Hamites
Encyclopedia
Hamites 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 wide use as a "catch all" for heteromorph ammonites of the superfamily Turrilitaceae
that do not neatly fit into the more derived
groupings. In an attempt to identify clades within the genus, it has been divided up into a series of new genera or subgenera by different palaeontologists
, including Eohamites, Hamitella, Helicohamites, Lytohamites, Planohamites, Psilohamites, and Sziveshamites.
The type species is Hamites attenuatus from the early Albian, named by James Sowerby
in his Mineral Conchology of Great Britain of 1814, although the genus itself was created by James Parkinson
in his 1811 book Organic Remains of the Former World. This James Parkinson is best known as the first scientific description of a disease he called the Shaking Palsy, now referred to as Parkinson's disease
in his honour.
http://homepage.mac.com/nmonks/Projects/ammonitereconstr.html. No Hamites had spines or other such ornamentation on the shell, but several species appear to have developed apertural modifications when mature; that is, once the ammonite had grown to its final size, the aperture became constricted and was bounded by one or two thickened ribs, known as collars. These have been observed on other ammonites as well, and are assumed to be signs of sexual dimorphism
.
The open shell of these ammonites would have made them poor swimmers because of drag
, but beyond that fact, very little is certain about their mode of life. It is widely assumed that they were plankton
ic, perhaps catching small prey in the manner of jellyfish
, but repaired shell damage apparently caused by crabs may indicate that they spent at least some time close to the sea floor.
groups of heteromorph ammonites. The genus rapidly diversified
during the Albian
into a number of morphologically distinct lineages that seem to have given rise to at least three other families of heteromorphs, the Baculitidae, Turrilitidae
, and Scaphitidae
. The lineage that gave rise to the helical Turrilitidae, for example, had a shell that initially grew as a helix before straightening out; the Turrilitidae thus appear to have been derived from neotenic Hamites that retained the helically-coiled juvenile morphology of Hamites into adulthood.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of heteromorph ammonite
Ammonite
Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...
that evolved late in the Aptian
Aptian
The Aptian is an age in the geologic timescale or a stage in the stratigraphic column. It is a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma , approximately...
stage of the Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
and lasted into the Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...
stage of the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
. The genus is almost certainly paraphyletic but remains in wide use as a "catch all" for heteromorph ammonites of the superfamily Turrilitaceae
Turrilitaceae
The Turrilitacaeae is a diverse superfamily of Cretaceous ammonites generally considered as heteromorphic and commonly included in the suborder Ancyloceratina...
that do not neatly fit into the more derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
groupings. In an attempt to identify clades within the genus, it has been divided up into a series of new genera or subgenera by different palaeontologists
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, including Eohamites, Hamitella, Helicohamites, Lytohamites, Planohamites, Psilohamites, and Sziveshamites.
The type species is Hamites attenuatus from the early Albian, named by James Sowerby
James Sowerby
James Sowerby was an English naturalist and illustrator. Contributions to published works, such as A Specimen of the Botany of New Holland or English Botany, include his detailed and appealing plates...
in his Mineral Conchology of Great Britain of 1814, although the genus itself was created by James Parkinson
James Parkinson
James Parkinson was an English apothecary surgeon, geologist, paleontologist, and political activist. He is most famous for his 1817 work, An Essay on the Shaking Palsy in which he was the first to describe "paralysis agitans", a condition that would later be renamed Parkinson's disease by...
in his 1811 book Organic Remains of the Former World. This James Parkinson is best known as the first scientific description of a disease he called the Shaking Palsy, now referred to as Parkinson's disease
Parkinson's disease
Parkinson's disease is a degenerative disorder of the central nervous system...
in his honour.
Morphology and ecology
Hamites species are characterised by a shell that began with an open, sometimes helical, regular spiral that either opened into a single large hook http://homepage.mac.com/nmonks/Projects/ammonitereconstr.html, or else formed three parallel shafts that gave the mature shell the approximate appearance of a paper clipPaper Clip
"Paper Clip" is a 1995 episode of The X-Files television series. It was the second episode broadcast in the show's third season. Paper Clip concludes the story regarding the agents' possession of a digital tape containing government secrets on extraterrestrials.- Plot :Continuing from the previous...
http://homepage.mac.com/nmonks/Projects/ammonitereconstr.html. No Hamites had spines or other such ornamentation on the shell, but several species appear to have developed apertural modifications when mature; that is, once the ammonite had grown to its final size, the aperture became constricted and was bounded by one or two thickened ribs, known as collars. These have been observed on other ammonites as well, and are assumed to be signs of sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...
.
The open shell of these ammonites would have made them poor swimmers because of drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
, but beyond that fact, very little is certain about their mode of life. It is widely assumed that they were plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic, perhaps catching small prey in the manner of jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
, but repaired shell damage apparently caused by crabs may indicate that they spent at least some time close to the sea floor.
Phylogeny
The genus Hamites is of particular interest to palaeontologists because the species included in the genus span a wide range of morphologies including ones apparently similar to several more derivedDerived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
groups of heteromorph ammonites. The genus rapidly diversified
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...
during the Albian
Albian
The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous epoch/series. Its approximate time range is 112.0 ± 1.0 Ma to 99.6 ± 0.9 Ma...
into a number of morphologically distinct lineages that seem to have given rise to at least three other families of heteromorphs, the Baculitidae, Turrilitidae
Turrilites
Turrilites is a helically coiled ammonoid cephalopod from the lower part of the Upper Cretaceous ; generally included in the Ancyloceratina. Previously it was included in the ammonoid suborder, Lytoceratina....
, and Scaphitidae
Scaphites
Scaphites is a genus of extinct cephalopod belonging to the family of heteromorph ammonites . They were a widespread genus that thrived during the Cretaceous period....
. The lineage that gave rise to the helical Turrilitidae, for example, had a shell that initially grew as a helix before straightening out; the Turrilitidae thus appear to have been derived from neotenic Hamites that retained the helically-coiled juvenile morphology of Hamites into adulthood.