913 Osborn, 1918 surrounding areas, such as the nearby and partially co- Referred specimens Fig. 2 eval Barstow Formation, which is approximately 19–13 Ma (Loughney and Smiley 2019). —WSC 8801 lower left p2; WSC 8816 upper left DP4; WSC 8826 upper right M3; WSC 8911 MATERIALS AND METHODS wornOccurrence upper tooth;— WSC 8912 partial dentary; WSC 8913 We collected over a dozen equid teeth over the course worn upper tooth; WSC 9969 upper DP?. of six collection trips from January 2018 to January 2020. Cajon Pass, San Bernardino National For- These are housed at the Western Science Center (WSC) in est, San Bernardino County, California. Exact ,locality data Hemet, California and were collected under United States are on file at the USFS and WSC. Subdivision Tcv5, Cajon Forest Service (USFS) permits R5-SBNF-MGM-FY18-001in situ Valley Formation (Woodburne and Golz 1972 Morton and and R5-SBNF-MGM-FY19-001. The teeth were collected Miller 2003); middle Miocene, late Hemingfordian–middle from the ground surface as loose float and from BarstovianDescription NALMA; Tcv5 (unit 5) spans approximately WSC Locality No. 381, a conglomerate sandstone bed 16.5–14 Ma (Liu 1990). within unit 5 of the Cajon Valley Formation (Morton and —Several teeth from a small brachydont Miller 2003) (Fig. 1). This formation consists of beds of horse were collected by WSC personnel from the Cajon STONEBURG ET AL.—MIOCENE EQUIDS FROM THE CAJON VALLEY FORMATION, CALIFORNIA, USA

Figure 2 Archaeohippus mourningi A. B. C. . Teeth of from the Cajon Valley Formation, San Bernardino County, CA. Lower left p2 of WSC 8801. Upper left DP4 of WSC 8816. Upper right M3 of WSC 8826. Scale bar=1 cm. Description

Valley Formation, in subdivision Tcv5. These teeth, based —WSC 8933 and WSC 8934 exhibitScaphohip fewer- Archaeohippusupon their small size, lack of cement, and absence of a pusthan ten plications and moderately oval-shaped isolated well-developed crochet, are best referred to the genus protocones, supporting referral to the genus (Pagnac 2005) (Fig.Ar. mourningi2). WSC 8826, the (Pagnac 2006). Furthermore, metric dataScaphohippus (Supple- only adult molar in the WSC sample, lacks a crochet mental Material 1) indicate that the occlusal surfacesAcritohip of- completely and so is referred to (Pagnac pusall referred teeth are consistent in size with 2005). BecauseAr. mourningi of size and brachydont condition, all rather than the larger penecontemporaneous other referred specimensAr. mourningi in the WSC sample are also (Kelly 1995), with its larger occlusal surface (Pagnac referred to . Pagnac and Reynolds (2010) 2006: figure 3). Additional cranial and dental diagnostic also described material from the CajonPara- features described by Pagnac (2006) cannot be assessed Valleyhippus Formation,mourningi including a juvenile skull from Tcv5. toat thisScaphohippus time. MerriamArchaeohippus (1913) originally named1918 this species as Having demonstratedScaphohippus that our sample intermontanus is best referredS. (?) , which was then transferred to the sumani , the question of which speciesMerychip it is be- genus by Osborn ( ). puscomes paramount. and Scaphohippus sumani Scaphohippus were originally referred to the genus MerychippusSCAPHOHIPPUS sumani Pagnac, 2006 by Merriam (1915) before being referred to the new (Merriam 1915) Pagnac 2006 Merychippus intermontanus genus by Pagnac (2006), who defined theS. Scaphohippus sumaniMerriam, 1915 intermontanustwo species by differences in tooth morphology and Scaphohippus intermontanusMerriam, 1915 Pagnac, 2006 temporal and geographic range. In terms of range, Pagnac, 2006 is restricted to the Barstow Formation Referred specimensFigs. 3, 4 of Southern California, stratigraphicallyScaphohippus located sumani in the is informal faunal subdivisions Barstow Fauna and Second —WSC 8922 partial right den- Division Fauna (Pagnac 2006). tary, p4–m3; WSC 8933 upper right DP3; WSC 8934 up- assigned a much larger range; found throughout Califor- perOccurrence left M2; WSC— 9618 highly worn upper left DP2; WSC nia, Nebraska, New Mexico, and Colorado, it is also found 9967 lower left p4. in the Barstow Formation, in the Barstow Fauna and Cajon Pass, San Bernardino National For- Second Division (Pagnac 2006). This resultsS. intermontanus in the two est, San Bernardino County, California. Exact locality data, species overlapping stratigraphically and geographically. are on file at the U.S. Forest Service and WSC. Subdivision According to previous descriptions, S. sumani Tcv5, Cajon Valley Formation (Woodburne and Golz 1972 dentition exhibits “higher crown height and simpler Morton and Miller 2003); middle Miocene, late Heming- enamel complexity” (Pagnac 2006: p. 49) than . fordian–middle Barstovian NALMA; Tcv5 (unit 5) spans Merriam (1915) in part used complexity of plications approximately 16.5–14 Ma (Liu 1990). (“enamel bordering fossettes,” the phrase then used by 4 PALEOBIOS, VOLUME 38, FEBRUARY 2021

Figure 3 Scaphohippus sumani A–C D–E A. B. C. D. . Teeth of E. , comparing rows from the Cajon Valley ( ) and Barstow ( ) formations of San Ber- nardino County, CA. WSC 8922 partial right dentary, p4–m3. WSC 8934 upper left M3?. WSC 8933 upper right DP3. LACM 4941 upper left P4–M3. LACM 33847 upper left P4–M3. Scale bar=2 cm. Numbers above each tooth indicate number of inner plications. STONEBURG ET AL.—MIOCENE EQUIDS FROM THE CAJON VALLEY FORMATION, CALIFORNIA, USA

Figure 4 Scaphohippus sumani A. S. sumani B. S. intermontanus C. S. intermontanus . Teeth of . Holotype of , showing P4–M3 from right to left, UCMP 21422. Holotype of showing P2–M3 from right to left, UCMP 21400. Referred specimen of palate (Pagnac 2006) showing P1–M3 from right to left, UCMP 316891. Scale bar=5 cm. Numbers above each tooth indicate number of inner plications. of S. sumani S. sumani S. intermontanus S. intermontanus Merriam to describe plications) in order to distinguishin (UCMP 21422; Fig. 4A, C). In the holotype of S. intermontanus from . Pagnac (2006) quantiS.- (UCMP 21400), P4 has four plications. sumanified this, stating that the average number of plications Given the considerable overlap in the number of plica- is two, with a maximum of four; for tions observed between the two species, we regard the both S. sumani, the averageS. number intermontanus is three with a maximum of difference between four and five plications as having no seven. However, complex inner plications can be found in taxonomic significance. Although this feature might be and , depending onS. intertooth- a species-level difference, it might also result from other montanusposition and wear state (Figs. 3, 4). For example, in the sources of variation, such as stratigraphic placement right tooth row of UCMP 316891, assigned to of specimens, population variation, and taphonomic , P4 displays five plications, as in the holotype artifacts. 6 PALEOBIOS, VOLUME 38, FEBRUARY 2021

Anchippus brevidens Parahippus brevidens Specimens of both species display an isolated pro- Marsh, 1874 tocone on the upper molars that connects to the pro- Referred specimens Gidley, 1907 toconule with increased wear; however, only a 5–10% Fig. 5 difference in wear (with an upper limit of 30% of wear) Occurrence— —WSC 8914 upper left M1; WSC is used to characterize this particular morphological 8918 lower right p3; WSC 9968 highly worn lower molar. difference between the dentitions of the two species Cajon Pass, San Bernardino National For- (Pagnac 2006). It is possible that other factors, such as est, San Bernardino County, California. Exact locality data, stratigraphic, ecological, or population differences could are on file at the U.S. Forest Service and WSC. Subdivision account for this minor dissimilarity in the appearance of Tcv5, Cajon Valley Formation (Woodburne and Golz 1972 certain occlusal surface features, rather than it being a Morton and Miller 2003); middle Miocene, late Heming- defining feature of two separate taxa. fordian–middleDescription Barstovian NALMA; Tcv5 (unit 5) spansAr. These observations from both species lead us to con- mourningiapproximately 16.5–14 Ma (LiuS. sumani 1990). clude that previously proposed differences in enamelS. —A brachydont horse larger than intermontanuscomplexity and to wear S. sumani states are not sufficient to distin- but smaller than is represented by guish between the two taxa. We refer all specimens of several specimens that include a well-preserved upper , which has taxonomic priority left M1 (WSC 8914), a lower right p3 (WSC 8918),Parahip and- (Merriam 1915). pusa worn lower molarOsborn or 1918 premolar (WSC 9968) (Fig. 5). Parahippus brevidens WSC 8914 exhibits two characters of the genus PARAHIPPUS Leidy, 1858 (following : p. 74): 1) “protocone and (Marsh 1874) Gidley, 1907 hypocone relatively large as compared with the simple

Figure 5 Parahippus brevidens A–C D E A. B. WSC . Teeth of C. from theParahippus Cajon Valley Formation,D. San Bernardino County, California ( ); the E.Mascall Formation, Grant County, Oregon ( ); and Temblor Formation, Fresno County, California ( ). WSC 8914 upper left M1. 8918 lower right p3. WSC 9968 highly worn lower tooth. YPM 11274 (holotype), upper left M1 or P4. LACM 1146 upper M?. Scale bar=1 cm. STONEBURG ET AL.—MIOCENE EQUIDS FROM THE CAJON VALLEY FORMATION, CALIFORNIA, USA

7 Osborn 1918 Protohippus avus Anchippus lophoid protoconule and metaconule”; and 2) “hypostyle brevidens( ). prominent, subtriangular” (Figs. 5, 6). According to the Marsh (1874) named andAnchippus , UCMP online database (https://ucmp.berkeley.edu/col- An. texanus, both from the MioceneParahippus of Oregon. However, lections/databases/Parahippus), material in the UCMP collection Gidley (1907)Anchippus transferred the type species of from the Cajon Valley Formation has been previously tion Parahippus (Leidy brevidens 1868), into , thus subsum- Parahippusidentified as by Michael Woodburne. How- ingProtohippus the genus avus into Parahippus and creating the new combina- ever, none of these teeth have been formally described as Osborn 1918, . He also placed the species . All Cajon Valley Formation specimens listed “ Pa.” brevidens , which later Pa.authors avus in the UCMP online database are, as of this publication,Pa. brevi- ( Bode 1933) continued. Downs (1956) currentlydens under the listing “Punchbowl Formation.” argued that was synonymous with . WSC 8914 bears closest resemblance to Osborn However, he based this on nomenclatural arguments, 1918. It exhibits complex plications on bothProtohippus sides of the and did not justify the synonymy withPa. morphological brevidens, we avusmetaloph (“metaloph ptychoid on both sides” Parahip[ - characters. Parahippus pus : p. 90]) (Figs. 5, Desmatippus6) in contrast with “ ” AnchippusAlthough we refer our specimens to Al (Marsh 1874) (later assigned to the genusD. crenidens acknowledge that the taxonomy of both and [Gidley 1907] or [MacFadden 1998]) is in need of revision (see discussion by - whichOsborn lacks these1918 plications entirely, and bright 1999) but is beyond the scope of the present work. (Scott 1893), which exhibits them only on the mesial side ( Osborn 1918). The protoconule does not align with DISCUSSION the protocone, resulting in the curvature of the mesial The differences among the horse teeth discussed in edge ( Osborn 1918). The metaconule is isolated, and this paper highlight the diverse ecology represented resembles an “ear-shaped lobe” with interior cement in sediments of theAr. Cajon mourningi Valley Formation (Fig. 7). with wear ( ). The tooth lacks a median ridge Pagnac and Reynolds (2010) previously discussed the between the metacone and the paracone, and the meta- high prevalence of Scaphohippus from sumani the Cajon Valley loph connects with the hypocone, but not the hypostyle Formation versus the coevalAcritohippus Barstow Formation, where it is comparatively rare. Pa. is brevidens present in both formations, but does not occur in the Cajon Valley Formation. The confirmation of in the Cajon Valley Formation extends the geographic range of this species southward by 400 km; the ,nearest previously described occurrence is in the Temblor For- mation north of Coalinga, California (Bode 1933 1935), at approximately 19–15 Ma (Bridges and Castle 2003) with the holotype described from the Mascall Formation of Oregon (Marsh 1874), at approximately 16–14.8 Ma of(Maquire Pa. brevidens et al. 2018) (Fig. 8). The ages of these Parathree- hippusformations brevidens are consistent with the known occurrences being roughly contemporaneous. is not found in the Barstow Formation (Pagnac 2005), again highlighting the differences in fauna between the two formations. There are numerous potential causes for these differences in faunal dispersal between these formations, which are less than 100 km from each other, and this is worth further research. In addition, collection biases could potentially affectPa. brevi our- densknowledge of California Miocene equids among these Figure 6. A. Parahippus brevidens various formations; it is entirely possible that B. might be discovered in Miocene deposits throughout Scaphohippus Upper molar sumani of labeled with anatomical features based upon WSC 8914. Upper the California coast. molar of labeled with anatomical fea- The presence of three small- to mid-sized equids tures based upon WSC 8934. and a distinct, consistent lack of larger equids such as 8 PALEOBIOS, VOLUME 38, FEBRUARY 2021

Figure 7

. A faunal comparison of horses and chalicotheres of the Cajon Valley Formation and the Barstow Formation. Chalicothere silhouette from PhyloPic, created by Zimices; license: https://creativecommons.org/licenses/by-nc/3.0/. Horse silhouette from PhyloPic, created by Heinrich Harder and vectorized by T. Michael Keesey; license: https://creativecommons.org/publicdomain/ mark/1.0/. STONEBURG ET AL.—MIOCENE EQUIDS FROM THE CAJON VALLEY FORMATION, CALIFORNIA, USA 9

Pa. brevidens

presence of suggest that the Cajon Valley Formation may have more in common with the Temblor and Mascall formations, apart from the presence of chalicotheres, than the geographically much closer Barstow Formation, which as previously noted is just under 100 km away. If this is not due to a temporal difference between our locality in the Cajon Valley Formation and the Barstow Formation,Pa. this brevidens could point toAr. an mourningi environmental in the disparity, perhaps with a similar factor contributing to the abundance of and Cajon Valley, Temblor, and Mascall formations. Further studies on the paleoflora and comparative isotope analysis might reveal more about the ecological nature of these formations.

ACKNOWLEDGEMENTS All collecting was conducted under USFS R5-SBNF- MGM-FY18-001 and USFS R5-SBNF-MGM-FY19-001. We thank Daniel Grijalva and Jay Marshall of the USFS for their help. We thank Sam McLeod and Vanessa Rhue for access to LACM; Gabriel-Phillip Santos for access to RAM collections; Crystal Cortez for access to the SBCM collections; and Darla Radford for assistance curating the WSC collection. Thanks are also extended to Alyssa Arre and Daniel Brinkman for photos of YPM specimens. Thank you to Eric Holt and Pat Holroyd for photos of Figure 8. Parahippus brevidens UCMP specimens. Thank you to WSC staff members and volunteers Leya Collins, John DeLeon, Joe Reavis and Published occurrences of Brett Dooley for excavation assistance and fossil prepa- in Oregon and California, now including the material from the Cajon Valley Formation. Horse silhouette from PhyloPic, ration. We also extend a special thank you to Kathleen created by Jay Matternes and vectorized by Zimices, with no Springer for her help in the genesis of this project. We changes except the color; license: https://creativecommons. thank the editors Peter Kloess and Diane M. Erwin and org/licenses/by/3.0/. Hypohippus affinis Megahippus mcken- three anonymous reviewers for their comments. All spe- nai cies outlines are from PhyloPic.org. (Leidy 1858) and (Tedford and Alf 1962) in the Cajon Valley Formation LITERATURE CITED may be due to sampling bias, but could also point to AlbrightBulletin III, of L. the Barry. Florida 1999. Museum Ungulates of Natural of the History Toledo Bend local interesting ecological comparisons with the otherwise fauna (late Arikareean, early Miocene), TexasMerychippus coastal plain. similar faunal list of the nearby Barstow FormationMoropus (Fig. Carnegie 42:1–80. Institution 7). Three specimens of chalicotheres have been describedPa. Bode,of Washington F.D. 1933. Anchitheriine Publication horses from the Zone brevidensin the Cajon Valley Formation, all referred to of the North Coalinga District, MerychippusCalifornia. sp. Marsh, 1877 (CoombsParahippus and Reynolds brevidens 2015). Like Carnegie 440:43–58. Institution of Washington , chalicotheres are absent in the Barstow For- Bode,Publication F.D. 1935. The fauna of the Zone, North Coal- inga District, California. mation (Pagnac 2005). has been , 453:66–96. previously found in units that lack chalicotheres includ- Bridges, R.A. and Castle, J.W. 2003. Local and regional tectonic con- ing the Mascall and Temblor formations (Bode 1935 trol onSedimentary sedimentology Geology and stratigraphy in a strike-slip basin: Downs 1956). However, in the Cajon ValleyPa. brevidens Formation on Miocene Temblor Formation of the Coalinga area, California, they co-occur. The Cajon Valley Formation,Hypohippus in contrast USA, 158:271–297. Megahippusto other described localities containingAr. mourningi Coombs, M.C., and R.E. Reynolds. 2015. Chalicothere material the Pacific Coast, lacks the larger equids and (Perissodactyla, Chalicotheriidae, Schizotheriinae) from late . The abundance of and the Hemingfordian and early Barstovian faunas of the Cajon Valley PALEOBIOS, VOLUME 38, FEBRUARY 2021

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