''Hildoceras bifrons'' is an extinct species 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) ...

in the family

Hildoceratidae Hildoceratidae is a family of ammonoid cephalopods from the Lower Jurassic, lower Pliensbachian (''Jamesoni'' zone) to lower Bajocian (maybe even upper Bajocian) substages, generally with strongly ribbed, involute shells. They are combined with ...

. It dates from about 175 million years ago in the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

when it was both widespread and common.

Fossil 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 ...

s have been found in North Africa and Europe, including several regions of England.

Origin of the name

The genus name has been given the name ''

Hildoceras ''Hildoceras'' is a genus of ammonite from the Jurassic period in the family Hildoceratidae. The shells are characterized by a narrow discoidal evolute shape, keeled venter, concave ribs along the outer flanks, and a shallow spiral groove r ...

'' in honour of St. Hilda of Whitby (614-680 AD). Legend has it that this lady was required to found an

abbey An abbey is a type of monastery used by members of a religious order under the governance of an abbot or abbess. Abbeys provide a complex of buildings and land for religious activities, work, and housing of Christian monks and nuns. The conce ...

on the cliffs above

Whitby Whitby is a seaside town, port and civil parish in the Scarborough borough of North Yorkshire, England. Situated on the east coast of Yorkshire at the mouth of the River Esk, Whitby has a maritime, mineral and tourist heritage. Its East Clif ...

, in the north of England. Finding the site to be infested by snakes (a devilish omen), she prayed to the Lord and the snakes coiled up and were turned to stone. She picked them up and threw them over the cliff, and that is why there are so many ammonite fossils in the rocks below the abbey. The specific name ''bifrons'' comes from Bifron, a demon, another name for the

Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter ...

god,

Janus In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...

Description

''Hildoceras bifrons'' has a slender, flattened, deeply embossed spiral shell with dense transverse ribbing and a deep groove that runs parallel to the spiral. The umbilicus is convex and has gently sloping sides. The aperture is circular. The fossils come in two sizes, the macroconch (female) ranging in size from in diameter and the microconch (male) which ranges from . When the animal was alive, the interior of the shell was divided into chambers which were partly filled with air for buoyancy. The soft parts of the body have not been preserved in the fossils known, but it is probable that ''Hildoceras bifrons'' was a

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

and caught its prey with

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s that projected from its aperture.

Distribution

Fossils of ''Hildoceras bifrons'' have been found in North Africa, the

Caucasus Mountains The Caucasus Mountains, : pronounced * hy, Կովկասյան լեռներ, : pronounced * az, Qafqaz dağları, pronounced * rus, Кавка́зские го́ры, Kavkázskiye góry, kɐfˈkasːkʲɪje ˈɡorɨ * tr, Kafkas Dağla ...

, Europe and Eastern England. It is particularly associated with the fossil beds in the

Whitby Mudstone Formation The Whitby Mudstone is a Toarcian (Early Jurassic; ''Falciferum''-''Bifrons'' in regional chronostratigraphy) geological formation in Yorkshire and Worcestershire, England.index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock Stratum, strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictiona ...

to help identify the

geological period The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

of the rocks. It dates 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 ...

in the

, some 184 to 175 million years ago.

Biology

There has been much debate as to the mode of life and swimming ability of ammonites. Their closest living relatives, the

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

es, are able to swim because their weight does not exceed their buoyancy, their apertures are suitably orientated and they have sufficient stability to maintain a vertical position. Careful examination of ''Hildoceras bifrons'' and some other ammonites makes it seem unlikely that they had a

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

lifestyle and it is believed that they were primarily

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

animals living on the seabed.

References

{{Taxonbar, from=Q16753811 Jurassic ammonites Cephalopods described in 1789 Fossil taxa described in 1789