The

fossilized A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

remains of fish

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the ...

s have an extensive record going at least as far back as the

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

and spanning into the

Cenozoic era The Cenozoic Era ( ; ) is Earth's current geological Era (geology), era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, Insect, insects, birds and flowering plant, angiosperms (floweri ...

. The eggs of many different fish

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

have contributed to this record, including

lobe-finned fish Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

, placoderms, and

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s. Occasionally eggs are preserved still within the mother's body, or associated with fossil

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

s. Some fossil eggs possibly laid by fish cannot be confidently distinguished from those laid by

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

s; for example, the

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. ''Ichnotaxon'' comes from the Ancient Greek (''íchnos'') meaning "track" and English , itself derived from ...

'' Mazonova'' is known from impressions of eggs which resemble eggs of both fish and amphibians. Paleontologist B.K. Hall has observed that the discovery of fossil fish eggs, embryos and

larvae A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect developmental biology, development such as insects, some arachnids, amphibians, or cnidarians typical ...

link the sciences of paleontology with evo-devo.

Fish taxa known from egg fossils

Cartilaginous fishes

Fossilized egg cases that may have been laid by an

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...

or chimaeroid are known from as far back as the Devonian period. Egg sacs that can more confidently be referred to those taxa are known from the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

, although no embryos are known from these. Shark eggs have been recognized in the fossil record. More than thirty fossil shark egg cases have been discovered at the Bear River Seep Deposit of Washington state in the US. These capsules are up to 5 cm in length and resemble those of modern catshark species in the genus '' Apristurus''. Since fossil teeth attributable to this genus go back to the late

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

epoch, an ancient ''Apristurus'' species might very well be the mother of the fossil egg cases. A less likely candidate mother would be the genus '' Scyliorhinus'', which also has a fossil record stretching back to the Eocene. However, modern ''Scyliorhinus'' egg capsules are smoother than the Bear River Seep Deposit fossils, casting doubt on this possibility.

Lobe-finned fishes

Some well-preserved embryos have been found with

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example bec ...

sacs in the Carboniferous actinistian '' Rhabdoderma exiguum''. These range in development from specimens still inside the egg to individuals who had partially resorbed their yolk sac. These specimens were part of the Mazon Creek fauna of

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...

Placoderms

One possible specimen of the Devonian

placoderm Placoderms (from Ancient Greek πλάξ 'plax'', ''plakos'''Plate (animal anatomy), plate' and δέρμα 'derma'''skin') are vertebrate animals of the class (biology), class Placodermi, an extinct group of prehistoric fish known from Pal ...

'' Cowralepis mclachlani'' contained many unhatched egg sacs.

Confounding factors

Some fossil eggs possibly laid by fish can't be confidently distinguished from those laid by

s. Examples are known from various Carboniferous and

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 ...

lagerstatten that preserve the eggs of aquatic vertebrates. The identity of the fossils' egglayers can't be confidently ascertained.

Footnotes

References

* Cloutier, R. 2010. The fossil record of fish ontogenies: insights to developmental patterns and processes. ''Semin Cell Dev Biology'' 21: 400–413. * Treude, T., Kiel, S., Linke, P., Peckmann, J., and Goedert, J.L. 2011. Elasmobranch egg capsules associated with modern and ancient cold seeps: A nursery for non-seep marine predators. ''Marine Ecology Progress Series'' 437: 175–181. {{eggs .