Oncolite

{{Distinguish|Oolite}}

{{Short description|Trace fossil}}

Image:Oncolites.jpg (Silurian), with shell fragments as nuclei. The large flower is 2 cm in diameter.]]

Image:Bactroptyxis trachaea 041811.jpg gastropod Bactroptyxis trachaea (Normandy, France).]]

Oncolites are sedimentary structures composed of oncoids, which are layered structures formed by the growth of one or more species of microorganisms,{{cite journal |last1=Zafar |first1=Tehseen |last2=Frontalini |first2=Fabrizio |last3=Rehman |first3=Hafiz Ur |last4=Khan |first4=Danish |last5=Ullah |first5=Zaheen |last6=Rahim |first6=Hamad ur |last7=Oyebamiji |first7=Abiola |last8=Hussain |first8=Zahid |last9=Song |first9=Shuguang |last10=Farhan |first10=Muhammad |last11=Ahmed |first11=Mohamed S. |last12=Sami |first12=Mabrouk |last13=Fathy |first13=Douaa |title=Understanding the origin of Cambrian marine oncoids: A petrologic, geochemical, and isotopic perspective |journal=Sedimentary Geology |date=June 2024 |volume=466 |pages=106638 |doi=10.1016/j.sedgeo.2024.106638 |bibcode=2024SedG..46606638Z }} usually containing cyanobacteria.{{cite journal |last1=Hägele |first1=Daniela |last2=Leinfelder |first2=Reinhold |last3=Grau |first3=Jürke |last4=Burmeister |first4=Ernst-Gerhard |last5=Struck |first5=Ulrich |title=Oncoids from the river Alz (southern Germany): Tiny ecosystems in a phosphorus-limited environment |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=August 2006 |volume=237 |issue=2–4 |pages=378–395 |doi=10.1016/j.palaeo.2005.12.016 |bibcode=2006PPP...237..378H }} Oncolites are very similar to stromatolites, but, instead of forming columns, they form approximately spherical structures.{{cite journal |last1=Corsetti |first1=Frank A. |last2=Awramik |first2=Stanley M. |last3=Pierce |first3=David |title=A complex microbiota from snowball Earth times: Microfossils from the Neoproterozoic Kingston Peak Formation, Death Valley, USA |journal=Proceedings of the National Academy of Sciences |date=15 April 2003 |volume=100 |issue=8 |pages=4399–4404 |doi=10.1073/pnas.0730560100 |doi-access=free |pmid=12682298 |pmc=153566 |bibcode=2003PNAS..100.4399C }} The oncoids often form around a central nucleus, such as a shell fragment,{{cite journal |last1=Gutschick |first1=R. C. |last2=Perry |first2=T. G. |title=Sappington (Kinderhookian) Sponges and Their Environment |journal=Journal of Paleontology |date=1959 |volume=33 |issue=6 |pages=977–985 |jstor=1300833 }} and a calcium carbonate structure is deposited by encrusting microbes. Oncolites are indicators of warm waters in the photic zone, but are also known in contemporary freshwater environments.{{cite book |doi=10.1007/978-3-642-52335-9 |page=32 |title=Calcareous Algae and Stromatolites |date=1991 |isbn=978-3-642-52337-3 |editor-last1=Riding |editor-first1=Robert }} These structures rarely exceed 10 cm in diameter.

Oncolites may have either a porostromate or spongiostromate texture. Most oncolites are spongiostromate, having no recognisable cellular texture or microstructure. Porostromate oncolites are mostly unknown during the Precambrian; since the Eocene they have mostly been confined to freshwater environments.{{cite book |doi=10.1007/978-3-642-67913-1_1 |chapter=Spongiostromate vs. Porostromate Stromatolites and Oncolites |title=Phanerozoic Stromatolites |date=1981 |last1=Monty |first1=C. L. |pages=1–4 |isbn=978-3-642-67915-5 }}