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 in ...

remains of

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

laid by

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

. 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 in ...

of cephalopod eggs is scant since their soft, gelatinous eggs decompose quickly and have little chance to fossilize. Eggs laid by

ammonoids Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

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 Mya. The J ...

Kimmeridge Clay The Kimmeridge Clay is a 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 is the major source rock for North ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. 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 be ...

and

aerobic bacteria Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cell ...

. 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 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 orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

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 the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

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 The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk. In gastropods In gastropods, the ...

of a harpoceratid dating back to the

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

age of the

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

Baltic region The terms Baltic Sea Region, Baltic Rim countries (or simply the Baltic Rim), and the Baltic Sea countries/states refer to slightly different combinations of countries in the general area surrounding the Baltic Sea, mainly in Northern Europe. ...

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

Ceratites ''Ceratites'' is an extinct genus of ammonite cephalopods. These nektonic carnivores lived in marine habitats in what is now Europe, during the Triassic, from the upper-most Anisian to the lower Ladinian age.Sepkoski, JacSepkoski's Online Genus D ...

'' from the

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

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

, which dated to the Upper

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...

of the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

period. An additional less plausible case has been reported from

Kamchatka The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) 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 wes ...

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 younger 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 ''creta'', the ...

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 Lias may refer to: Geology * Lias Formation, a geologic formation in France *Lias Group, a lithostratigraphic unit in western Europe * Early Jurassic, an epoch People * Godfrey Lias, British author * Mohd Shamsudin Lias (born 1953), Malaysian ...

, 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 Carbonized were a Swedish avant-garde metal band formed 1988 in Saltsjöbaden. The band was formed by Lars Rosenberg in 1988, with Dismember vocalist Matti Kärki. Joined by drummer Piotr Wawrzeniuk, the trio was completed by Therion's Christo ...

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 a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

eggs in the Lower and Upper

of the Dorset Coast in

. The fossil sites were within the

Jurassic Coast The Jurassic Coast is a World Heritage Site on the English Channel coast of southern England. It stretches from Exmouth in East Devon to Studland Bay in Dorset, a distance of about , and was inscribed on the World Heritage List in mid-December ...

World Heritage Site A World Heritage Site is a landmark or area with legal protection by an international convention administered by the United Nations Educational, Scientific and Cultural Organization (UNESCO). World Heritage Sites are designated by UNESCO for h ...

. 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 ''Aulacostephanus'' is an extinct ammonoid cephalopod genus from the Upper Jurassic Tithonian belonging to the perisphinctoidean family Aulacostephanidae. ''Aulacostephanus'' produced a discoidal, strongly ribbed, evolute shell of moderate s ...

'' 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 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 ...

cephalopods.

Belemnoids

Unlike ammonites, no eggs of

belemnites Belemnites may refer to: *Belemnitida Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. ...

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