''Pliolymbus'' is a fossil genus of

grebe Grebes () are aquatic diving birds in the order Podicipediformes . Grebes are widely distributed freshwater birds, with some species also found in marine habitats during migration and winter. Some flightless species exist as well, most notably ...

known from the Late

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Early Pleistocene The Early Pleistocene is an unofficial sub-epoch in the international geologic timescale in chronostratigraphy, being the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently estimated to span the time ...

of United States and Mexico. It is known from a single species, ''P. baryosteus''.

History

The specimens were collected in the summer of 1950 from Kansas, United States by

Claude W. Hibbard Claude W. Hibbard, popularly known as Hibbie (March 21, 1905 in Toronto, Kansas – October 9, 1973 in Ann Arbor, Michigan), was an American paleontologist. Born on a farm in rural southeastern Kansas as the oldest of six children, Hibbard g ...

and the species was named in 1967 by Bertram G. Murray. The binomial nomenclature for the species means “heavy bone Pliocene diver” in reference of the age and skeletal features of the grebe. Howard (1969) would published an specimen collected from

Lake Chapala Lake Chapala ( es, Lago de Chapala, ) is Mexico's largest freshwater lake. It lies in the municipalities of Ocotlán, Jalisco, Ocotlán, Chapala, Mexico, Chapala, Jocotepec, Poncitlán, and Jamay (municipality), Jamay, in Jalisco, and in Venu ...

from a series of field surveys in the region from 1926, 1958 and 1963.

Description

A total of six specimens of ''Pliolymbus'' have been recovered. The holotype consists of the anterior portion of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...

(

UMMP The University of Michigan in Ann Arbor is home to a number of museums. Located on the university's Central Campus are University of Michigan Museum of Natural History; the University of Michigan Museum of Art; the Kelsey Museum of Archaeology; Si ...

51839). The additional specimens are a humerus (UMMP 51840), proximal of a

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

(UMMP 51841), the shaft of a

tarsometatarsus The tarsometatarsus is a bone that is only found in the lower leg of birds and some non-avian dinosaurs. It is formed from the fusion of several bones found in other types of animals, and homologous to the mammalian tarsus (ankle bones) and meta ...

(UMMP 51844), and a more incomplete specimen that has more complete remains of coracoids, an ulna and

carpometacarpus The carpometacarpus is a bone found in the hands of birds. It results from the fusion of the carpal and metacarpal bone, and is essentially a single fused bone between the wrist and the knuckles. It is a smallish bone in most birds, generally flatt ...

, a near complete cervical vertebrae and various portions of raddii, scapula, femur,

synsacrum The synsacrum is a skeletal structure of birds and other dinosaurs, in which the sacrum is extended by incorporation of additional fused or partially fused caudal or lumbar vertebrae and it can only be seen in birds. Some posterior thoracic vert ...

, two ribs and an unidentified bone (UMMP 27173). ''Pliolymbus'' was a genus of small grebe with an incredibly robust skeleton, more-so than the skeletons of extant grebes. The coracoid is similar in size to the least grebe and is more robust and has a broader base like those in the

red-necked grebe The red-necked grebe (''Podiceps grisegena'') is a migratory aquatic bird found in the temperate regions of the northern hemisphere. Its wintering habitat is largely restricted to calm waters just beyond the waves around ocean coasts, although ...

(''Podiceps grisegena'') and

black-necked grebe The black-necked grebe or eared grebe (''Podiceps nigricollis'') is a member of the grebe family of water birds. It was described in 1831 by Christian Ludwig Brehm. There are currently three accepted subspecies, including the nominate subspecie ...

(''Podiceps nigricollis''). The distal end of the humerus is about the same size as the white-tufted grebe, but the humerus shaft is stouter. The ulna is also longer than the

least grebe The least grebe (''Tachybaptus dominicus''), an aquatic bird, is the smallest member of the grebe family. It occurs in the New World from the southwestern United States and Mexico to Argentina, and also on Trinidad and Tobago, the Bahamas and the ...

(''Tachybaptus dominicus'') and

white-tufted grebe The white-tufted grebe (''Rollandia rolland''), also known as Rolland's grebe, is a species of grebe in the family Podicipedidae. Found in the southern half of South America, its natural habitat is freshwater lakes, ponds and sluggish streams. ...

(''Rollandia rolland'') but short in comparison to the black-necked grebe. The robustness of the ulna however is comparable to the female members of the black-necked grebe. The carpometacarpus is comparable to the white-tufted grebe in length but the diameter of it is closer in size to female black-necked grebes, yet it is short overall in comparison with other grebes. A Mexican specimen consisting of the upper right portion of the coracoid has also been recorded (

LACM The Natural History Museum of Los Angeles County is the largest natural and historical museum in the western United States. Its collections include nearly 35 million specimens and artifacts and cover 4.5 billion years of history. This large coll ...

2891).

Classification

''Pliolymbus'' has no close living relatives among the modern grebes, being part of an extinct lineage of grebes that died out by the end of the Pliocene. However Olson (1985) question the validity of the genus and suggest ''Pliolymbus baryosteus'' might be a species of ''

Podiceps ''Podiceps'' is a genus of birds in the grebe family. The genus name comes from Latin ''podicis'', "rear-end" and ''ped'', "foot", and is a reference to the placement of a grebe's legs towards the rear of its body. It has representatives breed ...

'' instead.

Paleobiology

''Pliolymbus'' is from the

Piacenzian The Piacenzian is in the international geologic time scale the upper stage (stratigraphy), stage or latest age (geology), age of the Pliocene. It spans the time between 3.6 ± 0.005 year#SI prefix multipliers, Ma and 2.588 ± 0.005 Ma (million yea ...

age of the Pliocene to the

Gelasian The Gelasian is an age (geology), age in the international geologic timescale or a stage (stratigraphy), stage in chronostratigraphy, being the earliest or lowest subdivision of the Quaternary Period/System and Pleistocene Epoch/Series. It spans ...

age of the Pleistocene from the Fox Canyon section of the

Rexroad Formation The Rexroad Formation is a geologic formation in Kansas. It preserves fossils dating back to the Neogene period. These fossils include two types of skunk (''Spilogale rexroadi'' and '' Brachyprotoma breviramus''), a tree bat (''Lasiurus fossil ...

from Kansas and Lake Chapala from

Jalisco Jalisco (, , ; Nahuatl: Xalixco), officially the Free and Sovereign State of Jalisco ( es, Estado Libre y Soberano de Jalisco ; Nahuatl: Tlahtohcayotl Xalixco), is one of the 31 states which, along with Mexico City, comprise the 32 Federal En ...

References

{{Taxonbar, from=Q84236158 Podicipediformes Prehistoric bird genera