Thefossilized remains of fisheggs have an extensiverecord going at least as far back as theDevonian and spanning into theCenozoic era. The eggs of many different fishtaxa have contributed to this record, includinglobe-finned fish,placoderms, andsharks. Occasionally eggs are preserved still within the mother's body, or associated with fossilembryos. Some fossil eggs possibly laid by fish cannot be confidently distinguished from those laid byamphibians; for example, theichnogenusMazonova 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 andlarvae link the sciences of paleontology withevo-devo.[1]
Fossilizedegg cases that may have been laid by anelasmobranch orchimaeroid are known from as far back as the Devonian period. Egg sacs that can more confidently be referred to those taxa are known from theCarboniferous toOligocene, although no embryos are known from these.[2]
Shark eggs have been recognized in the fossil record. More than thirty fossil sharkegg cases have been discovered at the Bear River Seep Deposit of Washington state in the US.[3] These capsules are up to 5 cm in length and resemble those of moderncatshark species in the genusApristurus.[4] Since fossil teeth attributable to this genus go back to the lateEocene epoch, an ancientApristurus species might very well be the mother of the fossil egg cases. A less likely candidate mother would be the genusScyliorhinus, which also has a fossil record stretching back to the Eocene. However, modernScyliorhinus egg capsules are smoother than the Bear River Seep Deposit fossils, casting doubt on this possibility.[5]
Some well-preserved embryos have been found withyolk sacs in the CarboniferousactinistianRhabdoderma exiguum.[6] These range in development from specimens still inside the egg to individuals who had partially resorbed their yolk sac. These specimens were part of theMazon Creek fauna ofIllinois.[7]
One possible specimen of the DevonianplacodermCowralepis mclachlani contained many unhatched egg sacs.[2]
Some fossil eggs possibly laid by fish can't be confidently distinguished from those laid byamphibians. Examples are known from various Carboniferous andTriassiclagerstatten that preserve the eggs of aquatic vertebrates. The identity of the fossils' egglayers can't be confidently ascertained.[2]
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| Biology | |
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