Cephalopod egg fossils are 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 eggs laid by

cephalopods A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

. The

fossil record 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 ...

of cephalopod eggs is scant since their soft, gelatinous eggs decompose quickly and have little chance to fossilize. Eggs laid by ammonoids are the best known and only a few putative examples of these have been discovered. The best preserved of these were discovered in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

Kimmeridge Clay The Kimmeridge Clay is a sedimentary rock, sedimentary deposit of fossiliferous marine clay which is of Late Jurassic to lowermost Cretaceous age and occurs in southern and eastern England and in the North Sea. This rock formation (geology), form ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. Currently no belemnoid egg fossils have ever been discovered although this may be because scientists have not properly searched for them rather than an actual absence from the fossil record.

Taphonomy

Ammonite eggs in well-aerated sea bottoms probably would have quickly been broken down by

scavengers Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding ...

and

aerobic bacteria An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. The ability to exhibit aerobic respiration may yield benefits to the aerobic organism, as aerobic respiration yields more energy than anaerobic ...

. Fossil evidence supports this general idea since swarms of hatchling ammonitellae fossils are known although there are no associated egg fossils. One of the eggs preserved in the Kimmeridge Clay ammonite egg cluster K1486 bears

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

on its surface. Since phosphate is mobile only in organic form this suggests the eggs were already decaying before fossilization. The fact that the ammonite eggs survived decomposition to become fossilized suggests two possibilities. The first is that the eggs were transported from the well-aerated location they were originally laid in to low-

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

waters near the sea floor. The second possibility is that the eggs were laid in waters whose oxygen abundance varied, possibly by season.

Cephalopod taxa

Ammonoids

Multiple instances of fossil ammonite eggs have been observed in the fossil record. However, prior to 2009 only two credible examples of ammonite eggs had been reported to the scientific literature. The first was an apparent clutch of eggs preserved in the sediment that filled in the living chamber of a harpoceratid dating back to the

Toarcian The Toarcian is, in the International Commission on Stratigraphy, ICS' geologic timescale, an age (geology), age and stage (stratigraphy), stage in the Early Jurassic, Early or Lower Jurassic. It spans the time between 184.2 Megaannum, Ma (million ...

age of the

period. This specimen was discovered in a concretion incorporated into glacial drift that came from the

Baltic region The Baltic Sea Region, alternatively the Baltic Rim countries (or simply the Baltic Rim), and the Baltic Sea countries/states, refers to the general area surrounding the Baltic Sea, including parts of Northern, Central and Eastern Europe. Un ...

. The ammonite itself was a fully grown individual with a macroconch shell. The second possible example was another adult macroconch of '' Ceratites'' from the

Muschelkalk The Muschelkalk (German for "shell-bearing limestone"; ) is a sequence of sedimentary rock, sedimentary rock strata (a lithostratigraphy, lithostratigraphic unit) in the geology of central and western Europe. It has a Middle Triassic (240 to 230 m ...

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, which dated to the Upper

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

of the

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 ...

period. An additional less plausible case has been reported from

Kamchatka The Kamchatka Peninsula (, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and western coastlines, respectively. Immediately offshore along the Pacific ...

where an egg clutch was purportedly associated with a '' Desmophyllites'' dating back to the early Triassic. However a later summary of ammonite embryos from the same age and location does not mention any eggs being known and ''Desmophyllites'' is a

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

genus, so this report is not reliable. The first two possible ammonite egg specimens were found in sediments filling the interior of the body chambers of ammonite shells. Lehmann's specimen from the Lias, reported in 1966, was a pouch of about 50 empty egg capsules lying in the innermost part of a mature macroconch's body chamber. The specimen was not filled with sediment, but rather crystalline calcite. The eggsac had been carbonized completely, destroying its fine detail. Other than the eggs, the specimen does not preserve any of the ammonite's soft parts. The eggs preserved within this shell probably did not belong to the shell's occupant since the eggs probably would have been removed with the rest of the body. The shell itself of Lehmann's specimen belonged to the genus '' Eleganticeras''. The researchers described the other specimen, reported by Muller in 1969 from the Trias, as a "carbonized ghost" that preserved very little detail, not even showing signs of individual egg capsules. Its outline was a similar shape to an axe-head, like Lehmann's. The specimen was within the sedimentary infill of a mature macroconch. There were no sign of any other soft parts. The egg sac is positioned near the opening and oriented in a way that might suggest it was attached to the shell. These specimens have been used to support the contention that macroconch ammonite shells were those of females. Apart from these specimens this would just be a general assumption. In 2009, Steve Etches, Jane Clarke, and John Callomon reported the discovery of eight clusters of

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

eggs in the Lower and Upper

of the Dorset Coast in

. The fossil sites were within the Jurassic Coast

World Heritage Site World Heritage Sites are landmarks and areas with legal protection under an treaty, international treaty administered by UNESCO for having cultural, historical, or scientific significance. The sites are judged to contain "cultural and natural ...

. The eggs are subspherical to spherical in shape. Some are isolated but some were also found in association with the shells of perisphinctid ammonites. They were interpreted by the researchers as ammonite eggs sacks and are the best preserved specimens of such known to science. The parents of the egg sacks are thought to be two local ammonite genera co-occurring with the eggs, '' Aulacostephanus'' and '' Pectinatites''. The Kimmeridge Clay cephalopod eggs are regarded as ammonite eggs based on "strong" circumstantial evidence. These spawning grounds were sometimes small geographic areas that were used continuously for long periods of time. Ammonite taxa that remained in one place for a long period of time are called to that place. This leaves the most probable identity of the egglayers as ''Aulacostephanus'' and ''Pectinatites'' since they were the ammonites eudemic to the region at the time the egg fossils formed. The researchers described the Kimmeridge Clay ammonite eggs as offering "the best insights so far" into the embryonic phase of the ammonite life cycle. Since the Kimmeridge Clay is so thoroughly studied the environment and depositional context is better understood for these ammonite eggs than those reported in the previous two examples. The eggs provide evidence that ammonites had similar life cycles to modern neritic cephalopods.

Belemnoids

Unlike ammonites, no eggs of belemnites are currently known from the fossil record, although Steve Etches, Jane Clarke, and John Callomon have observed that this may be due to a lack of people actually looking for them.

Footnotes

References