''Didymoceras nebrascense'' was an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

heteromorph The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and bec ...

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

from the upper

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campanian s ...

age (around 83 to 70 million years ago). It was

sexually dimorphic Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

, with two adult sizes averaging at and high for females and males respectively. It exhibited three distinct growth stages. The first growth stage was composed of one or two straight sharply bending sections and a gently curved third. The second growth stage is composed of around three and a half loosely coiling whorls. The last (adult) growth stage is composed of a U-shaped bend facing upwards.

Description

''Didymoceras nebrascense'' is relatively easily identifiable. It has three growth stages. The initial growth stage produces two more or less straight sections connected with each other by a sharply bent elbow. The sections are parallel to each other or diverging at an angle of 10° to 30°. The second, larger straight section is elliptical in cross-section, higher than it is wide. It connects to a third more broadly curving elbow. Like the second straight section, its cross section is also elliptical. Occasionally, some individuals only has one straight section and one curved section for the first growth stage. The middle growth stage begins at the end of the last curved limb of the first growth stage. It is composed of three and a half loosely coiled whorls. The whorls are usually widely separated, but some individuals display whorls that are just barely touching each other. The ribbing becomes denser than in the first growth stage and its cross-section is more or less circular. The third growth stage is that of the adult. It is composed of a single distinctive upward-facing U-shaped whorl. Ornamentation of ''Didymoceras nebrascense'' consists of a double row of nodes running along the ventral side. They are usually evenly matched with each other, but can sometimes alternate briefly. The straight sections of the first growth stage shows evidence of small sharp spines on the tubercles. The coiling can be to the left or right. ''Didymoceras nebrascense'' is

, with adults showing two average heights - the smaller male adults (

microconch Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

s) had an average height of ; while the larger female adults ( macroconchs) had the average height of .

Taxonomy

''Didymoceras nebrascense'' was originally described (tentatively) in 1856 as ''Ancyloceras nebrascensis'' by the

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

Fielding B. Meek Fielding Bradford Meek (December 10, 1817 – December 22, 1876) was an American geologist and a paleontologist who specialized in the invertebrates. Biography The son of a lawyer, he was born in Madison, Indiana. In early life he was in bu ...

and

Ferdinand V. Hayden Ferdinand Vandeveer Hayden (September 7, 1829 – December 22, 1887) was an American geologist noted for his pioneering surveying expeditions of the Rocky Mountains in the late 19th century. He was also a physician who served with the Union Ar ...

. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

was collected from Iquor Creek of

Cheyenne County, Nebraska Cheyenne County is a county in the U.S. state of Nebraska. As of the 2010 United States Census, the population was 9,998. Its county seat is Sidney. The county was formed in 1871 and named for the Cheyenne Native American tribe. In the Nebrask ...

. It was poorly preserved and incomplete, being a mere fragment. This led other workers to erroneously classify further discoveries of the same species as new species - notably ''Emperoceras beecheri'' and ''Helicoceras simplicostatum'' by

Alpheus Hyatt Alpheus Hyatt (April 5, 1838 – January 15, 1902) was an American zoologist and palaeontologist. Biography Alpheus Hyatt II was born in Washington, D.C. to Alpheus Hyatt and Harriet Randolph (King) Hyatt. He briefly attended the Maryla ...

(1894) and

Robert Parr Whitfield Robert Parr Whitfield (27 May 1828 – 6 April 1910) was an American invertebrate paleontologist and a curator of geology at the American Museum of Natural History. Largely self-educated Whitfield rose into paleontology as an assistant of James ...

(1902) respectively. It has been classified under various genera by different authors, reflecting the constantly shifting opinions when it comes to

heteromorph ammonite The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and bec ...

s. Kennedy ''et al.'' (2000) placed it under the genus ''

Didymoceras ''Didymoceras'' is an extinct genus of ammonite cephalopod from the Late Cretaceous epoch (approximately 76 Ma). It is one of the most bizarrely shaped genera, with a shell that spirals upwards into a loose, hooked tip. It is thought to have drif ...

'', which in turn is classified under the family

Nostoceratidae Nostoceratidae is a diverse Family (biology), family of heteromorph ammonites found throughout the oceans of the world during the Late Cretaceous. The nostoceratids are famous for the bizarre coiling of their shells. Many genera, such as ''Yezo ...

under the diverse suborder of heteromorph ammonites,

Ancyloceratina The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and beca ...

Distribution

''Didymoceras nebrascense'' occurs in the western edge of what was once an ancient sea that stretched from northern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...

to northeastern

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columbi ...

in the upper

age (around 83 to 70 million years ago) of the

late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

. A putative specimen from the

Mishash Formation The Mishash Formation is a Late Cretaceous (Campanian-Maastrichtian) geologic formation in Israel. The formation is correlated with the Duwi Formation of Egypt and the Amman and Ruseifa Formations of Jordan.Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...

has also been recorded in 1969.

References

{{Taxonbar, from=Q5274267 Nostoceratidae Campanian life