Heteromorph Ammonites
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The Ancyloceratina were a diverse suborder 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 ...
most closely related to the ammonites of order
Lytoceratina Lytoceratina is a suborder of Jurassic and Cretaceous ammonites that produced loosely coiled, evolute and gyroconic shells in which the sutural element are said to have complex moss-like endings. Morphologic characteristics Shells are generally ...
. They evolved during the Late
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. ...
but were not very common until the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
period, when they rapidly diversified and became one of the most distinctive components of Cretaceous marine faunas. They have been recorded from every continent and many are used as zonal or
index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...
s. The most distinctive feature of the majority of the Ancyloceratina is the tendency for most of them to have shells that are not regular spirals like most other ammonites. These irregularly-coiled ammonites are called heteromorph ammonites, in contrast to regularly coiled ammonites, which are called homomorph ammonites.


Biology

The biology of the heteromorph ammonites is not clear, but one certainty is that their uncoiled shells would have made these forms very poor swimmers. Open shells, particularly ones with spines and ribs, create a lot of drag; but more importantly, the orientation of the shell, with the body hanging below the buoyant part of the shell, would have created a serious impediment to efficient swimming. It's more likely these ammonites either drifted in the
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
, collecting small animals on long
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 like modern
jellyfish Jellyfish, also known as sea jellies or simply jellies, are the #Life cycle, medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animal ...
, or else they crawled along the sea floor feeding on sessile or slow-moving animals such as
clam Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...
s.


Morphology

Ancyloceratida varied widely in size, ranging from diminutive ''Ptychoceras'', which was barely long, to ''Baculites'' and '' Diplomoceras'', which could grow to in length. Some species were very widely distributed, for example some species of ''Hamites'' can be found in
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,
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,
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, and
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. Others, like those inhabiting the
Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, or the Western Interior Sea) was a large inland sea (geology), inland sea that existed roughly over the present-day Great Plains of ...
that covered much of the US, were much more localized. In the more primitive forms, the shell departs only slightly from being a perfect spiral, with only the last, outermost whorl being open, forming a hook underneath the main spiral. In such forms the spiral was the chambered,
buoyant Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...
part of the shell, and the hook was the living chamber in which the soft body of the ammonite resided. Examples of such types were '' Ancyloceras'', '' Protanisoceras'', and ''
Tropaeum ''Tropaeum'' is an extinct genus of ammonites found throughout the oceans of the world during the Early Cretaceous. As with many other members of the family Ancyloceratidae, there was a trend among species within this genus to uncoil somewhat, ...
''. The more advanced heteromorphs departed from such forms radically. The shells of '' Ptychoceras'' consisted of three or four shafts squashed together and connected with tight, 180 degree bends. Members of the genus ''
Hamites Hamites is the name formerly used for some Northern and Horn of Africa peoples in the context of a Scientific racism, now-outdated model of dividing humanity into different races; this was developed originally by Europeans in support of coloni ...
'' were much larger but had a similar sort of shape, though the shafts were open so that the whole thing looked rather like a big
paper clip A paper clip (or paperclip) is a tool used to hold sheets of paper together, usually made of steel wire bent to a looped shape (though some are covered in plastic). Most paper clips are variations of the ''Gem'' type introduced in the 1890s or ...
. Many of the earlier heteromorph forms had regularly coiled shells barely distinguishable from the homomorph ammonites (for example, the Lower Cretaceous genus '' Deshayesites''). Some offshoots of the uncoiled varieties even went back to being regularly coiled. Most notable among these were the ''
Scaphites ''Scaphites'' is a genus of heteromorph ammonites belonging to the Scaphitidae family. They were a widespread genus that thrived during the Late Cretaceous period. Morphology ''Scaphites'' generally have a chambered, boat-shaped shell. The ini ...
'', ''
Hoploscaphites ''Hoploscaphites'' is an extinct ammonite genus from the Upper Cretaceous and the Lower Paleocene, included in the family Scaphitidae. Morphology It is considered by some to be a subgenus of ''Scaphites''. Like ''Scaphites,'' the shell of ''Hop ...
'' and their relatives, which were mostly regular spirals in shape except for a very slight hook at the end. The Late Cretaceous enjoyed the widest variety of heteromorphs, including the straight-shelled ''
Baculites ''Baculites'' is an extinct genus of heteromorph ammonite cephalopods with almost straight shells. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the K-Pg mass extinction event, was named by ...
'' and '' Sciponoceras''; the helically coiled '' Turrilites''; and the bizarre ''
Nipponites ''Nipponites'' is an extinct genus of heteromorph ammonites. The shells of ''Nipponites'' (primarily ''N. mirabilis'') form "ox-bow" bends, resulting in some of the most bizarre shapes seen among ammonites. The ecology of ''Nipponites'', as wit ...
'', which looked more like a ball of string than anything else. Some forms combined different coiling modes. For example, ''
Nostoceras ''Nostoceras'' is an extinct genus of ammonites. The etymology of the name ''Nostoceras'' comes from "nostos" meaning return and "ceros" meaning horn, named as such by Alpheus Hyatt because it bends back on itself. Taxonomy ''Nostoceras'' is the ...
'' started off with a helix like a ''Turrilites'', but had a planar hook hanging underneath, more like an ''Ancyloceras''. The thin shelled heteromorphic ammonites probably lived at depths .


Classification

Suborder Ancyloceratina * Superfamily
Ancyloceratoidea Ancyloceratoidea, formerly Ancylocerataceae, is a superfamily of typically uncoiled and loosely coiled heteromorph Ammonoidea, ammonoids established by Alpheus Hyatt in 1900, that may contain as many as 11 families, depending on the classificat ...
** Family Acrioceratidae ** Family
Ancyloceratidae Ancyloceratidae is a family of heteromorphic Ammonitida, ammonites that lived during the Early Cretaceous. Their shells begin as a loose spiral with whorls not touching which then turns into a straight shaft that ends in a J-shape hook or bend ...
** Family Bochianitidae ** Family
Crioceratitidae Crioceratitidae is an extinct cephalopod family belonging to the subclass Ammonoidea and included in the order Ammonitida. Genera * '' Acantholytoceras'' Spath, 1932 * ''Balearites ''Balearites'' is an extinct ancyloceratin genus included i ...
** Family Hamulinidae ** Family
Hemihoplitidae Hemihoplitidae is an extinct family of ammonoid cephalopods belonging to the superfamily Ancyloceratoidea. Fossils of species within this genus have been found in the Cretaceous rocks of southeastern France, Mexico, Slovakia, South Africa and Tri ...
** Family Heteroceratidae ** Family Himantoceratidae ** Family Labeceratidae ** Family Macroscaphitidae ** Family Ptychoceratidae * Superfamily Deshayesitoidea ** Family Deshayesitidae ** Family
Parahoplitidae Parahoplitidae is an extinct family of Cretaceous Ammonitida, ammonites with stoutly ribbed, compressed, generally involute shells lacking or with only minor tubercles included in the Deshayestoidea, a superfamily now separated from the Hoplitaca ...
* Superfamily Douvilleiceratoidea ** Family Astiericeratidae ** Family Douvilleiceratidae ** Family Trochleiceratidae * Superfamily
Scaphitoidea Scaphitidae is a family of extinct cephalopods belonging to the family of heteromorph ammonites (suborder Ancyloceratina). There is a possible fossil record of them being the last ammonites with fossils dating to the Danian of the Paleocene in D ...
** Family
Scaphitidae Scaphitidae is a family of extinct cephalopods belonging to the family of heteromorph ammonites (suborder Ancyloceratina). There is a possible fossil record of them being the last ammonites with fossils dating to the Danian of the Paleocene in D ...
* Superfamily
Turrilitoidea Turrilitoidea is a diverse superfamily of Cretaceous ammonites generally considered as heteromorphic and commonly included in the suborder Ancyloceratina. Shells of this diverse group do not coil planospirally, as typical for most ammonitida, am ...
** Family
Anisoceratidae Anisoceratidae is an extinct Family (biology), family of heteromorph ammonites which belong to the Ancyloceratina superfamily Turrilitoidea. Members of the family range is from the lower Albian to the upper Turonian. The family is possibly deriv ...
** Family Baculitidae ** Family Diplomoceratidae ** Family
Hamitidae ''Hamites'' (" hook-like") is a genus of heteromorph ammonite that evolved late in the Aptian stage of the Early Cretaceous and lasted into the Cenomanian stage of the Late Cretaceous. The genus is almost certainly paraphyletic but remains in wi ...
** Family
Nostoceratidae Nostoceratidae is a diverse Family (biology), family of heteromorph ammonites found throughout the oceans of the world during the Late Cretaceous. The nostoceratids are famous for the bizarre coiling of their shells. Many genera, such as ''Yezo ...
** Family Turrilitidae


Footnotes


References

* * Pasch, A. D., K. C. May. 2001. Taphonomy and paleoenvironment of hadrosaur (Dinosauria) from the Matanuska Formation (Turonian) in South-Central Alaska. In: ''Mesozoic Vertebrate Life''. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press. Pages 219–236.


External links


Scientific report on the functional morphology of Cretaceous heteromorph ammonites
* {{Taxonbar, from=Q290948 Ammonitida suborders Prehistoric animal suborders Mollusc suborders Late Jurassic first appearances Paleocene extinctions