First record of a complete humerus of Agriotherium (mammalia-ursidae) from the early-late Hemphillian of Zacatecas, Mexico

Abstract

A complete humerus referred to Agriotherium is described, collected from early-late Hemphillian deposits from Zacatecas. Agriotherium is widely represented by isolated molars, mandibles, and maxillae in early-late Hemphillian faunas of Eurasia and North America. In the literature, postcranial elements are scarce and briefly described with little detail. The greatest diversity is known from the Langebaanweg quarry in South Africa; however, the only complete specimen is from Mexico. The proximal end is described, and the humerus shares similarities with the description of the distal end from South Africa, in which the medial epicondyle and crest of the lateral epicondyle are reduced, which can be considered as a limitation in the hunting of larger prey for food. This implies that Agriotherium was not strictly carnivorous but was a predator-scavenger with an omnivorous diet that included plants and fruits.

Full text

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Carranza-Castañeda and Wang
RMCG | v. 40 | núm. 2 | www.rmcg.unam.mx | DOI: http://dx.doi.org/10.22201/cgeo.20072902e.2023.2.1738
Carranza-Castañeda, O., Wang, X., 2023, First record of a complete humerus of Agriotherium (mammalia-ursidae) from the early-late Hemphillian of Zacatecas,
Mexico: Revista Mexicana de Ciencias Geológicas, v. 40, núm. 2, p. 214-226.
ABSTRACT
A complete humerus referred to Agriotherium is described,
collected from early-late Hemphillian deposits from Zacatecas.
Agriotherium is widely represented by isolated molars, mandibles,
and maxillae in early-late Hemphillian faunas of Eurasia and North
America. In the literature, postcranial elements are scarce and briefly
described with little detail. The greatest diversity is known from the
Langebaanweg quarry in South Africa; however, the only complete
specimen is from Mexico. The proximal end is described, and the
humerus shares similarities with the description of the distal end from
South Africa, in which the medial epicondyle and crest of the lateral
epicondyle are reduced, which can be considered as a limitation in the
hunting of larger prey for food. This implies that Agriotherium was not
strictly carnivorous but was a predator-scavenger with an omnivorous
diet that included plants and fruits.
Key words: Ursidae; Agriotherium; carnivore, humerus; early-late
Hemphillian; Juchipila basin; Zacatecas; Mexico.
RESUMEN
Se describe un húmero completo referido a Agriotherium, recolectado
en depósitos de edad lo más tardío del Henfiliano temprano de Zacatecas.
Agriotherium está ampliamente representado por molares, mandíbulas
y maxilares aislados en las faunas de edad Henfiliano temprano-tardío
de África, Eurasia y América del Norte. En la literatura, los elementos
poscraneales son escasos y descritos someramente con poco detalle. La
mayor diversidad se conoce de la cantera de Langebaanweg en Sudáfrica,
sin embargo, el espécimen de México es el único completo. Se describe
el extremo proximal de Agriotherium que era desconocido; comparte
similitudes con la descripción del extremo distal del espécimen de
Sudáfrica, en el cual el epicóndilo medial y la cresta lateral del epicóndilo
lateral están reducidos, esto se considera una limitación durante la caza
de presas grandes para conseguir alimento. Esto implica que Agriotherium
no era estrictamente carnívoro, sino también un depredador carroñero
con una dieta omnívora que incluía plantas y frutas.
Palabras clave: Agriotherium; carnívoro; húmero; Henfiliano temprano-
tardío; cuenca de Juchipila; Zacatecas; México.
INTRODUCTION
The extinct giant bear Agriotherium in the late Miocene and
Pliocene was well dispersed with a nearly worldwide distribution in
Africa, Eurasia, and North America except for the island continents of
Australia and South America (Jiangzuo and Flynn, 2020). Yet, its fossils
are quite rare, often consisting of fragmentary dental remains. Rarely
documented are its postcranial skeletons, and a complete humerus
has never been reported. For an ursid reaching to a body size of 400
kg and with controversial diet (Sorkin 2006a; Oldfield et al., 2012), its
limb bones contain much-needed information about its locomotion
as well as predatory behaviors.
The faunas of North America have had different displacement
episodes to the south during the Barstovian - Clarendonian -
Hemphillian ages, probably influenced by climatic changes, and
arrival of new invaders from Euro-Asia or due to greater competition
for food during these ages in the faunas of North America (Jiangzuo
and Wang, 2022).
The continuous research in the Juchipila basin, state of Zacatecas,
has demonstrated these southern mammalian movements in early
Hemphillian age in central Mexico, with its importance and diversity
of mammals contained in the sedimentary sequence described in
previous works (Carranza-Castañeda et al., 2013; Tseng et al., 2017;
Carranza-Castañeda, 2022; Carranza-Castañeda et al., 2022). This work
presents the latest discovery, a humerus, the only complete element
known in the early Hemphillian deposits that has been referred to
Agriotherium, in deposits dated to 6.35±0.38 Ma (U/Pb method).
This Mexican specimen possibly represents an individual of the first
migration of ursids from Asia to North America in the late Miocene
Hh3 age (Jiangzuo and Wang, 2022).
Records of Agriotherium in Mexican faunas
The oldest record in North America was described from an
m1 collected in Tehuichila deposits, state of Hidalgo (Freudenberg,
1910, p.205) named Hyaenarctus (=Agriotherium, Stock, 1950).
According to Frick (1926), Freudenberg considered it an ancestral to
the arctotheres of South America, in his interpretation "to have had
unreduced premolars”. Dalquest (1986), only mentioned the records
of Agriotherium in Mexican faunas. The Yepomera fauna in the
northwestern Chihuahua is the most important Hemphillian-Blancan
fauna because of its diversity, Agriotherium cf. schneideri is part of the
REVISTA MEXICANA DE CIENCIAS GEOLÓGICAS
v. 40, núm. 2, 2023, 214-226
DOI: http://dx.doi.org/10.22201/cgeo.20072902e.2023.2.1738
First record of a complete humerus of Agriotherium (mammalia-ursidae) from
the early-late Hemphillian of Zacatecas, Mexico
Oscar Carranza-Castañeda1, and Xiaoming Wang2,3,*
1 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230, Querétaro, Querétaro, Mexico.
2 Department of Vertebrate Paleontology, Natural History Museum of Los Angeles, 900 Exposition Blvd, Los Angeles, CA 90007, USA.
3 Institute of Vertebrate Paleontology and Paleoanthropology, 142 Xizhimenwai Dajie, Western District Beijing 100044, China.
* carranza@geociencias.unam.mx; ** xwang@nhm.org

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First record of Agriothrium from the early-late Hemphillian, Zacatecas, Mexico
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associated fauna (Merriam et al. 1916; Stock 1950). A further record of
Agriotherium cf. A. schneideri has been found from the late Hemphillian
fauna of Matachic, state of Chihuahua (Lindsay et al., 2006).
During a new research supported by Universidad Autónoma del
Estado de Hidalgo, in the Zacualtipan basin, state of Hidalgo, lower
molars and other fragments referred to Agriotherium, plus horse teeth
of Calippus and the peccary Prosthennops sp., were discovered in the
Zietla deposits of early-late Hemphillian age (Carranza-Castañeda,
1991, 1994). Another report, of Tecolotlán basin state of Jalisco,
isolated molars of Agriotherium that were collected in San José deposits,
referred to late Hemphillian (Carranza-Castañeda and Miller, 2004).
In San Miguel Allende basin a complete jaw and maxillary, that surely
belong to the same individual, were collected in the Rinconada locality
in the same stratigraphic level (Carranza-Castañeda, 1992; Miller and
Carranza-Castañeda, 1996; Carranza-Castañeda, 2006), Figure 1a.
Agriotherium in faunas of North America
Agriotherium is widely represented in the late Miocene faunas at
ages ranging from 7 to 3.6 Ma (Tedford et al., 2004). The only middle-
Blancan record is known from Hagerman fauna, Idaho (Samuels et al.,
2009). Most records of Agriotherium are described based on isolated
upper and lower molars, plus maxillae, and mandibles (Sellars, 1916;
Dalquest, 1986; Everhart and Hawkins, 2020). Almost all these reports
do not mention postcranial elements. Schultz and Martin (1975)
described and illustrated a complete radius (UNSM 6013) from the
late Hemphillian, Ash Hollow Formation in Ogallala Group (Hunt,
1998). From the Edson Quarry, Kansas, Harrison (1983) described
Agriotherium based on an upper canine, including the so far widest
representation of postcranial elements, such as radius, ulna, metacarpus
III and IV as well as part of a metapodial; however, she only describes
a distal fragment of a humerus with part of the lateral epicondylar
crest, that according to the brief description matches with the Mexican
specimen.
Hendey (1976, 1980), mentions a collection of postcranial ele-
ments known, all referred to Agriotherium africanum including from
Langebaanweg Quarry in South Africa: skull, jaws, upper and lower
molars, and different postcranial elements, also scapula and humerus
fragments that correspond to different individuals. However, the
comparisons with the humerus presented in this work, are made with
a fragment of humerus illustrated in Hendey (1980, p. 32, fig. 15),
and with information provided from Samuels et al., (2013). The main
objective of this paper is to describe this new specimen (MPG 5676)
from the Zac Juch 47 El Resbalón locality (Figure 1a), which is the
only complete known humerus in the early-late Hemphillian faunas.
ABBREVIATIONS
Jal: Jalpa; Juch: Juchipila; Tab: Tabasco; LACM: Los Angeles
County Museum of Natural History; MPGJ: Museo de Paleontología
Geociencias Juriquilla, Querétaro, México; TMM: Texas Memorial
Museum, University of Texas at Austin, Jackson School of Geosciences;
Zac: Zacatecas; FMNH: Field Museum Of Natural History, Chicago;
MLP: Museo de la Plata; USNM: United States National Museum;
UCMP: University of California Museum of Paleontology; UNAM:
Universidad Nacional Autonoma de Mexico; USA: United States of
America; km: kilometer; m: meter; cm: centimeter; mm: millimeter.
METHODS
The humerus described in this work was prepared in Laboratory of
Paleontology of Centro de Geociencias, Juriquillla, UNAM, Querétaro,
and is cataloged and housed in the Paleontological collections of the
same Institution. According to different authors (Hendey, 1980; Wolff
et al., 1973; Sorkin, 2006a) no complete humerus of Agriotherium
is known. In general, it is represented by fragmented material from
different individuals. The Mexican specimen was compared with the
illustration of Hendey (1980, fig. 15 L45063), and fragmented humerus
of Arctodus from La Brea Tar Pits and Museum, Los Angeles. Other
comparisons were made with TMM M-3773 Ursus arctos, TMM
M-137 Ursus americanus and recent material of TMM M-7503 Ursus
maritimus specimens catalogued in Vertebrate Paleontology collections
of Jackson School of Geosciences, University of Texas and UCMP
22362 type specimen of Indarctos oregonensis. The measurements
mentioned were taken according to the illustrations of Crouch (1969).
Illustrations of other consulted publications were considered (Sorkin,
2006b); Davis, 1964; Torres Perez-Hidalgo, 1988; Samuels et al., 2013).
The measurements mentioned are in millimeters, as defined in Figure
2 of this paper. Numbers between square brackets are identifications
of the measurements shown in Table 1.
BIOSTRATIGRAPHY OF JUCHIPILA BASIN
The stratigraphy of the Juchipila basin has been logged by Aranda-
Gómez (described in Carranza-Castañeda et al., 2013), in selected areas
where the greatest diversity and number of fossil mammals from the
early Hemphillian have been recovered.
The Juchipila basin (Figure 1) has an extension of 165 km long
and 15 km wide, with its long axis trending NNE. The Miocene-early
Pliocene continental deposits that fill the basin were informally
described as Juchipila Formation by Lahiere (1992) and López (1991).
The deposits that fill the basin are from lacustrine, alluvial, floodplain,
and pyroclastic origin. The fossil mammals are found in these deposits
throughout the basin, although important localities are around the
town of Huanusco (Zac Tab 32 and Zac Tab 37, Figure 1b). In the
central part of the basin, El Mixtón deposits are found (Zac Juch 30,
Figure 1b), which contains the greatest diversity of mammals. South of
Jalpa city is Cofradía (Zac Jal 42, Figure 1b), where records of Cosoryx
sp. and Gomphotherium hondurensis were recovered.
The stratigraphic sequence that contains the greatest diversity of
early late Hemphillian mammals is found south of Juchipila city. The
most important localities of this stratigraphic sequence are: El Resbalón
(Zac Juch 47), La Copa (Zac Juch 48) and El Epazote (Zac Juch 51), due
to the diversity of recovere d mammals: Sphenophalos, Gomphotherium
hondurensis, Neohipparion trampanense, Dinohippus interpolatus,
Calippus hondurensis, Enhydritherium terranovae, Alforjas taylori,
Megalonichid? Teeth (Carranza-Castañeda et al., 2013; Tseng et al.,
2017). Recently, in El Resbalón locality, we collected the southernmost
record of Agriot herium of the early-late Hemphillian age known outside
of the USA faunas.
The stratigraphic sequence of El Resbalón is 45–55 m thick. The
strata lie horizontal without evidence of folding, however, laterally the
strata have significant changes in lithology. The base of the sequence
is a layer of clayey sand with a thickness of 10 m, a layer of dark gray
volcanic ash laterally interrupts the sequence. Towards the top, the
sequence is an alternation of clay strata with different degrees of
compaction and thickness. The Agriotherium humerus was recovered
at 15 m from the bottom of the section (Figure 1c).
The ash below the strata, bearing Agriotherium fossil, was
analyzed by 207Pb/235U method, and gave an age of 6.35 ± 0.38 Ma. In
the last 22 m towards the top, the sequence is an alternation of clay,
sandy-gravel, and clay layers, with different degrees of comp action and
thickness.

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200 0 200 400
kilometers
Yepómera
116º W 112º 108º 104º 100º 96º W
32º N
28º
24º
20º
16º N
Yepómera
ZACATECAS
Rinconada,
Gto.
Rinconada,
Gto.
Tehuichila and
Zietla, Hgo.
Tehuichila and
Zietla, Hgo.
Matachic
Town
Cenozoic volcanic rocks
Lake and fluvial
deposits (Miocene -
Pliocene)
Gravel deposits
(Pleistocene?)
Modified from Servicio Geológico Mexicano, 1998
T
Ja
H
Ju
M
SR
Zac Juch 30
I
I
I
I
I103° W
103° W
22° N
21°
30’ N
30’ N
21°
30’ N
5 km
Zac Tab 32
Zac Tab 32
Zac Tab 37
Zac Tab 37
Zac Tab 34
Zac Tab 34
A
Normal fault
Juchipila river
Strike and dip of
bedded units
Strike ofvertical beds
Zac Juch 47
Zac Juch 47
Zac Juch 48
Zac Juch 48
Amoxóchitl
Zac Jal 42
Zac Jal 42
Zac Jal 57
Zac Jal 57
Te
López’ strat. section
Vertebrate fossil
localities
Achoquén dam
Juchipila
formation
Tecolotlán, Jal.
Tecolotlán, Jal.
JUCHIPILA
JUCHIPILA
– Agriotherium sp.
– Calippus hondurensis
– Megalonichidae
– Cosoryx sp.
– Sphenophalos sp.
– Enhydritherium terranova
– Calippus hondurensis
– Calippus hondurensis
0
55
m
15
– Dinohippus interpolatus
– Gomphotherium hondurensis
6.35 ± 0.38 Ma
6.35 ± 0.38 Ma
5.59 ± 0.11 Ma
5.59 ± 0.11 Ma
Mexico City
Mexico City
Atemajac
a)
b)
c)
– Dinohippus interpolatus
– Neohipparion trampanense– Neohipparion trampanense
– Alforjas taylori
Zac Juch 51
Zac Juch 51
Figure 1. a) Map of Mexico, showing the states with records of Agriotherium. b) Geologic map of the Juchipila basin, with the location of the most important localities by their diversity of mammals (modied from
Aranda-Gomez in Carranza-Castañeda et al., 2013). c) Composite biostratigraphic column in El Resbalón section, showing the approximate position of Agriotherium and associate fauna.

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10 cm
0
5
5
1
6
6
3
3
4
4
2
a) b)
The highest ash in the sequence, analyzed by the U/Pb method,
gave an age of 5.59 ± 0.11 Ma. The stratigraphic sequence ends in a
caliche layer less than 50 cm thick. It is important to point out that
Calippus molars are found throughout the entire sequence (Carranza-
Castañeda et al., 2013), Figure 1c.
Order Carnivora
Family Ursidae Fischer von Waldheim, 1817
Genus Agriotherium Wagner, 1837
Agriotherium sp.
Figures 2a, 3, 4 and 5a, 6 and Table 1.
Material. MPGJ 5676 left humerus, in good condition without
distortion.
Locality and Age. Locality El Resbalón (Zac Juch 47, Figure 1), of
Juchipila basin, state of Z acatecas, Mexico. Age early-late Hemphillian.
Description. MPGJ 5676. The humerus is a long, stout and almost
straight bone. The head, that occupies all the proximal end, is distinctly
large and rounded. The part that articulates with the scapula is slightly
convex with rounded shape projecting posteriorly to the shaft.
The anterior part the bicipital groove is limited by a shallow fossa
between the tuberosities; which are offset to the internal side (Figure
2a measurement [1], and Figure S1a of the Supplementary Material).
The anteroposterior axis (Figure 2a and S1a) from the middle of
the bicipital groove to the posterior edge is long ([2] in Figure 2a), and
the transverse axis including the posterior extension of the tuberosities
almost have the same dimension ([3] in Figure 2a).
The lateral tuberosity is not completely ossified (Figure 2a, and
S1a), forming an elongated-convex and robust structure; it protrudes
slightly from the articular surface, and is partially incomplete although
it is possible to visualize the continuity through the posterior middle
part of the articular surface of the head ([5] in Figure 2a).
The medial tuberosity is a smaller and low structure that project
slightly from the surface of the head, continues toward posterior part
by a thin projection around the margin of the head ([6] in Figure 2a).
The upper lateral side of the proximal end is not complete (Figure
3a and 3b, and S1d); however, it is possible to observe a deep concave
depression surface for insertion of the infraspinatus muscle. The medial
side of the proximal end has a wide, concave, and elongate impression
(Figure 3b and S1d2) with multiple foramina and scars, for insertion
of the subscapularis muscle.
The diaphysis is long, almost straight, and robust (Figure 4, and
Figure S2 of the Supplementary Material). The anterior side has a flat
triangular shape arrangement by the cephalo-humeral surface, which
originates from the base of the lateral tuberosity ([10] in Figure 4), and
the widest part is in the middle ([9] in Figure 4); it is a long, rough, and
flat structure bounded in the inner side throughout by the prominent
Figure 2. a) MPGJ 5676, Agriotherium le humerus, proximal view. e bicipital groove is wide, a fossa separates the tuberosities. It is remarkable the large size
of the medial tuberosity that continues by a thin projection around the margin of the head. Note the dierences in the hemispherical shape of the head. b) Recent
Ursus maritimus humerus, with the lateral tuberosity larger and higher, the medial tuberosity greatly reduced. e head is more ovoid and there is no fossa between
the tuberosities. All measurements are in Table 1.

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Numbers
in Figures
Agriotherium
MPGJ 5676
Ursus
maritimus
TMM M-7503
Ursus
maritimus*
USNM 218230
Ursus
americanus*
USNM 275184
Ursus
arctos*
LACM 30466
Ursus
arctos*
LACM 30586
Ursus
arctos°
FMNH 27268
Arctodus
simus°
FMNH-M 24880
Actotherium
angustidens +
MLP 35-IX.26
[1] Maximum distance between
the posterior part of the
tuberosities.
90 85       
[2] Maximum anteroposterior
axis of the head.
98 91       156
[3] Transversal axis including
the extensions of
tuberosities.
96 87       130
[4] Maximum transversal axis
of articulation of the head.
84 77       
[5] Anteroposterior length of
the lateral tuberosity.
86 85       
[6] Length of the medial
tuberosity including the
rim.
86 45       
[7] Greatest anteroposterior
width of the sha.
74 5043 43.33 30.82 35.06   91
[8] Maximum length of the
humerus including the
proximal and distal ends.
507 428 418.96 350.9 284.81 301.45 383 580 620
[9] Maximum transversal
width of the sha in the
middle of the deltoid
surface.
-63 66       84
[10] Maximum length of
the delto-pectoral ridge
including the tuberosity.
334 245 288.81 230.33 191.35 209.95 291 370 
[11] Maximum high of the
external epicondylar crest.
142 158       
[12] Greatest distal width
between the epicondyles.
126 144 95.38 73.73 62.48 63.27   
[13] Height of the nutritive
foramen.
225 151       
[14] Transversal width of the
lateral epicondylar crest.
85 -       
[15] Maximum width of
distal end between the
epicondyles.
125 123 144.41 104.67 85.4 89.4   185
Table 1. Measurement and comparisons between Agriotherium (MPGJ 5676) from El Resbalón locality and bears cited; Samuels et al., 2013*, Sorkin 2006a°, Soibelzon
et al., 2011+. Square braquets apply only for the measurements of Agriotherium. Measurements in mm.
MPGJ: Museo de Paleontología Geociencias Juriquilla, Querétaro, México; TMM: Texas Memorial Museum, University of Texas at Austin; USNM- United States National Museum; LACM: Los Angeles
County Museum of Natural History; FMNH- Field Museum of Natural History, Chicago; MLP- Museo de la Plata.

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10
0
cm
a) b)
fossa separated from the radial fossa by a convexity, which is small
and shallow, the lateral epicondylar ridge begin in the border of the
radial fossa.
In the posterior side, the olecranon fossa is deep and moderately
wide, and oriented towards the lateral epicondyle (Figure 6).
The nutrient foramen is a little higher in the posterior face (Figure
4b [13]), and there is no evidence of a scar of entepicondylar foramen
(Figure 4b).
Taxonomic remarks. While describing a new species, Agriotherium
hendeyi, from the late Hemphillian Quiburis Formation of Arizona,
Jiangzuo and Flynn (2020) provided the most recent taxonomic
treatment of North American Agriotherium, largely based on a
hypothesized transformation series in the metaconid-entoconid
complex in the lower molars. Although they did not treat in detail
all previously named species of Agriotherium from North America,
they concluded that there are only two valid species, A. hendeyi and
A. schneideri, with the former confined to Arizona only and the
latter including the rest of North American records. Furthermore, A.
hendeyi is a small species, substantially smaller than A. schneideri. By
these criteria, our Zacatecas humerus likely belongs to A. schneideri,
as all known dental materials from Mexico were referred to the latter
(Carranza-Castañeda and Miller, 2004).
From the early Hemphillian (Hh2) of North America, Jiangzuo
and Hulbert (2021) also recognized another large ursid, Indarctos,
including humeri from the Rattlesnake Formation of Oregon (Merriam
et al., 1916), Hi Level Quarry of Nevada, and Withlacoochee River
4A of Florida. Relationships between Indarctos and Agriotherium are
not clear, but in general, Indarctos tends to be more hypocarnivorous
with a distinct M2 talon and may belong to a distant relative of the
and robust deltoid crest that ends in the delto-pectoral tuberosity. The
pectoral ridge merges in external border of the lateral tuberosity and
between them is a flat surface for the insertion of the cephalo-humeral
muscle (= clavotrapecius + clavodeltoid). The upper part is slightly
wider and more convex outwards, because the origin of the pectoral
crest denotes that the surface is broader and lightly convex (Figure
4a and S2).
Halfway down the diaphysis, the cephalo-humeral crest begins to
reduce (Figure 4a [10]) and becomes narrower due to the convergence
of deltoid and pectoral crests, which join a little more than halfway up
the diaphysis and form the delto-pectoral tuberosity, ending below the
pointed border at the upper external lateral epicondylar ridge.
The distal end is wider, the articular surface is the most developed
part, the transverse axis across the epicondyles is 75 % narrower
(Figure 4a [15]).
The medial epicondyle is a prominently marked convex structure
that projects out of the shaft (Figure 4a, 5a, S1c and S2a), with several
concave scars for the muscle insertion of the wrist and digits. Its
separation from the lateral epicondylar ridge is the widest part of the
distal end. The lateral epicondylar ridge is distinctly thin and triangular
in outline. It is located above the radial fossa. The external border of this
structure is curved and thick with two deep grooves in the middle for
the extensor muscles of the hand. It joints with the shaft a little above
where the delto-pectoral tuberosity ends. This structure occupies about
32 % of total length of the shaft (Figure 4a [11], and the total distance
between the epicondyles represents the widest part of the distal end
(Figure 4a [15], 5a, S1c and S2a).
The internal condyle in the Mexican specimen is a concave surface
where the ulna articulates; above is the larger and deep coronoid
Figure 3. MPGJ 5676 Agriotherium le humerus. a) Upper lateral side with depression surface for insertion of infraspinatus muscle. b) Medial side with attachment
for subscapularis muscle.

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6"
3"
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10
c)
8
8
10 cm
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0
1"
11
11
10
10
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1
1"
14
15
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a) b)
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8
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giant panda clade Ailuropodinae (Abella et al., 2012; Qiu et al., 2014),
whereas others regarded Agriotherium to be descended from Indarctos
(Hendey, 1980). D enta lly, A. schneideri may also have mixed characters
of both Agriotherium and Indarctos (Hunt, 1998; Jiangzuo et al., 2019).
Regardless the relationship (or lack thereof) of North American
Agriotherium and Indarctos, based on dental morphology, all known
Mexican late Miocene ursids are clearly identifiable as belonging to
A. schneideri.
COMPARISON
The main objective of this work is to describe the humerus of
Agriotherium collected in El Resbalón locality, the only complete
specimen known in faunas of the early-late Hemphillian and
equivalent beds in Mexico, Africa, and Eurasia. Records of humerus
of Agriotherium are very scarce, an also the proximal end is always
missing. We made comparisons with humeri from recent bears housed
Figure 4. MPGJ 5676 Agriotherium le humerus. a) Anterior view: the cephalo-humeral surface, form the delto-pectoral tuberosity. e medial epicondyle is
convex and prominent, and the lateral epicondylar ridge is wider and occupied the 32% height of the diaphysis. b) Posterior view: olecranon fossa reduced, and
straight. c) Lateral view: the head is hemispherical in shape, protrudes posteriorly. All measurements are in Table 1.

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10 cm
0
a) b)
in the collection of the Jackson School of Geosciences, University of
Texas. In Ursus americanus (TMM M-137, Figure 7) from Alaska, the
medial condyle is long and ends in a notable hook. In Ursus arctos
(TMM M-3773), a young animal, the proximal epiphysis is still
separated from the diaphysis, and the medial condyle is prominent.
In an adult individual (LACM 625, collection of the Los Angeles
Museum), the medial condyle is long and ends in a hook. The same
structure is well developed and is pointed in Indarctos oregonensis
(UCMP 22362, Figure 8).
When compared the proximal end in b oth specimens, Ursus arctos
and Ursus americanus, its medial tuberosity is small and the lateral
tuberosity is larger, the bicipital groove is narrower in the anterior
part; these are the main differences with the Mexican specimen. In
Ursus maritimus, the lateral tuberosity is larger and slightly higher, is
wide and form a rounded-concave depression with multiple foramina,
ending in the middle of the head. The bicipital groove is narrower and
relatively deep. The medial tuberosity is smaller and narrower in Ursus
maritimus, and the lateral tuberosity is larger than in the Mexican
specimen (Figure 2).
In the upper lateral side in Ursus maritimus the impression for
the infraspinatus muscle is wide and deep, occupying all the lateral
side. In Agriotherium from México, this structure is incomplete, but it
is possible to see that it is rounded and deep (Figure 3a dotted line).
In the proximal medial side, the fossa for the subscapularis muscle is
wide and directed toward the posterior part of the shaft and is larger,
in contrast to the Mexican specimen. The head of the humerus that
articulates with the scapula is more rounded in the specimen from
Mexico (Figure 3b and S1d2).
Hendey (1980) describes a humerus fragment of Agriotherium
from the Langebaanweg quarry in South Africa. It is important to point
out that the form and structures are like those mentioned in this work
for the complete humerus of Zac Juch 47 from El Resbalón, including
measurements provided for the two specimens, they are within the
range of measurements of the Mexican specimen and the differences
are not greater than 10 %.
In the Hendey illustration (Hendey, 1980), the medial epicondyle
where the insertions for the phalangeal and metacarpal muscles of
the hand are found, is comparable in shape and size with the Mexican
specimen (Figure 4a and S2a).
In Ursus maritimus, its large and tapered medial epicondyle
suggests that the extensor muscles of the hand and fingers are more
developed and efficient during hunting, as the metacarpal and
phalangeal muscles are more important for prey retention (Sorkin,
2006a). This shows that these bears were more active predators and
Figure 5. a) MPGJ 5676 le humerus of Agriotherium anterior view: the medial epicondyle is convex and protrude from the sha. e condyloid fossa is large and
deep; two grooves meet in the middle, the sha end little above the deltoid tuberosity. b) TMM M-7503, Ursus maritimus anterior view: the medial epicondyle is
large and hooked; the lateral epicondylar crest is concave in all the extension; no grooves are present along the edge.

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In Ursus arctos, the crest of the lateral epicondyle is slightly concave,
ending just above the delto-pectoral tuberosity. In all these bears the
grooves in the lateral epicondylar ridge are absent (Sorkin 2006a, fig.
11 B). Modern tremarctine bears (Vela-Vargas et al., 2021) and their
extinct relative Arctodus (such as a humerus from La Brea Tar Pits
and Museum, we examined), preserve the entepicondylar foramen,
which is not present in Agriotherium and has not been considered in
this comparison (Figure 7).
When comparing Indarctos orogenesis (UCMP 22362) with the
anatomical structures of Agriotherium, they have great similarity.
The measurements illustrated in Figure 8 were calculated using the
scale provided in the photograph. The most important thing is that
the measurements obtained are none greater than 10%, which allows
objectively considering the size of Indarctos and their comparison
with Agriotherium. The biggest difference is the presence in Indarctos
of an entepicondylar foramen which does not exist in Agriotherium
(Jiangzuo and Hulbert, 2021). Also, this foramen is present in Indarctos
atticus from Attica, Greece (Roussiakis, 2001). In general, all the
anatomical characters of Indarctos match with the Mexican specimen,
the differences fall in the individual variability on two populations of
bears; however, the principal differences are the length the humerus, the
medial epicondyle shape and the presence of entepicondylar foramen
absent in the Mexican specimen, although in these specimens the sex
is unknown.
According to the description of the Pleistocene bears of Spain
(Torres Perez-Hidalgo, 1988), the lateral epicondylar ridge has a great
10 cm0 1 5
had a specifically carnivorous diet. In Agriotherium, the same muscles
may be reduced, which implies less efficiency in the retention of
large prey and the diet could be not only carnivorous, but could also
be supplemented with carrion, as well as some plants and fruits.
In contrast, some authors suggest that Arctodus simus was more
herbivorous (Emslie and Czaplewski, 1985). However, there are also
views that Arctodus simus was predominantly carnivorous but obtained
most of the animal material in its diet through scavenging (Matheus,
1995, 2003, Figures 4a, 7d, and S2a).
In Agriotherium from El Resbalón, the crest of the lateral
epicondyle begins at the external condyle, and the external border
limits the radial fossa. In the middle part, the humerus has a thick rim
with two wide grooves, which are not described as part of the same
structure in bears from the Pleistocene of North America or Europe, as
illustrated in Ursus maritimus (Figure 5b and 7d). In Ursus maritimus
(Figure 5b) the lateral epicondylar ridge is different from the Mexican
Agriotherium. The crest begins in the external condyle as a concave
structure throughout, which includes the radial fossa, continues
towards the proximal region, and ends in a ridge that curves towards
the shaft, ending in the same plane as the delto-pectoral tuberosity, and
the grooves are not present. However, in Ursus americanus the crest
is shorter, ending at the same height as the delto-pectoral tuberosity
(Figure 7a).
In contrast, in Ursus maritimus and other ursids such as Arctodus
simus, Ursus americanus and Ursus arctos, the medial epicondyle has
a similar shape and size, and is highly developed and hook shaped.
Figure 6. Distal end of the humerus: the olecranon fossa is observed wide and straight located between the epicondyles.

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a)
C)
10 cm
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c)
d)
b)
diversity of forms as observed in Agriotherium and its comp arison with
other bear materials. The crest has a similar shape but with different
size and structures that, undoubtedly, have implications in the use of
brachialis and brachioradialis muscles.
REMARKS ON DIET AND LOCOMOTION
With an average body mass estimated at 700–800 kg (Christiansen,
1999), the Pleistocene short-faced bear, Arctodus simus, and it’s even
larger South American relative, Arctotherium angustidens, were the
largest terrestrial carnivorans in the Americas (Soibelzon and Schubert,
2011) and were important predators during the Ice Ages. The late
Miocene-Pliocene Agriotherium is similarly gigantic and may have
served as top predators of their time. Its much wider distributions, from
Africa through Eurasia to North America, also suggests that it may
have played an important ecological role in a wide range of habitats.
Hannold et al . (2021) recently studied carbon isotope geochemistry
of Agriotherium schneideri from the latest Hemphillian Yepómera site
in Chihuahua, Mexico. It has a mean δ13C value of -5.7±0.3 ‰ from
Figure 7. a) TMM M-3773, Ursus arctos; b) LACM 625, Ursus arctos. c) TMM M-137, Ursus americanus; d) TMM M-7503, Ursus maritimus. In a, c and d), the
medial condyle is developed, forming a hook. e lateral epicondylar crest is concave throughout its length, and it does not have grooves. In d) the delto-pectoral
tuberosity is longer than the epicondylar crest.

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538.35 mm
538.35 mm
298.89 mm
298.89 mm
66.50 mm
66.50 mm
163.77 mm
163.77 mm
10 cm
0
enamel samples (n=2). Correcting for a carnivorous lifestyle and
associated +8 ‰ enrichment adjusting for body size, Hannold et al.
(2021) arrived at a mena δ13C diet of -13.7±0.3 ‰ and concluded
that Agriotherium may have consumed preys of C4 diet, although
they cautioned that isotopic signatures of modern ursid analogues
were not known well enough to be certain of the conclusion. Miller
and Carranza-Castañeda (1996), on the other hand, suggested that
Agriotherium was a cursorial predator-scavenger and an omnivore
with a wide-ranging diet, as well as able to crush bones. Indeed, it is a
safer assumption of an omnivore diet for Agriotherium given that living
ursids are all omnivorous except for the polar bears and giant pandas.
As the proportions of meat and plant consumptions are intensely
debated for the Pleistocene big, short-faced bear Arctodus simus, with
a mass estimated of 590–630 kg by Kurtén (1967) and of 700–800 kg
by Christiansen (1999) for males, is considered an hypercarnivore.
Figueirido et al. (2010) and Soibelzon and Schubert (2011) describe
the largest bear from South America Arctotherium angustidens, with
a body mass estimated between 1588–1749 kg, and suggest that it was
the largest known bear and probably the most powerful terrestrial
carnivoran of the late Cenozoic.
The diet of Agriotherium will likely remain controversial, partly
for lack of a modern ursid analogue. Presence of a premasseteric fossa
in the lower jaws of both Arctodus and Agriotherium, used as evidence
of large amount of plant materials in their diets (Sorkin, 2006a), is
tantalizing but its muscular functions remain unexplored.
With a total humeral length of 507 mm (Table 1), Agriotherium
schneideri has comparable front limb length of Arctodus simus
(e.g., Emslie and Czaplewski, 1985; Richards and Turnbull, 1995;
Christiansen, 1999; Sorkin, 2006a; Figueirido et al., 2010), although
our Mexican specimen is smaller than the largest individuals of the
latter. As suggested by Sorkin (2006a), these two giant bears may
share ecological similarities such as diet and locomotion. However,
detailed anatomical differences as outlined above are likely inherited
from distinct evolutionary lineages that went t heir separate ways many
million years ago.
CONCLUSIONS
The MPGJ 5676 humerus is the only complete humerus of
Agriotherium known. It shows great similarity with the characteristics
in L 45063 mentioned by Hendey (1980). In particular, the Mexican
specimen shares many similarities with the only known distal end
from South Africa especially on the medial epicondyle, where the
metacarpal and fingers muscles are inserted, and on the size and shape
of the lateral epicondylar crest where the brachialis and brachioradialis
muscles originate allowing the rotation of the arm. The reduction of
these structures is a factor that limited the effectiveness of Agriotherium
for hunting and retention of large prey. These factors influenced the
diet of Agriotherium that was not strictly carnivorous (Sorkin, 2006a;
Hendey, 1980; Oldfield et al., 2012).
ACKNOWLEDGMENTS
Research on the late Neogene mammalian fauna in the central
region of Mexico has cont inue d with the support of the direction of the
Centro de Geociencias, Campus Juriquilla, of the Universidad Nacional
Autónoma de México. Grateful for the support of the DGAPA, UNAM
Project IN102817, funded research in the central region of Mexico and
a grant from the National Science Foundation (EAR EAR-1949742).
I am grateful to Dr. Chris Sagebiel, curator of the Vertebrate collection,
and Kenneth Bader, manager of the Osteology Laboratory, Jackson
School of Geosciences, University of Texas at Austin, gratitude for all
the facilities that provided the comparison material for this research.
I especially appreciate the invaluable participation of Eng. J Jesús
Silva Corona during the collection of the Agriotherium specimen,
the edition of the manuscript and illustrations of this work. To an
anonymous reviewer, thank you for your contribution to improve this
manuscript. To Dr. Joshua X. Samuels, for his critical review of the
manuscript and his contribution of information that improved this
work, as well as the photos of Indarctos Oregonensis UCMP 22362
that served to compare the specimen from Mexico. Our thanks to
Biologist Hilda Troncoso Altamirano who worked on the preparation
of the specimen. Dr. Carlos Ortega, for the analysis and dating of the
Figure 8. UCMP 22362 Indarctos oregonensis anterior view of the humerus:
the main dierence is the presence of the entepicondylar foramen, absent
in Agriotherium. e measurements provided were calculated based on the
scale of the photograph.

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ash associated with the humerus. Master in IEVEA Cesar Contreras
Zamora for the Technical Support in computing processes and Eng.
Bernardino Rodríguez Morales, for their collaboration in the adapta-
tion of the Museum for the preservation of the fossils in the collection.
SUPPLEMENTARY MATERIAL
For a better view, Figures S1 and S2 illustrate, in high definition
and without dimension lines, the figures of the Mexican Agriotherium
shown in this paper. Figure S1 joins Figures 2a, 3, 5a and 6, while
Figure S2 shows the Figure 4 of the previous text. This material can be
downloaded at www.rmcg.unam.mx, in the abstract page of this paper.
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Manuscript received: january 10, 2023
Corrected manuscript received: may 2, 2023
Manuscript accepted: may 3, 2023