The first postcranial oreodont (mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance

Journal Pre-proof The first postcranial oreodont (mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance Ismael Ferrusquía-Villafranca, José E. Ruiz-González PII:

S0895-9811(19)30435-3

DOI:

https://doi.org/10.1016/j.jsames.2019.102388

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SAMES 102388

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Journal of South American Earth Sciences

Received Date: 21 August 2019 Revised Date:

9 October 2019

Accepted Date: 10 October 2019

Please cite this article as: Ferrusquía-Villafranca, I., Ruiz-González, José.E., The first postcranial oreodont (mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance, Journal of South American Earth Sciences (2019), doi: https:// doi.org/10.1016/j.jsames.2019.102388. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

The first postcranial oreodont (Mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance Ismael Ferrusquía-Villafranca*, José E. Ruiz-González

Instituto de Geología, Universidad Nacional Autónoma de México, Circuito de la Investigación S/N, Coyoacán, Ciudad de México, C.P. 04510, México *

Corresponding author.

E-mail addresses: [email protected] (I. Ferrusquía-Villafranca), [email protected] (J.E. RuizGonzález).

ABSTRACT

In this study, the first postcranial skeleton of an oreodont (Merycoidodontidae, Artiodactyla) found in Mexico is described and discussed. This family was very successful, quasi endemic of North America, and lived from the Late Duchesnean North American Land Mammal Age, acronymed NALMA (Late Eocene) to the Late Hemphillian NALMA (early Late Miocene), roughly between 38 Ma to 5.2 Ma. Oreodonts were diverse (~27 genera, 69 species), and fairly abundant. However their record in Mexico is modest: two genera in northeastern Chihuahua (Rancho Gaitán local fauna, Late Duchesnean), one genus in Aguascalientes (Zoyatal local fauna, early Hemingfordian), one genus in northwestern Oaxaca (Yolomécatl local fauna, latest Uintanearliest Duchesnean) and one genus in central Oaxaca (Suchilquitongo local fauna, latest Arikareean-Early Hemingfordian). Therefore, the finding in Tehuitzingo, southern Puebla of an articulated postcranial skeleton referred to the Merycoidodontidae, described below, is quite important and significant. The fossil material was recovered from the Tehuitzingo Formation, which largely fills the namesake basin; such geologic unit is a thick lacustrine-fluvial, fine-grained, indurated sequence that includes strata of silicified limestone and siltstone/sandstone interbedded by tuff/ignimbrite sheets, one of them yielded a K-A date of 29.9 ± 0.8 Ma (Early Oligocene), confirmed by the

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29.96 Ma age of the overlying Puerto El Gato Formation. The fossil locality lies near Atopoltitlán, and yielded a medium-sized mammalian, articulated, postcranial skeleton. The recovery, preparation and study of the material were performed using standard paleontological methods and techniques. The fossil exhibits among others, the following features: Anteroposteriorly wide ribs, astragalus with double trochlea, ecto- and mesocuneiform fused, phalanx II smaller than the others, and hoofed ungual phalanges. Such characters set it in the artiodactyl Family Merycododontidae, making it the first of its kind in Mexico. The size of the postcranial skeleton corresponds to that of the Whitneyan-Late Arikareean Eporeodon; however, the lack of skull and jaws prevents a positive assignment to this or other merycoidodont genus. Finally, the Tehuitzingo species was tropical, probably cursorial, (?herding), and lived in an open country ecologic setting, with ready access to water supply. Unusual taphonomic conditions led to its preservation as an articulated, headless carcass.

Keywords: Arikareean; Mexico; Oligocene; Oreodont; Puebla; Tehuitzingo

1. Introduction

Juan Darío González-Herrera, a schoolboy from Atopoltitlán, Tehuitzingo, Puebla (Fig. 1) found the specimen in the namesake arroyo. His family contacted the senior author by e-mail, informing him about this interesting find; he agreed to visit the locality, and collect the specimen, which was a difficult task (see Material and Methods below), because it was embedded in a very hard mudstone; once collected, the specimen (now included in a large block) was brought to the Laboratorio de Paleontología de Vertebrados, Instituto de Geología, UNAM in Mexico City. The geologic setting of the locality suggested that the unit bearing the postcranial skeleton was at least of Late Paleogene age; the specimen is the first and only known vertebrate fossil of this age in the region (southern Puebla and surroundings), and in the country for this matter. The scientific importance of the specimen led to its thorough paleontological study, which is the subject matter of this report. The results reiterate the importance of the specimen, who turned out to be the first and only fossil vertebrate from this basin, and the first oreodont postcranial skeleton from the Paleogene-Neogene of Mexico.

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Other colleagues (e.g., Rodríguez-Torres, 1970; España-Pinto, 2001; Silva-Romo, 2010; Silva-Romo et al., 2018) independently worked on the detailed geologic study of the Tehuitzingo Basin, as part of a regional analysis of southeastern Mexico’s Paleogene basins, to disclose their tectonic relationships and evolution. We benefited from their work, which provided a geochronologic insight for the paleontological study.

2. Material and methods

Figure 2 shows the way the specimen, IGM 3495-1, looked like in the site before collecting. Notice that the carcass was peneplained by erosion, exposing the vertebrae (C3–C6– T1–T10) as if they would have been cut along a parasagittal section (i.e., a section parallel to the antero-posterior axial plane, but somewhat off). To remove the specimen, a 0.85 x 0.65 x 0.75 m block weighting at least ~400 kg was carved from the ground; subsequently a backhoe loader machine was used to place it in the field vehicle. Although the block seemed strong enough to hold in one piece, actually upon returning, it was found that it had some fissures, and eventually cracked into several small blocks, unfortunately breaking some bones (particularly the long ones). Understandingly, the preparation of the specimen took a long time. The material recovered is depicted in Figure 3, and consists of: (1) part of the right hemithorax [IGM 3495-1]; (2) three articulated vertebral centra [IGM 3495-2]; (3) five articulated vertebral centra [IGM 3495-3]; (4) three associated vertebral centra [only the middle is complete, IGM 3495-4]; (5) combination of humerus, radius and ulna (all missing the distal epiphyses) anomalously associated to part of the scapula [IGM 3495-5]; (6) nearly complete, articulated tetradactyl forelimb including the humerus, radius, ulna carpus, metacarpus and digits [IGM 3495-6]; (7) right femur articulated to the ilium and ischion [IGM 3495-7]; (8) articulated femur with the proximal part of the tibia and rotula [IGM 3495-8]; (9) distal portions of the tibia and fibula articulated with the tarsus and digits of a tetradactyl foot [IGM 3495-9]; (10) distal part of a left tibia [IGM 3495-10]; and (11) an assortment of incomplete digits and small bone fragments [IGM 3495-11, -12, and -13]. The anatomical nomenclature used is the standard for mammals (but see Hildebrand, 1954; Getty, 1975; Munthe, 1989; Evans, 1993, López-Plana et al., 2000). All measurements

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were taken with a dial caliper accurate to the 0.1 mm, and are expressed in mm. Given that the Tehuitzingo specimen belongs to a species referable to the Merycoidodontidae, we compare its limb measurements with those of broadly contemporaneous merycoidodont species (included in sixteen genera) to assess which species is/are about the size of the Tehuitzingo taxon (see Table 1). The taxonomically informative relationship of the skull basal length and selected appendicular elements length (see the Schultz and Falkenbach monographs of 1940 to 1968) cannot be established here for lack of the skull. To assess cursoriality, we rely on anatomical criteria (e.g., the length of manus and pes), because the specimen appendicular skeleton is incomplete for any given limb, it was not possible to calculate rate relationships between its skeletal components.

3. The study area

The Tehuitzingo area (Figs. 1 and 4) lies in southern Puebla, between 18°13’30” – 18°20’30” N Lat. and 98°14’30” –98°19’30” W Long. (INEGI, 1999), it includes some 114 sq. km of low relief country (set between 1,077–1,372 masl) draining to the north. The namesake town, and the villages Atopoltitlán, San Miguel Buenavista, and Tecolutla are included in this area. Geologically, this territory is chiefly constituted by the Tehuitzingo Formation (CalderónGarcía, 1956; Silva-Romo et al., 2018), which is a lacustrine-fluvial sequence largely consisting of variously colored (light gray, pistachio green, pink to brick red) mudstone, fine-grained sandstone, lacustrine limestone (it may be partly silicified), felsic unwelded to welded tuff sheets (intercalated in the sequence without any discernible pattern, one of them is the dated Atopoltitlan Ignimbrite, in the upper part of the unit), and ledge-forming dark chert set in thin to mediumly thick (and even massive) strata gently dipping (~15°) to the northeast; a litharenitic conglomerate occurs in the southwest, underlaying the Tehuitzingo Formation, which largely fills the namesake basin; such structure is north-south elongate (~27 km N-S long and ~10 km E-W wide), and was developed in the Acatlán Complex through left strike-slip faulting (Silva-Romo et al., 2018).

3.1. Comment on the environmental interpretation

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The stratigraphic occurrence and makeup of the Tehuitzingo Formation suggest that it records ephemeral pond sedimentation, probably fed by hydrothermal springs associated to pervasive regional volcanism. The possibility of pond coalescing at high stand levels, thereby forming an extensive shallow lake, is not excluded.

3.2. Geologic Age

The lack of fossils and of radio isotopic dates produced uncertainty about the age of the Tehuitzingo Formation, which was placed in the Pleistocene (Calderon-García, its author, 1956), or in the Plio-Pleistocene. However, Silva-Romo (2010) and Silva-Romo et al. (2018) reported an age of 29.9 ± 0.8 Ma (K-Ar in biotite) from the Atopoltitlán Ignimbrite, a pyroclastic sheet placed in the upper part of this formation (about 80 m below the fossiliferous stratum), which puts it in the Early Oligocene (i.e., the early Arikareean North American Land Mammal Age). This assessment is corroborated by the fact that the overlying Puerto El Gato Formation, of felsic pyroclastic composition, yielded an U/Pb age of 29.96 ± 0.8 Ma (Silva-Romo et al. 2018, p. 184– 185 and figs. 6–7). The biochronologic date afforded by the oreodont postcranial skeleton is congruent with the radio-isotopic dating.

3.3. The Fossil Locality It lies in the Arroyo Atopoltitlán (18o 16’ N Lat. and 98o 17’ W Long.), about 900 m NNE of the namesake village (Fig. 4), placed ~80 m above the K-Ar dated stratum, floored there by a continuous section of felsic, vitric ash-fall and ash-flow tuff sheets interbedded by moderately lithified, red, fine-grained, silicic-tuffaceous, tabular cross-bedded sandstone, and ledge-forming dark chert set in medium to thick strata, gently dipping (~15°) northeast.

4. Systematic paleontology Class MAMMALIA Linnaeus, 1758 Order ARTIODACTYLA Owen, 1848 Suborder TYLOPODA Illiger, 1811

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Superfamily Oreodontoidea Leidy, 1869 Family Merycoidodontidae Thorpe 1923: 239 [sensu Stevens and Stevens, 2007] Figures 2, 3, 5–11

Description— The specimen is a partial postcranial skeleton (Fig. 3) of a medium-sized mammal (height at shoulders ~45–50 cm at least) without predation and/or scavenging marks, and includes a good part of the vertebral column (excluding the majority of caudal vertebrae), the right dorsal part of the thorax (the ventral one is missing); a fragment of the left scapula; the forelimbs (the right is nearly complete), the left consists of the proximal part of radius and ulna), the hindlimbs (both femora are nearly complete, the right one is still articulated to an ilium fragment), the other shows its proximal part broken, but its detached part exhibits a fragment of the head articulated to an acetabulum fragment, the distal part is articulated to the proximal half of the tibia, the left tibia proximal part is present; its distal half is articulated to the pes, whose phalanges are largely missing, but fragments of two remain. The description follows:

Vertebrae and ribs. Out of the 30 precaudal vertebrae present in most artiodactyls (formula: C7, T13, L7, S3, Ca20–23; Getty, 1975), the specimen has 21 (Figs. 5.1–3, 6); excluding the thoracic ones associated to ribs (Fig. 6), the remainder vertebrae are hard to place in their proper anatomical region, because none has the spine, pre- and postzygapophyses preserved, and simply consist of parasagittal sections of the centra, so they were discriminated on size as follows: the three larger ones (Table 1) included in a small block of matrix are interpreted as cervical, (Fig. 5.1; notice that the atlas and axis are not preserved), but their particular anatomical position cannot be ascertained, their prezygapophyses (anterior articular facet) is slightly convex, whereas their postzyapohyses (posterior articular facet) is gently concave; the three middle-sized ones (Table 1) included in another block of matrix (Fig. 5.2) are regarded as lumbars, their centrum sections are similar in shape to those of the cervical, two of them show in the antero-ventral corner a small bone fragment, interpreted as part of the ventral portion of the transverse apophysis; the smaller, isolated one, also included in a block of matrix, is considered as caudal (Fig. 5.3), perhaps the first one, because its size is only slightly smaller than the lumbar vertebrae.

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Thorax. Figure 6 shows a parasagittal section of the thoracic vertebrae probably corresponding to T1-10; they have been peneplained by fluvial erosion, so that the left part of the thorax was removed. What remains includes only the centra (Table 1) and part of the zygapophyses; the prezygapophyses are antero-dorsally faced, whereas the postzygapophyses faced postero-ventrally; they show also a small part of the ventral portion of the transverse apophyses. The right half of the thorax is partly preserved exposing eight ribs, which are fairly strong, antero-posteriorly wide, about twice as much as those of a dog or sheep of the same size (Getty, 1975); the capitulum articulated to the corresponding vertebral facet is visible only in the three anterior ones. Besides oreodonts (e.g. Eporeodon, Promerycochoerus, Thorpe, 1937), antero-posteriorly wide ribs are common among other mammals, in the pilosan edentates, Hippopotamus and in Toxodon. The closely related oreodontoid family Agriochoeridae (Agriochoerus and Protoreodon) bear slender ribs Ludtke (2007, figs. 11.1 and 11.2). The oblique setting of the ribs (~40°) respect to the vertebral column is a post mortem artifact, because the ribs normal setting is nearly perpendicular (~90°) to the column. Notice that the anterior part of this block shows a detached, articulated, complete manus (see below), allochtonously placed close to the thorax, which indicates partial dismembering of the carcass before burial. It should be noted that agriochoerids bear claws on the ungual phalanx (Ludke, 2007), a fact that precludes assigning the Tehuitzingo specimen to this taxon. Forelimb. The clavicles and right scapula are not preserved; the left one (Fig. 7) is allochthonously associated with the humerus, radius and ulna; it consists of the anterior part of the scapula (excluding the glenoid cavity and the neck) that includes a small portion of the spine (without its tuberosity), which is rather thin, and part of the supraspinose fossa and its anterior border. The specimen also includes an almost complete, articulated right forelimb skeleton (Fig. 8, lateral view). The humerus resembles that of a Recent peccary or pig (Getty, 1975) but slender, it lacks the proximal part (head and trochanters), its diaphysis (shaft) shows a well developed musculo-spiral sulcus, the condylean crest. The humeral distal part exhibits a large olecranon fossa (= sulcus), but the fossa itself and the supratrochlear foramen are still covered by sediment; the same occurs with the coronoid fossa; both lateral and mesial condyles and epicondyles are prominent, well preserved and form the humeral trochlea (largely covered), which is articulated

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with the ulna and radius. Both are complete, also stout, unfused, separated all along by a narrow interosseus space. The ulna (Figs. 7 and 8) bears a large olecranon (relatively wider than that of a pig of similar size), as well as a strong tuberosity and a large semilunar neckline bordered by well developed labia, its length (from the anconeal process up) is ~13% the total ulnar length, which is relatively shorter than that of a pig (Getty, 1975), but comparable to that of Canis (cf., Evans, 1993); the ulnar distal extreme is large too and articulates with the carpus. The radius (still articulated to the humerus, ulna and carpus, Fig. 8) is complete, slightly anteriorly convex; its length is about the same as that of Merycoidodon culbertsoni and M. (Paramerychocoerus) major (Table 1; Schultz and Falkenbach, 1968); however other limb bone measurements in both species are different, thus precluding assignment to either. The diaphysis of the radius is nearly cylindrical rather than transversely elongated as in sheep and deer; its glenoid cavity is covered; the distal extreme is wide (wider than the proximal end), and articulates with the carporadial and intermediate carpal; grooves for the extensor digitalis, extensor carpo-radialis and abductor polllicis longus muscles are well developed.

Carpus and Metacarpus. The carpus is complete, articulated and partly embedded in sediment (Fig. 8); dorsally these elements are discernible: carporadial, carpal intermedius, carpoulnar, unciform and magnum; the limb is strongly flexed, so that the palmar side of the manus is covered. Metacarpals II and V are slightly shorter, and slender than III–IV (see Table 1); all are hemicylindrical (half caned) with the plantar side flat, and distally articulated with large dorsal sesamoid bones (Fig. 8).

Digits. They are fully preserved and include Digits II–V (Fig. 8), have the full threephalangeal set, and are somewhat spread (Fig. 8, interdigit angle ~20°) in a typical oreodont fashion i.e., with tetradactyl manus and pes moderately spread and all ungual phalanges touching the ground (Thorpe, 1937; Schultz and Falkenbach, 1968). Phalanx 2 in all digits is significantly shorter (about one third) than phalanges 1 and 3, as is common in other artiodactyls (Getty, 1975), but no so in canids e.g. Canis (Evans, 1993), where phalanges I–II have approximately

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equal length. Having phalanx II shorter than the others might have improved body weight support. Phalanx III in the four digits is hoofed (Fig. 8), gently curved length-wise (downward), tapering slightly toward the tip, which is round (not acute as in carnivores), the dorsal (upper) side is hemicylindrical (i.e., lower part flattish) and relatively wide; distally it shows a moderately long furrow (length ~40% of phalanx 3) on each side, which probably afforded additional anchorage for the hoof. Tetradactyl hoofed manus and pes is a distinct trait in merycoidodonts (Thorpe, 1937; Lander, 1998; and many others). The osteology of the detached manus (see above) is similar to the one described here. It should be noted that in the other oreodontoid family, Agriochoeridae, the ungual phalanges bear claws instead (Ludtke, 2007), which precludes assigning this specimen to them.

Pelvis and hindlimb. Pelvis. The specimen includes part of the pelvis (= innominated) consisting of the lower part of the ilium (Fig. 9), which lengthwise is nearly complete (~105 mm long, i.e., 60% of the femur length), missing only the coxal tuberosity, its lower border is gently curved downward, as in oreodonts and other artiodactyls; its mesial (= internal) face is moderately concave and the lateral is gently convex. The acetabulum is large and still articulated to the femoral head. The ischion (Fig. 9) is nearly complete (~45 mm long), its distal end is wider than the proximal (~24 mm versus 10 mm); its tuberosity is not as well developed as in pigs of the same size (45–50 cm height at shoulders; Getty, 1975). Hindlimb. Both right and left, nearly complete femora are preserved (Figs. 9 and 10) and resemble those of pigs, but less massive (i.e., with a lesser transverse diameter); the left one is still loosely articulated to the acetabulum, and the right one is articulated to the tibia. The head bears a large fovea and is partly outside the acetabular fossa, which suggests that before burial, the femur was becoming detached from the pelvis. The femoral diaphysis shows well developed labia (lateral and medial) which delimit a narrow rough area (cara aspera). The condyles are subequal (the lateral one is a little larger than the mesial); the trochlea is rather deep, delimited by high and narrow crest-like ridges. The patella (fairly large) is still articulated to the left femur (Fig. 10). Overall the femur of the Tehuitzingo specimen is similar to that of merycoidodonts (Schultz and Falkenbach, 1954, 1968).

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Tibia and fibula. Both tibiae (Figs. 9–10) are preserved, the left one consists of the proximal half, whereas the right is nearly complete, articulated to the femur and to the tarsus, but broken at the middle; the articular surface with the femur is broadly triangular, on the anterior part includes the proximal portion of the large and strong tibial crest, which reaches to half the length of the diaphysis, and is as broad as that of a pig. The rest of the articular surface includes two subequal condyles (the mesial is larger) separated by the tuberosity which stops the patella gliding. It should be noted that the medial and lateral vertices of the articular surface are supported by two very narrow crests that distally extend to a little less than half the tibial length. It should be noted that the proximal articular surface of the tibia is smaller than that of a pig, but much larger than that of a dog of similar size. The distal articular surface is much smaller than the proximal one, and transversely wider than antero-posteriorly, thus delimiting a rectangular transverse section, as in oreodonts (Thorpe, 1937) and pigs (but not so in dogs, whose distal end is squarish, Evans, 1993), and bears the mesial maleolus. The distal half of the fibula (Fig. 10) is preserved, it is delicate and separated from the tibia by a narrow interoseus space (that of pigs is wide; Getty, 1975), and distally includes the lateral maleolus, which articulates with the tibiotarsal, and with the tarso-fibular. The general appearance of the tibia and fibula closely resembles those of merycoidodonts (Schultz and Falkenbach, 1968).

Tarsus and metatarsus. Both tarsus and metatarsus are complete and still articulated (Figs. 10 and 11), which prevents a detailed description of individual bones. The calcaneum is large, its tuberosity and associated support are missing (by breakage), and its body is still articulated to the fibular maleolus, astragalus and cuboid. The astragalus partly shows the double trochlea, characteristic of artiodactyls, the distal trochlea is smaller than the proximal; the astragalus is smaller than that of a pig, and is articulated with the tibia, calcaneum, navicular and cuboid (in more advanced artiodactyls the navicular and cuboid are fused). The cuboid is large (Fig. 11), its proximal part articulated with the navicular, and its distal part is laterally articulated with the “composite” cuneiform, which actually consists of the meso- and ectocuneiforms fused, in a typical oreodont fashion (see Loomis, 1933, fig. 2). The metarsals articulate with the distal row of the tarsals (cuboid + “composite” cuneiform, Figs. 10–11). Metatarsals II–V are ~15% longer

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than metacarpals (Table 1), the elongation of both metacarpal and metatarsals (greater than in pigs and comparable to that of canids) strongly suggests some degree of cursoriality. Metatarsal II and V are subequal, shorter and less robust than metatarsals II and III, which are transversely wider, stronger and bear large dorsal sesamoids, whereas in the plantar side, a pair of sesamoid bone for each metatarsal is present. Only the first phalanx of digit V is preserved, its length is about 2/3 that of the corresponding metatarsal. Other two detached, incomplete phalanges are also present, but cannot positively be assigned to a particular digit.

5. Discussion

5.1. Taxonomic Assessment

A positive identification below the family level of the species represented by IGM 3495 is prevented by the lack of skull and jaws. Although a direct comparison with substantial, taxonomically identified postcranial material, such as that of the American Museum of Natural History in New York, could have advanced the level of identification to that of genus, lack of resources prohibited. The axial skeleton is like that of a generalized mammal, with a few specializations such as the relatively large antero-posterior width of the ribs. The appendicular skeleton corresponds to that of a cursorial mammal with elongate fore- and hindlimbs, as well as manus and pes (which have an even number of metapodials and digits), fused ecto- and mesocuneiform, a digitigrade stance, and hoofed digits. This unique combination of characters is observed in early artiodactyls, like oreodonts and tylopodans. Additional oreodont features include stout leg bones, unfused radius/ulna and tibia/fibula, reduced phalanx II (i.e., smaller than phalanges I and III), and hand/feet (actually fingers/toes) little spread (Thorpe, 1937; Schultz and Falkenbach, 1947, 1949, 1954, 1968, among others). Given these characters, the specimen is assigned

to

the

Oreodontoidea,

which

includes

the

Families

Agriochoeridae

and

Merycoidodontidae (McKenna and Bell, 1997; Lander, 1998; Stevens and Stevens, 2007). The lack of the pollex indicates membership with the latter (Thorpe, 1937; Lander, 1998). Given the good preservation of the specimen, the lost of the pollex by a taphonomic process is unlikely; further, no trace of articulation for the base of the pollex is present.

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Within the Merycoidodontidae, the species represented by the Tehuitzingo specimen is, on the basis of size (estimated from the length of the radius, ulna, metacarpal III, femur, tibia and metatarsal III, see Table 1) and stratigraphic range (that includes the 29.8 Ma datum of the Tehuitzingo Formation, and the 29.96 Ma of the overlying Puerto El Gato Formation) is larger than Leptauchenia decora (Leptaucheniinae, genus range middle Orellan-early middle Arikareean, Stevens and Stevens, 2007), smaller than most oreodonts included in Table 1, and about the same size as Prodesmatochoerus meekae (Miniochoerinae, subfamily range early middle Chadronian-early Whitneyan, Stevens and Stevens, 2007), Eporedon socialis, and E. thurstoni (Eporeodontinae, genus range middle Whitneyan-late Arikareean, Stevens and Stevens, 2007). Further taxonomic precision beyond this point is unwarranted on the basis of the available evidence (cranium and mandibles not preserved).

5.1.1. Paleobiologic aspects Cursoriality. The elongated limbs (particularly so manus and pes) of the species represented by the Tehuitzingo specimen is an adaptation for cursoriality (Thorpe, 1937; Hildebrand, 1954; Munthe, 1989). Having a relatively long tibia (~94% of femur length or better, see Getty, 1975) as observed in IGM 3495 (Table 1), is a means for limb elongation. The fact that forelimb skeletal elements are only moderately shorter than those of the hindlimbs, a common feature in open country mammals, enhances cursoriality; this condition is observed for instance in Canis and other canids (Hildebrand, 1954; Evans, 1993), as well as in other oreodonts (Thorpe, 1937). These and other facts lead us to suggest that the Tehuitzingo species probably had a certain degree of cursoriality, perhaps being as swift (or nearly so) as contemporaneous carnivores of the same size e.g. the hesperocyonine canids (Munthe, 1998). As occurs in cursorial mammals, the hindlimbs are the principal propelling elements during walking/running. Habitat and environment. Current tectonic and paleogeographic data (Scotese, 1997) indicate that Mexico (and consequently Puebla State where the Tehuitzingo specimen was found) lay by ~30 Ma in the same geographic position that it does at present, i.e., well within the Tropical Realm, so it was subdued to tropical conditions; other tropical merycoidodonts are those from the following local faunas (Fig. 1): (a) late Uintan-early Duchesnean Yolomecatl l.f., Merycoidodon sp. from the namesake formation, northwestern Oaxaca (Jiménez-Hidalgo et al.,

12

2015; Ferrusquía-Villafranca et al., 2016); (b) late Arikareean Suchilquitongo l.f., Merychyus aff. M. minimus from the namesake formation, central Oaxaca (Ferrusquía-Villafranca, 2001, 2006); (c) Hemingfordian, Zoyatal l.f., Merychyus elegans from the Zoyatal Formation, central Aguascalientes (Dalquest and Mooser, 1974; Stevens, 1977; Ferrusquía-Villafranca, 1990); (d) middle Arikareean Gaillard Cut l.f., Merycochoerus matthewi from the Cucaracha Formation, Panamá Canal Zone (Whitmore and Stewart, 1965; MacFadden, 2006). These occurrences indicate that although merycoidodonts were chiefly adapted to temperate environments, so that the bulk of their fossil record lies in temperate North America, where they are very abundant; by contrast, only a few genera could thrive in subtropical to tropical environments, where their record is scarce, fragmentary and scattered. Cursorial herbivorous mammals display a variety of group strategies that vary from solitary or nearly solitary individuals to large herds, the latter tend to live in open country/savanna communities, with ready access to water supplies (e.g., rivers, ponds and lakes); the geologic setting of the Tehuitzingo area is congruent with this interpretation. It follows then, that the Tehuitzingo species probably had these attributes too, and lived in an environment similar to the one described. Abundance. It is noteworthy that in temperate North America oreodonts are extremely common, becoming the most abundant components of Oligocene faunas (Schultz and Falkenbach, 1968; Stevens and Stevens 1977; Lander, 1998 among other authors), which stands in strong contrast with their meager record in Mexico-Central America. The most plausible explaination is that oreodonts were stenotopic temperate mammals, and that only a handful of them were capable of thriving in subtropical to tropical conditions, and their remains are scarce.

5.1.2. Taphonomy The fact that the oreodont specimen was rather well preserved, articulated and without predation and/or scavenging marks is not common, and requires an explanation. The following taphonomic hypothesis establishes the possible sequence of events that led to its burial and fossilization: The Tehuitzingo oreodont probably died on or close to the shore of a floodplain, got adrift, thus avoiding scavenging, rotted there, becoming inflated by decomposition gases. This process advanced, so that the head being heavier, became separated from the carcass; other parts

13

were also detached from it, but remained close producing “allochthonous” spatial association between unrelated skeletal elements, like the manus attached to the cervical vertebrae (Fig. 6), or produced unusual limb flexing positions (Fig. 8), or simply became lost. Eventually, whatever remained of the carcass came to rest on the bottom, becoming buried by fine-grained clastics laid down by fluvial/lacustrine sedimentation. Millions of years later, erosion uncovered the burial place, and partly peneplained the specimen, thus eliminating most of the thorax left half, exposing only a series of small spool-shaped, whitish squares (the thoracic vertebrae) that contrast with the pale red embedding rock.

5. Summary and conclusions

1. The Tehuitzingo Basin fill largely belongs to the Tehuitizingo Formation, a thick lacustrine-fluvial, fine-grained sequence interbedded by silicified limestone, chert and tuff/ignimbrite sheets, one of them, the Atopoltitlan Ignimbrite yielded a K-A date of 29.9 Ma, which places it in the Early Oligocene (i.e., in the early Arikareean North American Land Mammal Age); this datum is corroborated by the 29.96 Ma age of the overlying Puerto del Gato Formation. It bears near Atopoltitlán, Tehuitzingo, Puebla (northeastern Sierra Madre del Sur Morphotectonic Province), a locality that yielded a partial postcranial articulated skeleton of an oreodont, which is the first and only Paleogene-Neogene vertebrate found in this basin, and the first of its kind in Mexico. 2. The skeleton includes the thorax, other vertebrae, forelimbs/manus, part of the pelvis, hindlimbs/pes; both manus and pes are paired (tetradactyl), and the ungual phalanges of both fore- and hindlimbs are hoofed. These and other features allow to assign the Tehuitzingo mammal to the artiodactyl Family Merycoidodontidae (sensu Stevens and Stevens, 2007); further, its size corresponds to that of contemporaneous Eporeodon and Prodesmatochoerus, but the lack of skull and jaws prevents to positively assign it to either or other merycoidodont genus. 3. Oreodonts are nearly exclusive temperate North American mammals that lived in the late Duchesnean-early Hemphillian NALMAs, but were more diverse and successful during the late Uintan-Arikareean interval (Lander, 1998; Stevens and Stevens, 2007). In Mexico they are recorded in these four local faunas and a single occurrence: (a) Yolomécatl l.f., northwestern

14

Oaxaca (late Uintan-early Duchesnean, Jiménez-Hidalgo et al., 2015); (b) Rancho Gaitán l.f., Chihuahua (Chadronian, Ferrusquía-Villafranca, 1967); (c) Zoyatal l.f., Aguascalientes (early Hemingfordian, Dalquest and Mooser, 1974; Ferrusquía-Villafranca, 1990); (d) Suchilquitongo l.f., central Oaxaca (late Arikareean, Wilson and Clabaugh, 1970; Ferrusquía-Villafranca, 2003); and (e) Tehuitzingo single occurrence, Puebla (early Arikareean, this report). 4. The Tehuitzingo species was tropical, had some degree of cursoriality (?herding), and lived in an open country ecologic setting, with ready access to water supply. Unusual taphonomic conditions led to its preservation as an articulated, headless carcass.

Acknowledgments

The González-Herrera Family from Atopoltitlán, Puebla informed the senior author the occurrence of vertebrate fossil remains in the Arroyo Atopoltitlán, and provided many courtesies in our visit to recover these remains; the authors duly thank them for this. Mr. Gerardo ÁlvarezReyes, Technician of the Vertebrate Laboratory, Instituto de Geología, skillfully prepared the specimen. The authors also appreciate and thank Dr. Gary Morgan (New Mexico Museum of Natural History) his kind and encouraging review. In addition, two anonymous reviewers made valuable suggestions to improve the manuscript, and we duly appreciate and thank their effort. The student scholarship holders Rosa Olivia González-Oliva, Elsa Paulina Calvo-Ábrego and José Manuel Martínez-Montes, helped in preparing technical cards and retrieving information needed to develop the project. The Instituto de Geología, Universidad Nacional Autónoma de México supported the project. The Dirección General de Asuntos del Personal Académico de la UNAM (acronymed DGAPA), through Grant IN-107210 afforded additional economic support to carry out the project. The authors appreciate and thank both institutions for this.

References

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Ludtke, J.A., 2007. Family Agriochoeridae. In: Prothero, D.R., Foss, S.E. (Eds.), The evolution of Artiodactyls. Johns Hopkins University Press, Baltimore, Maryland, 151–158. MacFadden, B.J., 2006. North American Miocene Land Mammals from Panama. Journal of Vertebrate Paleontology, 26, 720–734. McKenna, M.C., Bell, S.K., 1997. Classification of mammals above the species level. Columbia University Press, New York. Marsh, O.C., 1875. Notice of new Tertiary mammals, IV. American Journal of Science, 9, 239– 250. Munthe, K., 1989. The skeleton of the Borophaginae (Carnivora, Canidae): Morphology and function. University of California Publications in Geological Sciences, 133, 1–115. Munthe, K., 1998, Canidae. In: Janis, J.M., Scott, K.M, Jacobs, L.L. (Eds.), Evolution of Tertiary mammals of North America, Vol. 1, Terrestrial carnivores, ungulates and ungulatelike mammals. Cambridge University Press, Cambridge, 124–143. Owen, R., 1848. Description of teeth and portions of jaws of two extinct Anthracotherioid quadrupeds (Hyopotamus vectianus and Hyop. bovinus) discovered by the Marchioness of Hastings in the Eocene deposits on the NW coast of the Isle of Wight: with an attempt to develop Cuvier's idea of the Classification of Pachyderms by the number of their toes. Quarterly Journal of the Geological Society of London, 4, 103–141. Rodríguez-Torres, R., 1970, Itinerario geológico México-Oaxaca. In: Segura-Vernis, L., Rodríguez-Torres, R. (Eds.), Libro-guía de la Excursión México-Oaxaca. Sociedad Geológica Mexicana, México, 4–32. Schultz, C.B., Falkenbach, C.H., 1940. Merychochoerinae, a new subfamily of oreodonts. Bulletin of the American Museum of Natural History, 77, 213–306. Schultz, C.B., Falkenbach, C.H., 1947. Merychyinae, a subfamily of oreodonts, Bulletin of the American Museum of Natural History, 88, 157–286. Schultz, C.B., Falkenbach, C.H., 1949. Promerycochoerinae, a new subfamily of oreodonts. Bulletin of the American Museum of Natural History, 93, 69–198. Schultz, C.B., Falkenbach, C.H., 1950. Phenacocoelinae, a new subfamily of oreodonts. Bulletin of the American Museum of Natural History, 95, 91–149.

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Schultz, C.B., Falkenbach, C.H., 1954. Desmatochoerinae, a new subfamily of oreodonts. Bulletin of the American Museum of Natural History, 105, 143–256. Schultz, C.B., Falkenbach, C.H., 1956. Miniochoerinae and Oreonetinae, two new subfamilies of oreodonts. Bulletin of the American Museum of Natural History, 109, 377–482. Schultz, C.B., Falkenbach, C.H., 1968. The phylogeny of the oreodonts, Parts 1 and 2. Bulletin of America Museum of Natural History, 139, 1–498. Scotese, C.R., 1997. Continental Drift (7th edition). PALEOMAP Project, Arlington, Texas. Scott, W.B., 1893. The Mammals of the Deep River Beds. The American Naturalist, 27, 659– 662. Servicio Geológico Mexicano (SGM), 1998. Carta Geológico-Minera Cuernavaca. México, Servicio Geológico Mexicano, E14-5, escala 250,000. Silva-Romo, G., 2010. Origen tectónico y evolución de la Cuenca Tehuitzingo-Tepexi, Estado de Puebla. Doctor’s Dissertation, Universidad Nacional Autónoma de México. Silva-Romo, G., Mendoza-Rosales, C.C., Campos-Madrigal, E., Hernández-Marmolejo, Y.B., de la Rosa-Mora, O.A., de la Torre-González, A.I., Bonifacio-Serralde, N., López-García, C., Nápoles-Valenzuela, J.I., 2018. Timing of the Cenozoic basins of Southern Mexico and its relationship with the Pacific truncation process: Subduction erosion or detachment of the Chortís

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https://doi.org/10.1016/j.jsames.2018.01.007 Stevens, S.M., Stevens, J.B., 2007. Family Merycoidodontidae. In: Prothero, D.R., Foss, S.E. (Eds.), The evolution of Artiodactyls. Johns Hopkins University Press, Baltimore, Maryland, 157–168. Thorpe, M.R., 1921. John Day Promerycochoeridae, with descriptions of five new species and one new subgenus. American Journal of Science, 1, 215–244. Thorpe, M.R., 1937. The Merycoidodontidae, an extinct group of ruminant mammals. Memoire, Peabody Museum of Natural History, 3, 1–428. Wilson, J.A., Clabaugh, S.E., 1970. A new Miocene formation and a description of volcanic rocks, northern Valley of Oaxaca. In: Rodríguez, R.T., Segura, V.L. (Eds.), Excursión México-Oaxaca, Libro-Guía. Sociedad Geológica Mexicana, México, 120–128.

19

Figure captions Fig. 1. Index map of Mexico-Central America showing the location of Puebla, Tehuitzingo area and local faunas: 1, Rancho Gaitán, Chihuahua; 2, Zoyatal, Aguas Calientes; 3, Suchilquitongo, central Oaxaca; 4, Yolomécatl, northwestern Oaxaca; 5, Cucaracha, Panama Canal Zone. Abbreviations: B. Belice; G. Guatemala; H. Honduras; ES. El Salvador; N. Nicaragua; CR. Costa Rica; P. Panama. Fig. 2. Appearance of the unprepared specimen (IGM 3495) in the site. It includes peneplained centra of cervical, thoracic, lumbar and caudal vertebrae, as well as two costal fragments. Anterior to the left. Scale bar = 50 millimeters. Fig. 3. Silhouette of an oreodont (Merycoidodon culbertsoni, after Thorpe, 1937) showing its skeleton, and the position of available remains of the Tehuitzingo specimen (in yellow), the unavailable ones are shown in white. Fig. 4. A, Geologic map. B, Satellite image of the Tehuitzingo area. Sources: Map slightly modified from SGM, 1998, and Silva-Romo et al., 2018. Image taken from Google Earth Pro, 2019. Fig. 5. IGM 3495-2-4. Peneplained centra of cervical (A), lumbar (B) and caudal vertebrae (C) of the Tehuitzingo specimen. Scale bar = 20 millimeters. Fig. 6. IGM 3495-1. Dorsal part of the right hemithorax. The oblique position of the ribs with respect to the vertebral column is a post morten artifact. Notice that the right hand is allochthonously associated with the thoracic vertebrae. Scale bar = 50 millimeters. Fig. 7. IGM 3495-5. Humerus (lacking the proximal end) articulated with the ulna and radius. Notice that the humerus is anomalously flexed becoming nearly parallel to the radius, and that a fragment of the scapula is also anomalously associated to the humerus. Scale bar = 20 millimeters. Fig. 8. IGM 3495-6. Right nearly complete articulated humerus, radius, ulna, carpus, metacarpus and fingers. Notice that the anomalous flexure of the hand (carpus-metacarpus-and fingers) with respect to the radius/ulna, is a post mortem artifact. Scale bar = 50 millimeters. Fig, 9. IGM 3495-7, -10. Part of the right ilium and ischion still articulated to the femur; below it (femur) the proximal portion of the corresponding tibia (IGM 3495-10) is pictured. Scale bar = 50 millimeters.

20

Fig. 10. IGM 3495-8,-9. Hindlimb bones: Left nearly complete hindlimb bones: The femur (IGM 3495-8) lacks the proximal end, it is distally articulated to the tibia, which is broken in two halves, the distal one (IGM 3495-9) is articulated with the tarsus-metatarsals. Scale bar = 50 millimeters. Fig. 11. Dorsal view of the tarsus (part of IGM 3495-9). Abbreviations: a, astragalus; ca, calcaneum; cu, cuneiform (ecto- and mesocuneiform fused); cb, cuboid; F, fibula; mtIII; third metatarsal; mtIV, fourth metatarsal; n, navicular; T, tibia. Scale bar = 10 millimeters.

Table Table 1. Selected length measurements of oreodont taxa broadly contemporaneous with the Tehuitzingo species. To avoid confusion, the synonymy of some genera (in bold type) is included in the body of the table.

21

Table 1 Selected length measurements of oreodont taxa broadly contemporaneous with the Tehuitzingo species. To avoid confusion, the synonymy of some genera (in bold type) is included in the body of the table. Merycoidodont taxa

Length of Length of

Meta-

Length of

Length of

of Ulna

Radius

carpal III

Femur

Tibia

162

(152)

60

153

127

48

Metatarsal III (max)

(max) Tehuitzingo oreodont species

Length of

Length

Leptauchenia Leidy, 1856 (synonyms:Cyclopidius Cope 1877)

g

Leptauchenia. decora a

84

60

---

86

79

---

Cyclopidius. densus a

118

84

---

(110)

101

---

Oreonetes Loomis, 1924 (synonyms: Platyochoerus Schultz and Falkenbach 1956; Stenopsochoerus Schultz and Falkenbach 1956; Miniochoerus Schultz and Falkenbach 1956) g Platyochoerus. platycephalus e

(116)

84

42.5

124.5

---

56.5

---

---

---

---

121.5

---

---

---

---

---

---

57.5

110

81.5

38 x

---

110.5

---

---

---

---

---

(124)

---

---

((95))

45 x

---

---

---

124.5

92

44.5 x

128

120

(52.5)

---

98.5

---

127

117

---

106

88.5

43.5

120

116

---

M. (P.) gracilise

100

71

35.5

102.5

101

(44)

M. (P.) ottensi e

(121)

88

---

125

125

P. heartensis P. hatcreek

e

e

Stenopochoerus. sternbergi e S. berardae

e

S. (Pseudostenopsochoerus) chadronensis

e

S. (P.) douglasensis e Miniochoerus. battlecreekensise M. (Paraminiochoerus.) affinis e

Prodesmatochoerus Schultz and Falkenbach, 1954 (synonyme of ?Merycoidodon Leidy 1848)

--g

Prodesmatochoerus. meekaed

153

114

---

159

139

---

Merycoidodon. culbertsoniia

133

99

---

155

132

---

102.5

81

---

115.5

109

---

M. gracilis

a

1

Eporeodon Marsh, 1875 (synonyms: Mesoreodon Scott 1893; Promerycochoerus Douglass 1901; Pseudodesmatochoerus Schultz and Falkenbach 1954; Desmatochoerus Thorpe 1921; Subdesmatochoerus Leidy 1848) g Eporeodon davisi f

---

---

---

---

143.5

---

---

---

69.5

187

172.5

79

---

---

---

(181.5)

---

---

177

134

---

((182.5))

---

---

191

144

---

192

181

---

157

117.7

---

168

153

---

151.3

126.5

---

154

153

---

180

137

---

190

177

---

(200)

145

---

170

200

---

172

136

---

183

166

---

189

140

---

190

165

77

172

134

63

175

---

65

((180))

135

69.5

185

164

---

---

146

---

194

163

---

200.5

149

72.5

203

175

75

245

175

---

260

191

---

270

200

---

270

220

---

234

174

---

265

192

---

---

---

---

250

---

---

---

151

---

232

---

---

232

184

79

---

---

---

178.5

128.5

62

198

---

---

P carrikeri c

227

158.5

77

238

191.5

71

P. latidens c

---

(178)

81

---

---

86

---

---

---

184

---

---

P. longiceps d

---

140

---

(192)

167

82

Desmatochoerus

---

---

---

206

177.5

78.5

E. (Paraeporeodon) pacificus

f

E. (P.) longifrons f E. (P.) leptacantus

f

E. major chekii a E. sociales

a

E. thurstoni

a

Mesoreodon chelonyx a M. megalodon M. scotti

a

a

M. cheeki c M. cheeki scotti

c

M. chelonyx c M. megalodon

c

M. megalodon sweeti

c

Promerycochoerus. carrikeri a P. gregory

a

P. gregoryi loomisi

a

P. montanus grandis P. vantasselensis

a

a

P. (Pseudopromerycochoerus) montanus c P. (Pseudopromerycochoerus) minor pygmyus c

Pseudodesmatochoerus hoffmani d

2

(Paradesmatochoerus), grangeri d D. (P.) monroecreekensis d D. (P.) wyomingensis

d

?D. (P.) thurstoni d D. c. gregoryi

d

D. h. niobrarensi

d

D. h. geringensis d Subdesmatochoerus sociales

d

197

149

72

200

172

73

---

134.5

68.5

---

---

81

151.5

126.5

---

154

153

---

226

155

82.5

220

170

71

225

---

---

204

176

77

208

154.5

77

199

175

81

---

---

---

161

143

---

Merycochoerus Leidy, 1858 (synonyms: Superdesmatochoerus Schultz and Falkenbach 1954; Megoreodon Schultz and Falkenbach 1954) g Merycochoerus magnus a M. matthewi

a

M. proprius a Superdesmatochoerus lulli

d

Megoreodon fricki d M. grandis

d

M. grandis loomisi

d

M. grandis loomisi d Oreodontoides Thorpe, 1921 Oreodontoides oregonensis b ?O. curtusb O. (Paroreodon) marshi

b

---

161

---

238

195

---

---

146

---

---

---

---

---

150

---

250

195

---

256

190

---

---

---

---

((235))

168

85.5

(217)

192

89.5

---

---

86

262

204

86

247

182

90

239.5

200

82

254.5

192

89

245

202

85.5

---

---

---

---

---

62.5

((120))

94.5

51.5

(127)

(116)

(58)

147

112.5

---

147.5

---

48.5

g

x

Symbols: ( ), Approximate. (( )), Estimated. Forefoot with five digits. All measurements in millimeters. Sources: a Thorpe, 1937. b Shultz and Falkenbach, 1947. c Shultz and Falkenbach, 1949. d Shultz and Falkenbach, 1954. e Shultz and Falkenbach, 1956. f Shultz and Falkenbach, 1968..g Lander, 1998.

3

The first postcranial oreodont (Mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance

In this study, the first oreodont postcranial skeleton found in Mexico is described.

The fossil was recovered from the Tehuitzingo Fm. (29.9 ± 0.8 Ma, Early Oligocene).

Oreodonts were diverse (~27 genera, 69 species), and fairly abundant.

Mexico’s record includes 2 genera in Chihuahua, 1 in Aguascalientes and 2 in Oaxaca.

The new oreodont was tropical, cursorial, ?herding, and lived in open country.

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CONFLICT OF INTEREST The authors not have conflict of interest. Ismael Ferrusquía Villafranca and José E. Ruiz González