''Chondrites'' is a trace fossil

ichnogenus An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' and ταξις, ''taxis'' meaning ...

, preserved as small branching burrows of the same diameter that superficially resemble the roots of a plant. The origin of these structures is currently unknown. ''Chondrites'' is found in marine sediments from the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ( ...

period of the Paleozoic onwards. It is especially common in sediments that were deposited in reduced-oxygen environments.

Morphology

There are four recognized ''Chondrites'' ichnospecies, however, aberrant forms of unknown

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. A ...

affinity have been discovered. The fossils are made of infilled dendritic rootlike burrows. The branching angles are 30° to 40°, while a shaft diameter varies between 0.1 mm and 10 mm, remaining constant within a single system. ''Chondrites'' is classified as a fodinichnion.

Occurrence

First appearing during the Cambrian, ''Chondrites'' is still produced today. It is one of the most common ichnotaxa throughout the fossil record and is widely distributed in all types of marine sedimentary rocks, including sandstone, shale, limestone and

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...

that formed in environments ranging from

subtidal The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

shelves to the

abyssal The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...

zone. The tracemaker has been able to tolerate highly variable redox conditions. For example, ''Chondrites'' is abundant almost to the exclusion of other ichnogenera in

Posidonia Shale The Posidonia Shale (german: Posidonienschiefer, also called Schistes Bitumineux in Luxembourg) geologically known as the Sachrang Formation, is an Early Jurassic (Toarcian) geological formation of southwestern and northeast Germany, northern Swi ...

, formed from laminated, black, carbonaceous clay, deposited in an

anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of diss ...

, reducing environment. In the oxic, extensively bioturbated units of

Austin Chalk The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas. The formation is made up of chalk and marl. Fossils The putative galloanser ...

, the fossil is very common as well.

Interpretation

Though the characteristic burrows are still produced today (in deep-sea deposits), no organism has ever been observed inside them. Several theories exist regarding the origin of these structures. While some authors hypothesize ''Chondrites'' to be the product of an

infaunal Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is ''funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zool ...

abyssal

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...

, others propose it to be formed by a chemosymbiotic organism, pumping methane and hydrogen sulfide from the sediments. Another study suggests it to be a

fecal Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

storage structure. The ichnogenus is found both in anaerobic, organic-rich sediments and in oxic layers, where it is almost invariably the last in the bioturbation sequence, i.e., it was placed deep within the sediments, away from oxidizing surficial and interstitial water. These suggest the tracemaker's ability to tolerate oxygen deprivation very well. Therefore, ''Chondrites'' can be used as an indicator of anoxia in sediments.

References

{{Taxonbar, from=Q1076049 Burrow fossils