Steadman, D.W., and Carranza-Castañeda, Óscar, 2006, Early Pliocene to Early Pleistocene 61 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. 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 correspon- den 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 where the Neotropical and Nearctic biotic regions meet in fascinating and complex ways (Wiseman, 1980; Ra- Few countries are blessed with as much diversity of mamoorthy et al., 1993). Like most groups of organ- plant and animal communities as Mexico. Spanning isms, the species richness of birds increases in Mexico about 18° of latitude (14.6°–32.6°N) and 30° of lon- with decreasing latitude (Escalante-Pliego et al., 1993). gitude (86.7°–117.2°W), and with extraordinary varia- Unfortunately for those interested in historic biogeog- tion in climate, elevation, and geology, Mexico is raphy of birds, avian paleontology in the northern Neo- tropics is an understudied topic. As part of an effort to improve the Neogene record of birds from Mexico 1Florida Museum of Natural History, P.O. Box 117800, University of Florida, and the southwestern United States, we report here Gainesville, Florida 32611, USA. E-mail address: [email protected] on a small collection of avian fossils from six sites in 2Centro de Geociencias, Universidad Nacional Autónoma de México, Apartado Postal the states of Guanajuato and Jalisco in central Mexico 1-742, Querétaro, Qro. 76001, Mexico. E-mail address: [email protected] (Figure 1). CARRANZA-CASTAÑEDA, ÓSCAR, AND LINDSAY, E.H. EDS., ADVANCES IN LATE TERTIARY VERTEBRATE PALEONTOLOGY IN MEXICO 2006 62 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 fos- tecus; the rhino Teleoceras fossiger; the lagomorph No- sils, especially horses (Equidae), these local faunas tolagus velox; the carnivores Agriotherium schneideri, date from the late Hemphillian to the late Blancan land Borophagus secundus, and Pseudaelurus sp.; and the mammal ages (Miller and Carranza-Castañeda, 1984; giant camel Megatylopus mathhewi. Carranza-Castañeda, 1989; Carranza-Castañeda and For the Blancan faunas, the dates range from 4.6 Espinosa-Arrubarrena, 1994; Bell et al., 2004). The ± 0.3 to 3.9 ± 0.3 Ma (fission-track) and 4.74 ± 0.14 to four sites from Guanajuato date to the early Blancan 3.36 ± 0.04 Ma (40Ar/39Ar). Nearly all the mammalian (GTO 4, GTO 11, GTO 14, GTO 26), whereas the two species and most genera from the late Hemphillian are sites from Jalisco date to the late Hemphillian (Jal Teco absent in the Blancan deposits. The equids are reduced 20) and late Blancan (Jal Teco 7). Fission-track and to Nannippus peninsulatus and Equus simplicidens. 40Ar/39Ar dates from volcanic ash in Guanajuato and The carnivores are represented by the canid Boropha- Jalisco were reported by Kowallis and others (1998). gus diversidens and the cat Felis studeri. The most im- For the late Hemphillian fauna, the dates range portant findings are the first records of Glossotherium from 4.8 ± 0.2 to 4.4 ± 0.3 Ma (fission-track) and 4.89 sp., Neochoerus cordobai, Glyptotherium sp., and the ± 0.16 Ma (single 40Ar/39Ar date). The mammalian pampatheriidae Plaina (Carranza-Castañeda and Mill- fauna collected in these areas corresponds to the latest er, 2004). These records of immigrants in the Blancan records of the equids Dinohippus mexicanus, Astrohip- deposits represent the beginning of the great biotic in- pus stockii, Neohipparion eurystyle, and Nannippus az- terchange between the Americas. UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO, INSTITUTO DE GEOLOGÍA AND CENTRO DE GEOCIENCIAS PUBLICACIÓN ESPECIAL 4 EARLY PLIOCENE TO EARLY PLEISTOCENE BIRDS FROM CENTRAL MEXICO 63 Osteological terminology generally follows Bau- 941.P4107, a nearly complete tibiotarsus from the 111 mel and Witmer (1993), supplemented as needed by Ranch local fauna, southern Arizona (middle Blancan) Howard (1929). Classification of Anatidae at the sub- that we refer to Aechmophorus rather than Podiceps familial and tribal levels follows Woolfenden (1961). because of the characters already mentioned, as well as having the gap in the typically podicipedid, two-part SYSTEMATIC PALEONTOLOGY crista fibularis located proportionately closer to the Order Podicipediformes distal end. The second fossil is AWC 12156, a distal Family Podicipedidae tibiotarsus from the El Golfo local fauna, northern So- Aechmophorus cf. elasson nora (early Irvingtonian). The sizes of the three fossils are within the range of variation in P. grisegena and Material. Proximal end of tibiotarsus, IGM 9131, Lo- are somewhat smaller than in modern specimens of A. cality GTO 11 Garbani (early Blancan), Rancho Viejo occidentalis (Table 1). They represent a small form of area, San Miguel de Allende graben, state of Guanajua- Aechmophorus that will be evaluated further in a fu- to. Distal end of tibiotarsus, IGM 9132, Locality Jal ture paper that focuses on the many specimens from Teco 7 Las Gravas (late Blancan), Tecolotlán basin, 111 Ranch, which include nearly all major post-cranial state of Jalico. elements. Remarks. The largest North American species of Comparisons and Discussion. The only Blancan spe- Podiceps, P. grisegena, resembles in certain skeletal cies of Aechmophorus that has been described is A. elements those of the genus Aechmophorus, which is elasson from the Hagerman local fauna, Idaho (early represented today by two morphologically nearly iden- Blancan), based on 11 specimens from four skeletal tical species, A. occidentalis and A. clarki. Specimen elements (coracoid, humerus, ulna, tarsometatarsus; IGM 9131 is referred to Aechmophorus rather than Murray, 1967). Chandler (1990) reported five fossils Podiceps because the fossa retrocristalis is shallower. (an ulna, femur, tibiotarsus, and two tarsometatarsi) of Specimen IGM 9132 is referred to Aechmophorus rath- A. elasson from the San Diego Formation, California er than Podiceps because the dorsal-medial and dorsal- (Blancan). These two sets of Blancan Aechmophorus lateral margins of corpus tibiotarsi are more rounded fossils, as well as those from Mexico, are slightly small- (less sharp), and because the corpus tibiotarsi is broad er than in available modern specimens of A. occidenta- relative to the overall size of the bone. lis. This contrasts with late Pleistocene (Rancholabrean IGM 9132 is very similar to two fossils from land mammal age) specimens of Aechmophorus from Arizona and northern Mexico. The first is MSM 00- Fossil Lake (Oregon) and the state of Mexico (Mex- 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 64 STEADMAN AND CARRANZA-CASTAÑEDA ico), which average larger than in modern specimens Nearly complete tibiotarsus (two pieces), IGM 9133-1, and have been assigned to the temporal subspecies A. o. 9133-5, Locality GTO 26 Zodiaco (Figure 2). Proximal lucasi (Howard, 1946; Brodkorb and Phillips, 1973). end of tarsometatarsus, IGM 9135, Locality GTO 4 Ar- Pliocene and Pleistocene grebes are in bad need royo El Tanque (early Blancan), Los Galvanes area, of review. All described North American extinct spe- San Miguel de Allende area. cies of Podiceps (see Becker, 1987; Chandler, 1990; Storer, 2001) are much smaller than the fossils from Remarks. We recognize Cygnus and Olor at the subge- Guanajuato and Jalisco, except that P. arndti from the neric rather than generic level, finding the osteological San Diego Formation may approach a small form of differences (except for the sternum and furcula) to be Aechmophorus in size. The two described extinct gen- more minor than in Woolfenden (1961).