INTRODUCTION Few countries are blessed with as much diversity

Steadman, D.W., and Carranza-Castañeda, Óscar, 2006, Early Pliocene to Early Pleistocene
birds from central Mexico, in Carranza-Castañeda, Óscar, and Lindsay, E.H., eds., Advances
in late Tertiary vertebrate paleontology in Mexico and the Great American Biotic Interchange:
Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias,
Publicación Especial 4, p. 61–71.
61
Early Pliocene to early Pleistocene birds from central Mexico
David W. Steadman1 and
Óscar Carranza-Castañeda 2
Abstract
We report six taxa of fossil birds from late Neogene (late Hemphillian to late Blancan)
local faunas in Guanajuato and Jalisco, central Mexico. They are a grebe (Aechmophorus
cf. elasson), a swan (Cygnus aff. buccinator), three ducks (cf. Anabernicula medium-large
species, cf. Tadornini small species, cf. Nettapus undescribed species), and an unknown
genus of eagle (Accipitridae). This is the first fossil record for the Americas for Nettapus,
a very small anatid that today is represented by three species in tropical Africa, Asia, New
Guinea, and Australia.
Key words: birds, early Pliocene, early Pleistocene, Hemphillian, Blancan, Guanajuato,
Jalisco, central Mexico.
RESUMEN
Se reportan seis géneros de aves del Neógeno tardío (Henfiliano tardío-Blancano temprano)
de las faunas locales de la cuenca de San Miguel de Allende, en el estado de Guanajuato, y
de la cuenca de Tecolotlán, en el estado de Jalisco, en el centro de México. Ellas corresponden a un colimbo (Aechmophorus cf. elasson), a un cisne (Cygnus aff. buccinator), a tres
ánades (una especie de tamaño medio a grande de cf. Anabernicula, una especie pequeña
de cf. Tadornini y una especie no descrita de cf. Nettapus), y a un género desconocido de
águila (Accipitridae). El registro fósil de Nettapus, un anátido especialmente pequeño, que
al presente está representado por tres especies en el África tropical, Asia, Nueva Guinea, y
Australia, es el primero hecho en América.
Palabras clave: aves, Plioceno temprano, Pleistoceno temprano, Hemphilliano, Blancano,
Guanajuato, Jalisco, Mexico central.
INTRODUCTION
Few countries are blessed with as much diversity of
plant and animal communities as Mexico. Spanning
about 18° of latitude (14.6°–32.6°N) and 30° of longitude (86.7°–117.2°W), and with extraordinary variation in climate, elevation, and geology, Mexico is
Florida Museum of Natural History, P.O. Box 117800, University of Florida,
Gainesville, Florida 32611, USA.
E-mail address: [email protected]
2
Centro de Geociencias, Universidad Nacional Autónoma de México, Apartado Postal
1-742, Querétaro, Qro. 76001, Mexico.
E-mail address: [email protected]
1
where the Neotropical and Nearctic biotic regions meet
in fascinating and complex ways (Wiseman, 1980; Ramamoorthy et al., 1993). Like most groups of organisms, the species richness of birds increases in Mexico
with decreasing latitude (Escalante-Pliego et al., 1993).
Unfortunately for those interested in historic biogeography of birds, avian paleontology in the northern Neotropics is an understudied topic. As part of an effort
to improve the Neogene record of birds from Mexico
and the southwestern United States, we report here
on a small collection of avian fossils from six sites in
the states of Guanajuato and Jalisco in central Mexico
(Figure 1).
Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico
2006
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Steadman and Carranza-Castañeda
Figure 1. Locality map of central Mexico, showing Guanajuato, Jalisco, and the location of the fossil sites mentioned in the text.
Based on biochronology of their mammalian fossils, especially horses (Equidae), these local faunas
date from the late Hemphillian to the late Blancan land
mammal ages (Miller and Carranza-Castañeda, 1984;
Carranza-Castañeda, 1989; Carranza-Castañeda and
Espinosa-Arrubarrena, 1994; Bell et al., 2004). The
four sites from Guanajuato date to the early Blancan
(GTO 4, GTO 11, GTO 14, GTO 26), whereas the two
sites from Jalisco date to the late Hemphillian (Jal Teco
20) and late Blancan (Jal Teco 7). Fission-track and
40
Ar/39Ar dates from volcanic ash in Guanajuato and
Jalisco were reported by Kowallis and others (1998).
For the late Hemphillian fauna, the dates range
from 4.8 ± 0.2 to 4.4 ± 0.3 Ma (fission-track) and 4.89
± 0.16 Ma (single 40Ar/39Ar date). The mammalian
fauna collected in these areas corresponds to the latest
records of the equids Dinohippus mexicanus, Astrohippus stockii, Neohipparion eurystyle, and Nannippus az-
tecus; the rhino Teleoceras fossiger; the lagomorph Notolagus velox; the carnivores Agriotherium schneideri,
Borophagus secundus, and Pseudaelurus sp.; and the
giant camel Megatylopus mathhewi.
For the Blancan faunas, the dates range from 4.6
± 0.3 to 3.9 ± 0.3 Ma (fission-track) and 4.74 ± 0.14 to
3.36 ± 0.04 Ma (40Ar/39Ar). Nearly all the mammalian
species and most genera from the late Hemphillian are
absent in the Blancan deposits. The equids are reduced
to Nannippus peninsulatus and Equus simplicidens.
The carnivores are represented by the canid Borophagus diversidens and the cat Felis studeri. The most important findings are the first records of Glossotherium
sp., Neochoerus cordobai, Glyptotherium sp., and the
pampatheriidae Plaina (Carranza-Castañeda and Miller, 2004). These records of immigrants in the Blancan
deposits represent the beginning of the great biotic interchange between the Americas.
Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias
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Early Pliocene to early Pleistocene birds from central Mexico
Osteological terminology generally follows Baumel and Witmer (1993), supplemented as needed by
Howard (1929). Classification of Anatidae at the subfamilial and tribal levels follows Woolfenden (1961).
63
Material. Proximal end of tibiotarsus, IGM 9131, Locality GTO 11 Garbani (early Blancan), Rancho Viejo
area, San Miguel de Allende graben, state of Guanajuato. Distal end of tibiotarsus, IGM 9132, Locality Jal
Teco 7 Las Gravas (late Blancan), Tecolotlán basin,
state of Jalico.
941.P4107, a nearly complete tibiotarsus from the 111
Ranch local fauna, southern Arizona (middle Blancan)
that we refer to Aechmophorus rather than Podiceps
because of the characters already mentioned, as well
as having the gap in the typically podicipedid, two-part
crista fibularis located proportionately closer to the
distal end. The second fossil is AWC 12156, a distal
tibiotarsus from the El Golfo local fauna, northern Sonora (early Irvingtonian). The sizes of the three fossils
are within the range of variation in P. grisegena and
are somewhat smaller than in modern specimens of A.
occidentalis (Table 1). They represent a small form of
Aechmophorus that will be evaluated further in a future paper that focuses on the many specimens from
111 Ranch, which include nearly all major post-cranial
elements.
Remarks. The largest North American species of
Podiceps, P. grisegena, resembles in certain skeletal
elements those of the genus Aechmophorus, which is
represented today by two morphologically nearly identical species, A. occidentalis and A. clarki. Specimen
IGM 9131 is referred to Aechmophorus rather than
Podiceps because the fossa retrocristalis is shallower.
Specimen IGM 9132 is referred to Aechmophorus rather than Podiceps because the dorsal-medial and dorsallateral margins of corpus tibiotarsi are more rounded
(less sharp), and because the corpus tibiotarsi is broad
relative to the overall size of the bone.
IGM 9132 is very similar to two fossils from
Arizona and northern Mexico. The first is MSM 00-
Comparisons and Discussion. The only Blancan species of Aechmophorus that has been described is A.
elasson from the Hagerman local fauna, Idaho (early
Blancan), based on 11 specimens from four skeletal
elements (coracoid, humerus, ulna, tarsometatarsus;
Murray, 1967). Chandler (1990) reported five fossils
(an ulna, femur, tibiotarsus, and two tarsometatarsi) of
A. elasson from the San Diego Formation, California
(Blancan). These two sets of Blancan Aechmophorus
fossils, as well as those from Mexico, are slightly smaller than in available modern specimens of A. occidentalis. This contrasts with late Pleistocene (Rancholabrean
land mammal age) specimens of Aechmophorus from
Fossil Lake (Oregon) and the state of Mexico (Mex-
SYSTEMATIC PALEONTOLOGY
Order Podicipediformes
Family Podicipedidae
Aechmophorus cf. elasson
Table 1. Measurements (in mm) of the distal tibiotarsus of modern and fossil grebes (all adults), with mean, range, and sample size. For
Podiceps grisegena, the 11 specimens are from Ontario, California, and Washington (3 males, 5 females, 3 unsexed). For Aechmophorus
occidentalis, the 8 specimens are from California (3 males, 3 females, 2 unsexed). MPGJ 901, Jal Teco 7 is from the Las Gravas local fauna,
Jalisco, late Blancan). MSM 00-941.P4107 is from the 111 Ranch local fauna, Arizona (early Blancan). AWC 12156 is from the El Golfo
local fauna, Sonora (early Irvingtonian).
Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico
2006
Steadman and Carranza-Castañeda
64
ico), which average larger than in modern specimens
and have been assigned to the temporal subspecies A. o.
lucasi (Howard, 1946; Brodkorb and Phillips, 1973).
Pliocene and Pleistocene grebes are in bad need
of review. All described North American extinct species of Podiceps (see Becker, 1987; Chandler, 1990;
Storer, 2001) are much smaller than the fossils from
Guanajuato and Jalisco, except that P. arndti from the
San Diego Formation may approach a small form of
Aechmophorus in size. The two described extinct genera, Pliolymbus and Pliodytes, are each monotypic
and of uncertain if not unlikely validity (Olson, 1985).
Pliolymbus baryosteus is known from nearly 20 bones
(12 skeletal elements) from the Blancan Fox Canyon local fauna, Rexroad Formation, Kansas (Murray, 1967).
This small species is likely to belong to Podiceps.
Pliodytes lanquisti is based on a coracoid from the late
Hemphillian Bone Valley local fauna, Florida (Brodkorb, 1953). Of the seven characters used to diagnose
Pliodytes, two agreed with the coracoid of Podilymbus, two with that of Colymbus (= Podiceps), and three
were said to be unique (Brodkorb, 1953). Aechmophorus was not considered in the comparisons. The grebe
fossils from Guanajuato and Jalisco represent a much
larger species than either Pliolymbus baryosteus or
Pliodytes lanquisti, regardless of their generic-level
classification.
In interior Mexico today, both Aechmophorus occidentalis and A. clarki are widespread migrants and
local residents as far south as Guerrero, including Guanajuato and Jalisco (Howell and Webb, 1995).
Order Anseriformes
Family Anatidae
Subfamily Anserinae
Tribe Cygnini
Cygnus (Olor) aff. buccinator
Material. Proximal end of carpometacarpus, IGM
9133-10, Locality GTO 26 Zodiaco (early Blancan),
Rancho Viejo area, San Miguel de Allende graben,
state of Guanajuato. Distal end of humerus, IGM 9134,
Locality GTO 14 (early Blancan), Cuesta Blanca, Rancho Viejo area, San Miguel de Allende graben. Distal
end of femur, IGM 9133-2, Locality GTO 26 Zodiaco.
Nearly complete tibiotarsus (two pieces), IGM 9133-1,
9133-5, Locality GTO 26 Zodiaco (Figure 2). Proximal
end of tarsometatarsus, IGM 9135, Locality GTO 4 Arroyo El Tanque (early Blancan), Los Galvanes area,
San Miguel de Allende area.
Remarks. We recognize Cygnus and Olor at the subgeneric rather than generic level, finding the osteological
differences (except for the sternum and furcula) to be
more minor than in Woolfenden (1961). The five fossils
of a large swan from Guanajuato are referred to Cygnus
(Olor) rather than Cygnus (Cygnus) because of 10 of
the 11 characters summarized in Table 2.
Comparisons and Discussion. The fossil humerus
from Guanajuato (IGM 9134, GTO 14) differs from that
of Cygnus (Olor) mariae from the Wickieup local fauna
(late Hemphillian), described by Bickart (1990), in having a deeper and more distinctly defined fossa musculo
brachialis, and in being larger (Table 3). It is likely that
the Guanajuato fossils are part of the lineage leading to
the extant C. (O.) buccinator. The only differences that
are apparent are the fossils’s generally slightly larger
size and the relatively wider condylus lateralis of the
fossil tibiotarsus. The available specimens of swan
from the 111 Ranch local fauna represent a different
species than that from Guanajuato, having characters
of Cygnus (Cygnus) in the coracoid, radius, femur, and
tibiotarsus. The subgeneric classification of Cygnus hibbardi (Brodkorb, 1958) from the Hagerman local fauna,
Idaho (early Blancan) is uncertain, being based on such
poorly preserved material (Bickart, 1990).
Today, Cygnus (Olor) occurs in the Americas as
two species, C. buccinator and the much smaller C. columbianus, each breeding at high latitudes and wintering, at least rarely, as far south as northern Mexico. In
the case of C. buccinator, the only modern records from
Mexico are from coastal northern Tamaulipas (Howell
and Webb, 1995:155). Cygnus (Cygnus) is confined to
the Old World today but had a Pleistocene presence in
North America as Cygnus paloregonus (Rancholabrean age, Fossil Lake, Oregon; Howard, 1946; Bickart,
1990) and as a possibly undescribed species from 111
Ranch, mentioned in the previous paragraph (DWS
personal observation).
Universidad Nacional Autónoma de México, Instituto de Geología and Centro de Geociencias
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Early Pliocene to early Pleistocene birds from central Mexico
A
B
C
65
D
Figure 2. Left tibiotarsus of swans in dorsal aspect. (A) Cygnus (Cygnus) olor ♂, modern specimen, UF 23254 (PB 19318), Denmark. (B)
Cygnus (Olor) columbianus ♂, modern specimen, UF 23236 (PB 18455), Maryland. (C) Cygnus (Olor) buccinator ♂, modern specimen,
UF 23256 (PB 18932), Wyoming. (D) Cygnus (Olor) aff. buccinator, fossil, IGM 9133-1 (IGCU 4816-1), GTO 26, Guanajuato. Scale is
50 mm.
Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico
2006
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Steadman and Carranza-Castañeda
Table 2. Osteological characters to distinguish Cygnus (Cygnus) from Cygnus (Olor). Based on modern specimens of two very large
species, C. (C.) olor (UF 23251, 23254) and C. (O.) buccinator (UF 23252, 23257). *, character that more or less agrees with those of
Woolfenden (1961).
Table 3. Measurements (in mm) of certain skeletal elements of swans. Data for Cygnus (Olor) mariae are from Bickart (1990).
*, measured with a ruler rather than calipers.
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Early Pliocene to early Pleistocene birds from central Mexico
Subfamily Anatinae
cf. Tribe Tadornini
cf. Anabernicula medium-large species
Material. Associated coracoid, humerus (two pieces),
ulna, radius, and manus phalanx, IGM 9133-3, 9133-4,
and 9133-6 to 9133-9, Locality GTO 26 Zodiaco (early
Blancan), state of Guanajuato.
Remarks. This material (Figures 3, 4) is from the right
shoulder and wing of a medium-to-large duck between
67
Tadorna tadorna and T. cana in size, or about the size
of Anas platyrhynchos. These associated bones are
tentatively referred to the Tadornini (based on modern
specimens of Tadorna tadorna and T. cana) rather than
other tribes of Anatinae (especially the Anatini, based
on modern specimens of Anas platyrhynchos, A. fulvigula, A. acuta, and A. strepera) because of the following
characters. Coracoid: agrees with Tadorna in characters 2, 3, and 5 of Woolfenden (1961), but agrees with
Anas in his characters 1 and 4. Humerus (no Woolfenden characters are applicable): attachment of anterior
Figure 3. The coracoid (A-C) and manus phalanx (D-F) of selected ducks. A, D. Tadorna cana ♀, modern specimen, UF 23335 (PB 21250),
captive. B, E. Tadornini medium-large species, fossil, IGM 9133-7 (IGCU 4816-7), IGM 9133-8 (IGCU 4816-8), GTO 26, Guanajuato. C,
F. Anas platyrhynchos ♀, modern specimen, UF 23437 (PB 24326), Michigan. Scale is 30 mm.
Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico
2006
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Steadman and Carranza-Castañeda
A B C
D E F GH I
Figure 4. The humerus (A-C), ulna (D-F), and radius (G-I) of selected ducks. A, D, G. Tadorna cana ♀, modern specimen, UF 23335 (PB
21250), captive. B, E, H. Tadornini medium-large species, fossil, IGM 9133-3 (IGCU 4816-3), IGM 9133-4 (IGCU 4816-4), IGM 91336 (IGCU 4816-6), IGM 9133-9 (IGCU 4816-9), GTO 26, Guanajuato. C, F, I. Anas platyrhynchos ♀, modern specimen, UF 23437 (PB
24326), Michigan. Scale is 30 mm.
articular ligament smaller and more perpendicular to
corpus humeri as in Anas; epicondylus dorsalis larger
as in Tadorna; distal shaft more circular in cross-section as in Anas. Ulna (unstudied by Woolfenden, 1961):
in anterior aspect, the dorsal margin of cotyla dorsalis
more rounded (less pointed) as in Tadorna; disto-ventral margin of cotyla dorsalis indistinct as in Tadorna.
Radius (unstudied by Woolfenden, 1961): very long
overall, relative to humeral length, as in Tadorna; ventral margin of distal 20% of corpus radii sharper but not
dorso-ventrally expanded, as in Tadorna.
Comparisons and Discussion. To summarize, these
associated fossils agree with the shoulder and wing elements of Tadorna in eight characters and with those
of Anas in four characters. The shoulder and wing elements of Tadornini are not highly distinctive from those
of the Anatini. The leg elements, which are not present
in the Guanajuato material, may have more diagnostic
characters (Woolfenden, 1961; Olson and Rasmussen,
2001). The extinct genus Anabernicula is by far the
commonly reported Neogene tadornine fossil in North
America (see reviews by Bickart, 1990; Olson and Ras-
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Early Pliocene to early Pleistocene birds from central Mexico
69
mussen, 2001). We refer the material from Guanajuato
to Anabernicula only tentatively at this time, pending
the ability to make direct comparisons. Bickart (1990)
questioned whether Anabernicula was not more closely related to true geese (Subfamily Anserinae) than to
any form of Anatinae. The material from Guanajuato
clearly represents the Anatinae rather than Anserinae,
regardless of tribal affinity. We agree with Olson and
Rasmussen (2001) that tadornines are members of the
Anatinae.
Material. Humeral end of coracoid, IGM 9136, GTO
11 Garbani (early Blancan), state of Guanajuato.
cf. Tribe Tadornini
cf. Tadornini small-medium species
Tribe Anatini
cf. Nettapus undescribed species
A B
C
D
Remarks. This specimen is from a small-to-medium duck about size of a male Anas clypeata. It is
smaller than in any extant form of Tadornini, but
we tentatively refer it to the Tadornini rather than
other tribe of Anatinae because of its deep sulcus
musculo supracoracoidei, especially toward the humeral end.
E
F
Figure 5. Right tarsometatarsus of small ducks in anconal. A, cf. Nettapus undescribed species, fossil, IGM 9137 (MPGJ 395), Jal Teco 20,
Jalisco. B, Nettapus auritus ♂, modern specimen, UF 38916 (PB 27308), Zimbabwe. C, Anas crecca carolinensis ♂, modern specimen,
UF 23418 (PB 30987), Florida. D, Callonetta leucophrys ♂, modern specimen, UF 23364 (PB 19411), captive. E, Bucephala albeola ♀,
modern specimen, UF 23685 (PB 39862), Massachusetts. F, Oxyura jamaicensis ♀, modern specimen, UF 40856, Florida. Scale is 20
mm.
Carranza-Castañeda, Óscar, and Lindsay, E.H. eds., Advances in late Tertiary vertebrate paleontology in Mexico
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Material. Distal end of tarsometatarsus, IGM 9137,
Locality Jal Teco 20 La Hacienda (late Hemphillian),
Tecolotlán basin, state of Jalisco (Figure 5).
Remarks. This specimen, which represents a species
smaller than any extant New World duck, is referred
to Nettapus rather than to other genera of small ducks,
such as Anas, Callonetta, Bucephala, Mergus, and Oxyura (Figure 5) because of the following characters: narrow overall; foramen vasculare distal relatively large;
foramen vasculare located relatively proximad; trochlea metatarsi medialis relatively narrow; trochlea metatarsi tertii relatively narrow; lateral surface of trochlea
metatarsi medialis less deeply excavated; and incisura
intertrochlearis lateralis relatively wide.
Comparisons and Discussion. This is the first evidence
of Nettapus in the New World, whether fossil or modern. Known as “pygmy-geese” in spite of being in the
Anatini, Nettapus is confined today to tropical Africa,
Asia, and New Guinea/Australia, with one species in
each of these regions. The Mexican fossil is very close
in size and qualitative features to the tarsometatarsus of
the smallest extant species, N. auritus of Africa.
Order Accipitriformes
Family Accipitridae
Genus uncertain
Material. Humeral end of scapula, IGM 9138, GTO 11
Garbani (early Blancan), state of Guanajuato.
Remarks. This specimen does not correspond qualitatively to the scapula of any available living genus of
hawk or eagle, including the Gypinae (accipitrid vultures). It is about the size of the scapula in Geranoetus melanoleucus, being larger than in any species of
Buteogallus, Buteo, or Spizaetus, and smaller than in
Haliaeetus, Aquila, or Harpia. No specimen of Harpyhaliaetus solitarius was available.
DISCUSSION
We have barely scratched the surface of the Late Neogene record of birds in Mexico. Even this small amount
of material, however, provides important new information. For example, the similarity in Aechmophorus
grebes from Guanajuato and Jalisco with those from local faunas from the San Diego Formation (California),
111 Ranch (Arizona), El Golfo (Sonora), and Hagerman (Idaho) is evidence of a single, widespread Blancan and Irvingtonian lineage. The Cygnus (Olor) swan
from Guanajuato is the southernmost record of any
swan, living or fossil, in the Americas. The Nettapus
pygmy-goose from Jalisco is the first American record
for a genus widespread today in the Old World tropics.
We hesitate to speculate whether Nettapus was once
widespread in North and Central America, or whether
it was confined in the Americas to tropical latitudes as
it is today in Africa, Asia, and New Guinea/Australia.
Acknowledgments
For helpful discussions and critical evaluations of the
manuscript, we thank Everett Lindsay and Richard
White. This research was supported in part by NSF
grant BSR-0228682 to DWS, NSF grant EAR 9902898
and The Center for the Field Research Institute, Earth
Watch.
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