Calathium

{{Short description|Extinct genus of organism}}

{{Automatic taxobox

| image = Calathium formosum.jpg

| image_caption = Calathium formosum

| taxon = Calathium

| authority = Billings 1865

| synonyms =

| synonyms_ref =

}}

Calathium is an extinct genus of organism found in marine beds of Ordovician age. Its classification is enigmatic: It has long been placed among the receptaculites,{{cite journal |last1=Fisher |first1=Daniel C. |last2=Nitecki |first2=Matthew H. |title=Standardization of the Anatomical Orientation of Receptaculitids |journal=Journal of Paleontology |date=January 1982 |volume=56 |issue=S13 |pages=1–40 |doi=10.1017/S0022336000061928|bibcode=1982JPal...56S...1F }}{{cite journal |last1=Church |first1=Stephen B. |title=A new Lower Ordovician species of Calathium , and skeletal structure of western Utah calathids |journal=Journal of Paleontology |date=July 1991 |volume=65 |issue=4 |pages=602–610 |doi=10.1017/S0022336000030699|bibcode=1991JPal...65..602C }} but it has also been described as a quasi-sponge,{{cite journal |title=Reported Silurian Occurrence of Calathium from the Thornton Reef, Illinois: A Correction |first1=Donald F. |last1=Toomey |first2=Jerome J. C. |last2=Ingels |journal=Journal of Paleontology |volume=38 |number=6 |year=1964 |pages=1102–1104 |jstor=1301646}} possibly akin to the archeocyathids or other hypercalcified sponge.{{cite journal |last1=Meng |first1=Miaomiao |last2=Fan |first2=Tailiang |last3=Duncan |first3=Ian |title=Sedimentary characteristics of the Lower to Middle Ordovician Calathium reefs in the northwestern Tarim Basin, NW China |journal=Carbonates and Evaporites |date=March 2021 |volume=36 |issue=1 |pages=4 |doi=10.1007/s13146-020-00665-7|bibcode=2021CarEv..36....4M }} The chief difference from archaeocyathids is that their walls were connected by rods rather than septae.{{cite journal |last1=Li |first1=Qijian |last2=Li |first2=Yue |last3=Wang |first3=Jianpo |last4=Kiessling |first4=Wolfgang |title=Early Ordovician lithistid sponge– Calathium reefs on the Yangtze Platform and their paleoceanographic implications |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=May 2015 |volume=425 |pages=84–96 |doi=10.1016/j.palaeo.2015.02.034|bibcode=2015PPP...425...84L }}

The organisms were important reef-forming organisms during the Ordovician, forming communities with lithistid sponges that gradually displaced the earlier microbial mounds.