''Heterastridium'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of marine

hydrozoa Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is a taxonomy (biology), taxonomic class (biology), class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline wat ...

n. It is the only accepted genus in the monotypic family Heterastridiidae. The fossils date from the Upper

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

. They are mostly discoid or spherical and some forms found in the

Karakorum Karakorum (Khalkha Mongolian: Хархорум, ''Kharkhorum''; Mongolian script:, ''Qaraqorum'') was the capital city, capital of the Mongol Empire between 1235 and 1260 and of the Northern Yuan, Northern Yuan dynasty in the late 14th and 1 ...

mountains are called Karakorum stones.Erik Flügel (1960) Heterastridium conglobatum conglobatum Reuss, an Upper Triassic Hydrozoan from the Petra-Tou-Roumiou Limestone of Pendakomo, Cyprus. Journal of Paleontology 34(1):127-132 They vary in diameter from 1 to 35 cm and appear to follow

Cope's rule Cope's rule, named after American paleontologist Edward Drinker Cope, postulates that population lineages tend to increase in body size over evolutionary time. It was never actually stated by Cope, although he favoured the occurrence of linear e ...

for the prehistoric climate.

References

Anthoathecata Prehistoric cnidarian genera {{paleo-invertebrate-stub