Additions and Comments on the Fossil Birds of Agate Fossil Beds National Monument, Sioux County, Nebraska

Robert M. Chandler
Department of Biological and Environmental Sciences
Georgia College & State University, Milledgeville, GA 31061-0490

Abstract—Fossils from Agate Fossil Beds National Monument in western Nebraska have been a rich source of paleontological studies for many years. Fossil bird discoveries from the Monument have been far fewer than mammals and their reports have been sporadic and scattered throughout the literature. Although less common than mammals, the paleoavifauna of the Monument is very interesting in its level of diversity, ecological indicators, and from the perspective of historical biogeography with Old and New World representatives. The paleoavifauna has representatives from at least six families in four orders.


It has been more than ten years since Becker (1987a:25) briefly reviewed the fossil birds of the "Agate Fossil Quarries." The specimens described herein were collected in 1908 by field crews from the University of Nebraska State Museum, but have never been identified or reported upon until now. This collection includes the first record of a crane, Gruiformes, and additional specimens of the fossil hawk, Buteo ales Wetmore. Buteo typhoius Wetmore should be removed from the list of species from Agate.

For historical accounts, the most comprehensive geologic data and the best paleoenvironmental interpretation for the Monument and surrounding fossil localities are discussed in the works of Robert M. Hunt, Jr. (1972, 1978, 1981, 1985, 1990, and 1995) and Hunt and Skolnick (1996).

Systematic Paleontology

[An asterisk (*) indicates genera or species described from the Agate Fossil Beds. The classification sequence followed below is that of the AOU Check-list, 6th edition. The following acronyms are used: American Museum of Natural History, Department of Vertebrate Paleontology (AMNH); Carnegie Museum of Natural History (CMNH); Harold J. Cook collection (HC); Museum of Comparative Zoology (MCZ); Princeton University Geological Museum (PUGM) now at Yale University, Peabody Museum; University of Nebraska State Museum (UNSM)].

Order Falconiformes

(hawks, eagles, falcons, and allies)

Family Accipitridae

(kites, hawks, eagles, and allies)

Subfamily Accipitrinae

(kites, hawks, and eagles)

Genus Promilio Wetmore 1958

(extinct kite)

Promilio efferus (Wetmore) 1923

Holotype.—AMNH 6299, left tarsometatarsus missing internal half of the shaft; Agate Fossil Beds National Monument, Upper Harrison (late Arikareean), Sioux Co., Nebraska.

Promilio efferus is the earliest record for a kite in North America (Brodkorb, 1964:274). Wetmore (1923:504) had tentatively placed efferus in the genus Proictinia Shufeldt (1915: 301) from the late Miocene [latest Clarendonian or earliest Hemphillian, Long Island local fauna, Phillips County, Kansas; see Steadman (1981:171) for comments on age of Long Island local fauna]. Later, Wetmore (1958:2) decided that Proictinia was more closely related to the Everglade Kite, Rostrhamus sociabilis, and that P. efferus was more like the Old World carrion eating kites in the genus Milvus. Promilio efferus is equivalent in size and is similar osteologically to species of Milvus. Wetmore (1958:3) placed efferus in the subfamily Milvinae, but in a new genus, Promilio, based on perceived differences.

Genus Buteo Lacepede, 1799 (hawks)

Buteo typhoius Wetmore, 1923

Holotype.—AMNH 1754, distal two-thirds of the right tarsometatarsus missing the trochlea for Digit II; from the Lower Snake Creek, Olcott Formation (early Barstovian), 23 miles south of Agate, Sioux Co., Nebraska.

Referred material.—HC 477, distal one half of the right tibiotarsus, Agate Fossil Beds National Monument, Stenomylus Quarry 1.5 miles east of Carnegie Hill and University Hill quarries (referred by Wetmore, 1928:149).

Discussion.—Buteo typhoius is osteologically very similar to its living congeners. Wetmore compared B. typhoius exclusively to the eastern Red-tailed Hawk, B. jamaicensis borealis, from which it can be distinguished by subtle osteological differences and its much larger size (approximately 50 percent; see Wetmore, 1923:485-492; 1928:149-150).

None the less, because of the disparity in age of the type locality of B. typhoius, which is the Lower Snake Creek, early Barstovian, (see Becker, 1987b for the interesting history and insight into the age of the locality) and the age of the Monument, late Arikareean, I do not think B. typhoius occurs at Agate. The tibiotarsus (HC 477) that Wetmore (1928:149) referred to B. typhoius most likely belongs to the next species B. ales (Wetmore). Therefore, B. typhoius should be stricken from the Monument species list.

*Buteo ales (Wetmore) 1926

Holotype.—CMNH 1828, complete right tarsometatarsus; Agate Fossil Beds National Monument, Carnegie Hill (Quarry No. 2).

Referred material.—UNSM 3001, hallux; UNSM 3002, right humeral shaft; UNSM 3004, left ulna missing the olecranon; UNSM 3006, right femur, proximal end; UNSM 5782, left ulna, distal one/quarter; UNSM 5783, right tarsometatarsus, distal end missing trochlea of Digit 4; UNSM 5784, left tibiotarsus, proximal two/thirds; UNSM 5785, right tarsometatarsus, distal one/quarter missing anterior half of trochlea of Digit 3; UNSM 5786, left humerus, distal end missing the entepicondyle.

Discussion.—Buteo ales (Wetmore, 1926:403) was a large hawk. The length of the holotype tarsometatarsus, CMNH 1828, is 90.2mm with a distal width of 16.2mm. A hawk of this size is at the top of the size range for the eastern Red-tailed Hawk, B. jamaicensis borealis, given by Friedmann (1950:239). With the identification and referral of the UNSM fossils to B. ales, the assignment of this species to Buteo is affirmed. The ulna, UNSM 3003, may appear to be too long for this species (164.0mm), but is within reason when sexual size dimorphism is taken into consideration for North American raptors (Snyder and Wiley, 1976).

Measurements of selected fossils.—hallux (UNSM 3001) length of cord = 24.0mm; ulna (UNSM 3004) length missing the olecranon = [164.0mm], proximal width = 15.2mm, distal width = 11.0mm, depth of external condyle = 11.3mm, mid-shaft width/depth = 7.1/7.2mm; ulna (UNSM 5782) distal width/depth of external condyle = 10.8/11.2mm; tibiotarsus (UNSM 5784) width/depth across proximal articular surface = 13.5/17.5mm, length of fibular crest = 28.5mm, width/depth of shaft below fibular crest = 7.9/6.9mm; tarsometatarsus (UNSM 5785) distal width = 17.1, depth of trochlea Digit 4 = 8.3mm, width of trochlea Digit 2 = 7.2mm; humerus (UNSM 5786) distal width missing entepicondyle = [21.0mm].

Accipitridae indeterminate

Wetmore identified three other fossils only to family rank with the following comments: HC 466, right ulna, proximal end about the size of a caracara, Polyborus (Wetmore, 1923:506); PUGM 12157, right tarsometatarsus, distal end which resembles a Marsh Hawk, Circus hudsonius (Wetmore, 1923:507); and CMNH 2207, large claw somewhat smaller than a Golden Eagle, Aquila chrysaetos (Wetmore, 1926:406). I have not seen any of these fossils, but the right ulna (HC 466) which is the size of a caracara should be re-examined with the UNSM ulnae referred to B. ales.

*Genus Palaeastur

*Species Palaeastur atavus Wetmore 1943

Holotype.—HC 693, right tarsometatarsus, distal one/third; Agate Fossil Beds National Monument, Stenomylus Quarry.

Discussion.—Wetmore (1943:230) likened this new genus and species of extinct hawk to the monotypic Black-and-
white Hawk-eagle (Spizastur melanoleucus). Spizastur melanoleucus is the smallest of the booted eagles, which are considered by Brown and Amadon (1968:22) to be "the most highly evolved members of the family and indeed of all birds of prey." They live in dense, humid evergreen, and semideciduous forests in Central and South America (Blake, 1977; Howell and Webb, 1995).

Order Galliformes

(grouse, quail, turkeys, and allies)

Family Cracidae

(curassows, guans, and chachalacas)

Genus Boreortalis

(extinct chachalacas)

*Species Boreortalis tantala (Wetmore) 1933

Holotype.—HC 498, right tibiotarsus, distal end; Agate Fossil Beds National Monument, Carnegie Hill. (This specimen is now in the AMNH collections).

Discussion.—In the Neogene there are five species of boreal chachalacas in the genus Boreortalis Brodkorb (1964:304-305). Boreortalis (Brodkorb, 1954:180) is closely related to Ortalis, the genus of living chachalacas found in southern Texas through Central America and into South America (Blake, 1977). It was Brodkorb's impression that chachalacas had a Nearctic origin and only during the Great American Biotic Interchange did they expand their range into South America. Although boreal today, the chachalacas from these North American fossil localities would have been much more tropical to subtropical in nature.

Family Phasianidae

(grouse, quail, and turkeys)

Subfamily Tetraoninae


Genus Palaealectoris

(extinct grouse)

*Species Palaealectoris incertus Wetmore 1930

Holotype.—MCZ 2190, left humerus, proximal one/half and distal end with part of shaft missing; Agate Fossil Beds National Monument.

Discussion.—This fossil grouse is between a Northern Bobwhite, Colinus virginianus, and the Spruce Grouse, Dendragapus canadensis, in size. Wetmore (1930:152-153) thought it similar to the Spruce Grouse, but only distantly related. Today, the Spruce Grouse is a member of the boreal species community. Palaealectoris may represent an ancestral grouse living in a subtropical environment before the dichotomy we see today between boreal species like the Spruce Grouse and the prairie grouse like the Greater Prairie-Chicken, Tympanuchus cupido.

Order Gruiformes

(cranes, rails, limpkins, and their allies)

Family Gruidae


Genus and species indeterminate

Referred material.—UNSM 3003, right ulna mid-shaft; UNSM 3005, right humerus, distal end; both are from Agate Fossil Beds National Monument, University Hill, University Quarry.

Discussion.—This right ulna (UNSM 3003) and humerus (3005) are the first crane fossils to be identified from the Monument. Although fragmentary, the fossils are from a species of crane approximately the size of the modern Sandhill Crane, Grus canadensis (Linnaeus). It should be kept in mind, however, that in the late Clarendonian there is an Old World crane (Balearica) in Nebraska (Feduccia and Voorhies, 1992:241), and the fossils from Agate may represent either of these cranes.

Order Charadriiformes

(thick-knees, plovers, sandpipers, and their allies)

Family Glareolidae

(coursers and pratincoles)

Genus Paractiornis

(extinct pratincole)

* Species Paractiornis perpusillus (Wetmore) 1930

Holotype.—MCZ 2191, left tarsometatarsus missing the intercotylar prominence; Agate Fossil Beds National Monument, Carnegie Hill.

Discussion.—Wetmore (1930:153) originally named and described this new genus and species as the earliest representative of the modern oystercatchers, Haematopodidae. However, Olson and Steadman (1978:972-976) have shown this species to be the first New World record for the Old World pratincoles (Glareolinae). Pratincoles are short-legged insect feeders, which feed on the wing much like swallows. Once again, we have a member of the paleoavifauna from Agate which is representative of a greater, worldwide avifauna still in existence at the end of the Oligocene and into the early Miocene.


The fossil birds of the Agate Fossil Beds National Monument are very interesting at several levels. First, they are interesting at an alpha taxonomic level as new genera and species. Secondly, birds are frequently used as environmental indicators of certain habitats and to evaluate the health of the environment. The birds from the Monument support Hunt's interpretation of the paleoenvironment as an area with ephemeral stream channels, open plains, but with riparian areas along the streams. Thirdly, it is significant that the paleoavifauna represents a worldwide fauna, which only recently has been recognized at other important localities like Quercy, France; Messel, Germany; Green River, Wyoming; and the Naze in England. Olson (1989:2023-2029) has dubbed this the global avifauna, of which our world today has only relictual distribution patterns of that once cosmopolitan avifauna.


I would like to thank Robert M. Hunt, Jr. for giving me the opportunity and encouragment to work on these very interesting fossils. I am also indebted to Vincent L. Santucci for allowing me the time and opportunity to submit this contribution on the fossil birds of Agate. Much thanks goes out to the
following curators, collection managers, and their institutions for helping me with loans of fossils and/or modern osteological specimens for this study: Robert M. Hunt, Jr. and R. George Corner, Nebraska State Museum, University of Nebraska; Larry D. Martin and M. Desui, Vertebrate Paleontology Department and the late Robert M. Mengel, the late Marion Jenkinson, Richard Prum, and Mark Robbins, Museum of Natural History, The University of Kansas; S. David Webb and Marc Frank, Division of Vertebrate Paleontology and David Steadman and Tom Webber, Division of Ornithology, Florida Museum of Natural History, The University of Florida. I would like to thank Linda D. Chandler, Dennis Parmley, William P. Wall, and the editors for critically reading one or more manuscripts and improving this paper.


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