Geosternbergia
Encyclopedia
Geosternbergia from the Late Cretaceous
geological period of North America
, was one of the largest pterosaur
genera and had a wingspan of up to 7.25 metres (23.8 ft).
Geosternbergia was among the largest pterosaurs, with the wingspan of most adults ranging between three and six meters (9–20 ft). While most specimens are found crushed, enough fossils exist to put together a detailed description of the animal.
The most distinctive characteristic of Geosternbergia is its cranial crest. These crests consisted of skull bones (frontals) projecting upward and backward from the skull. The size and shape of these crests varied due to a number of factors, including age, gender, and species. Male G. sternbergi, the older species of the two described to date, had a larger vertical crest with a broad forward projection, while G. maysei had a short, ronded vertical crest and was generally smaller. Females of both species were smaller and bore small, rounded crests. The crests were probably mainly display structures, though they may have had other functions as well.
Geosternbergia fossils are known from the Niobrara
and Sharon Springs Formation
s of the central United States
. Geosternbergia existed as a group for more than four million years during the late Coniacian
- early Campanian
stages of the Cretaceous
period. The genus is present in the lower the Niobrarra Formation except for the upper two; in 2003, Kenneth Carpenter
surveyed the distribution and dating of fossils in this formation, demonstrating that Geosternbergia sternbergi existed there from 88-85 million years ago, while the species later named G. maysei existed between 81.5 and 80.5 million years ago.
Geosternbergia is distinguished from Pteranodon mainly by its upright-crest. The lower jaw of G. sternbergi was 1.25 meters (4 ft) long. It was collected by George F. Sternberg
in 1952 and described by John Christian Harksen in 1966, from the lower portion of the Niobrara Formation. It was older than Pteranodon longiceps and is considered by Bennett to be the direct ancestor of the later species.
, in most major studies of pteranodontians through the 1990s. However, a 2010 review of the group by A.W.A. Kellner suggested that Pteranodon sternbergi was different enough from P. longiceps to belong in a distinct genus, to which Kellner also referred a new species, Geosternbergia maysei. Earlier, pterosaur researcher Chris Bennett had considered the G. maysei specimen an adult male P. longiceps.
. Skulls from the larger size class preserve large, upward and backward pointing crests, while the crests of the smaller size class are small and triangular. Some larger skulls also show evidence of a second crest that extended long and low, toward the tip of the beak, which is not seen in smaller specimens.
The gender of the different size classes was determined, not from the skulls, but from the pelvic bones. Contrary to what may be expected, the smaller size class had disproportionately large and wide-set pelvic bones. Bennett interpreted this as indicating a more spacious birth canal, through which eggs would pass. He concluded that the small size class with small, triangular crests represent females, and the larger, large-crested specimens represent males.
Note that the overall size and crest size also corresponds to age. Immature specimens are known from both females and males, and immature males often have small crests similar to adult females. Therefore, it seems that the large crests only developed in males when they reached their large, adult size, making the gender of immature specimens difficult to establish from partial remains.
The fact that females appear to have outnumbered males two to one suggests that, as with modern animals with size-related sexual dimorphism, such as sea lion
s and other pinniped
s, Geosternbergia might have been polygynous
, with a few males competing for association with groups consisting of large numbers of females. Similar to modern pinnipeds, Geosternbergia may have competed to establish territory on rocky, offshore rookeries, with the largest, and largest-crested, males gaining the most territory and having more success mating with females. The crests of male Geosternbergia would not have been used in competition, but rather as "visual dominance-rank symbols", with display rituals taking the place of physical competition with other males. If this hypothesis
is correct, it also is likely that male Geosternbergia played little to no part in rearing the young; such a behavior is not found in the males of modern polygynous animals who father many offspring at the same time.
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
geological period of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, was one of the largest pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
genera and had a wingspan of up to 7.25 metres (23.8 ft).
Description
The most distinctive characteristic of Geosternbergia is its cranial crest. These crests consisted of skull bones (frontals) projecting upward and backward from the skull. The size and shape of these crests varied due to a number of factors, including age, gender, and species. Male G. sternbergi, the older species of the two described to date, had a larger vertical crest with a broad forward projection, while G. maysei had a short, ronded vertical crest and was generally smaller. Females of both species were smaller and bore small, rounded crests. The crests were probably mainly display structures, though they may have had other functions as well.
Geosternbergia fossils are known from the Niobrara
Niobrara Formation
The Niobrara Formation, also called the Niobrara Chalk , is a geologic formation in North America that was laid down between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian ages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member...
and Sharon Springs Formation
Pierre Shale
The Pierre Shale is a geologic formation or series in the Upper Cretaceous which occurs east of the Rocky Mountains in the Great Plains, from North Dakota to New Mexico....
s of the central United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Geosternbergia existed as a group for more than four million years during the late Coniacian
Coniacian
The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous epoch or Upper Cretaceous series and spans the time between 89.3 ± 1 Ma and 85.8 ± 0.7 Ma...
- early Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...
stages of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period. The genus is present in the lower the Niobrarra Formation except for the upper two; in 2003, Kenneth Carpenter
Kenneth Carpenter
Kenneth Carpenter is a paleontologist. He is the museum director of the USU Eastern Prehistoric Museum and author or co-author of a number of books on dinosaurs and Mesozoic life...
surveyed the distribution and dating of fossils in this formation, demonstrating that Geosternbergia sternbergi existed there from 88-85 million years ago, while the species later named G. maysei existed between 81.5 and 80.5 million years ago.
Geosternbergia is distinguished from Pteranodon mainly by its upright-crest. The lower jaw of G. sternbergi was 1.25 meters (4 ft) long. It was collected by George F. Sternberg
George F. Sternberg
George Fryer Sternberg was a paleontologist best known for his discovery in Gove County, Kansas of the "fish-within-a-fish" of Xiphactinus audax. He was the son of Charles Hazelius Sternberg and nephew of Brig. Gen. George M. Sternberg...
in 1952 and described by John Christian Harksen in 1966, from the lower portion of the Niobrara Formation. It was older than Pteranodon longiceps and is considered by Bennett to be the direct ancestor of the later species.
Classification
Geosternbergia was traditionally considered a species, or occasionally subgenus, of the similar pterosaur PteranodonPteranodon
Pteranodon , from the Late Cretaceous geological period of North America in present day Kansas, Alabama, Nebraska, Wyoming, and South Dakota, was one of the largest pterosaur genera and had a maximum wingspan of over...
, in most major studies of pteranodontians through the 1990s. However, a 2010 review of the group by A.W.A. Kellner suggested that Pteranodon sternbergi was different enough from P. longiceps to belong in a distinct genus, to which Kellner also referred a new species, Geosternbergia maysei. Earlier, pterosaur researcher Chris Bennett had considered the G. maysei specimen an adult male P. longiceps.
Sexual variation
Adult Geosternbergia specimens may be divided into two distinct size classes, small and large, with the large size class being about one and a half times larger than the small, and the small being twice as common as the large. Both size classes lived along side each other, and while researchers had previously suggested that they represent different species, Christopher Bennett showed that the differences between them are consistent with the concept that they represent females and males, and that Geosternbergia species were sexually dimorphicSexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
. Skulls from the larger size class preserve large, upward and backward pointing crests, while the crests of the smaller size class are small and triangular. Some larger skulls also show evidence of a second crest that extended long and low, toward the tip of the beak, which is not seen in smaller specimens.
Note that the overall size and crest size also corresponds to age. Immature specimens are known from both females and males, and immature males often have small crests similar to adult females. Therefore, it seems that the large crests only developed in males when they reached their large, adult size, making the gender of immature specimens difficult to establish from partial remains.
The fact that females appear to have outnumbered males two to one suggests that, as with modern animals with size-related sexual dimorphism, such as sea lion
Sea Lion
Sea lions are pinnipeds characterized by external ear-flaps, long fore-flippers, the ability to walk on all fours, and short thick hair. Together with the fur seal, they comprise the family Otariidae, or eared seals. There are six extant and one extinct species in five genera...
s and other pinniped
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...
s, Geosternbergia might have been polygynous
Polygyny
Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...
, with a few males competing for association with groups consisting of large numbers of females. Similar to modern pinnipeds, Geosternbergia may have competed to establish territory on rocky, offshore rookeries, with the largest, and largest-crested, males gaining the most territory and having more success mating with females. The crests of male Geosternbergia would not have been used in competition, but rather as "visual dominance-rank symbols", with display rituals taking the place of physical competition with other males. If this hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
is correct, it also is likely that male Geosternbergia played little to no part in rearing the young; such a behavior is not found in the males of modern polygynous animals who father many offspring at the same time.