Ammonitida

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* Order: '''Ammonitida'''
* Order: '''Ammonitida'''
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The '''Ammonitida''' comprises advanced ammonoids from the Jurassic and Cretaceous periods, characterized by closely coiled, mostly planspral  shells, and sutures that often are hightly complex but in later forms tend to become simplified. Although referred to as a taxonomic order, or sometimes as a suborder, the Ammonitida appears very much to be a polyphylectic assemblage of grossly similar forms having widely separate origins.
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The '''Ammonitida''' comprises advanced ammonoids from the Jurassic and Cretaceous periods, characterized by closely coiled, mostly planispiral shells thaat are commonly thick, often but not universally with strong ornament, and ammonitic sutures that lack the moss-like complexity found in the [[Lytoceratida]] and in which saddles are only rarely phylloid or lobes bifid and tending to become simplified in later forms.  
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Three phylogenetic series  are presented in the Treatise (Part L, 1957), revised by Donovan et al (1981) with both the Phylloceratidae ([[Phyllocerida]]) and Lytoceratidae ([[Lytocerida]]) provided ancestral sources for included subtaxa.  The three phylogenetic groups are the stand alone Psilocerataceae,  Eoderocerataceae and derived groups, and Desmoceratacea and its derived groups. Evolutionary connections between any of these is ambiguous,  remaining  uncertain.
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==Taxonomy==
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Primary subdivisions are superfamilies, the: <br>
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:[[Acanthocerataceae]] <br>
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:[[Desmocerataceae ]] <br>
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:[[Engonocerataceae]] <br>
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:[[Eoderocerataceae]] <br>
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:[[Hammatocerataceae]] <br>
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:[[Haplocerataceae ]] <br>
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:[[Hildoderataceae]] <br>
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:[[Hoplitaceae]] <br>
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:[[Perisphinctaceae]] <br>
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:[[Psilicerataceae]] <br>
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:[[Pulchelliaceae]] <br>
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:[[Stephanocerataceae]]
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===Ammonitid phylogenies===
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Cretaceous superfamilies are the Acanthocerataceae, Desmocerataceae, Haplocerataceae, Hoplitaceae, Perisphinctaceae and Pulchelliaceae, with Haplocerataceae and Perisphinctaceae also found in the Jurassic. Those that are exclusively Jurassic are , in temporal sequence, the Psylocerataceae, Eocerocerataceae, Hilderataceae, Hammatocerataceae, and Stephanocerataceae.
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Three primary superfamilies are involved, the [[Psilocerataceae]], [[Eoderocerataceae]], and [[Desmoceratacea]].
 
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The Psilocerataceae comprise lower Jurassic ammonoids, derived from the Phylloceratidae, that unites the Psiloceratidae, Schlotheimiidae, Arietitidae, Oxynoticeratidae, and the Echioceratidae, and stands alone among the ammonitids.
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==Phylogeny ==
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The prevailing consensus is that the Ammonitida are derived from, or at least split from, the [[Lytoceratida]] near the beginning of the Jurassic Period, and not long after one or the other spit off from the [[Phylloceratida]] which extends throughout almost the entire Mesozoic.  
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The Eoderocerataceae is the first in a series of superfamilies that have their ultimate origin the the lower Jurassic Lytroceratidae, beginning with the Eoderoceratidae. The series includes the Hildercerataceae, Stephanocerataceae, Perispinctaceae and Haplocerataceae. The latter two extend well into the Cretaceous.  
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The Psilocerataceae which are confined to the Lower Jurassic are the earliest of the ammonitids. Their origin is unsure of although they are shown derived from the Phylloceratida in the older Treatise. Apparently they gave rise to no subsequent groups.
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The Desmoceratacea is the basal superfamily in a set of three, derived form the Phyloceratidae in the early Cretaceous. Core to this group is the Desmoceratidae which gave rise to the Upper Cretaceous Hoplitaceae and Acanthocerataceae.
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The main phyllogenetic sequence begins in the Lower Jurassic with the Eoderocerataceae which gave rise to the Lower and lower Middle Jurassic Hildocerataceae. The Hildocerataceae in turn gave rise to the mostly Middle Jurasic Stephanocerataceae and to the Middle Jurassic to Cretaceous Perisphinctaceae and Haplocerataceae. The Hammatocerataceae may be a subgroup of the Hildocerataceae.
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===Study and application===
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The origin of the mostly Lower Cretaceous Desmocerataceae is left uncertain, but a considered to have given rise to the Upper Cretaceous Hoplitaceae from which the Pulchelliaceae have been split, and to the Acanthocerataceae which includes some of the latest ammonites.                                                               
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Ammonitids (order Ammonitida) are best studied at lower taxonomic levels, families, genera, and from stratigraphic and geographic perspectives.  
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==Study and application==
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Ammonitids (order Ammonitida) are best studied at lower taxonomic levels, families, genera, and from stratigraphic and geographic perspectives. As with ammonoids in general, and unlike nautiloids which require longitudinal sectioning of the phramocone and siphuncle for certainty, ammonitids are amenable to external examination, especially if the suture is visible.
As with ammonoids in general, ammonitids have great application in stratigraphic zoning and age determination.  
As with ammonoids in general, ammonitids have great application in stratigraphic zoning and age determination.  
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=== ''References -''===
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=== ''References ''===
* Arkell, et al, Mesozoic Ammonoidea; Treatise on Invertebrate Paleontology, Part L (Mollusca 4, Ammonoidea) GSA and KU 1957.  
* Arkell, et al, Mesozoic Ammonoidea; Treatise on Invertebrate Paleontology, Part L (Mollusca 4, Ammonoidea) GSA and KU 1957.  
* Donovan, Callomon, and Howarth, 1981. Classification of the Jurassic Ammonitina in The Ammonoidea, The Systematics Association special volume no. 18. M.R  House and J.R, Senior (eds).     
* Donovan, Callomon, and Howarth, 1981. Classification of the Jurassic Ammonitina in The Ammonoidea, The Systematics Association special volume no. 18. M.R  House and J.R, Senior (eds).     
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Credits: [[user:John M|John M]] 12-01-11
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Credits: [[user:John M|John M]] 12-01-11, 6-13-12

Revision as of 20:12, 13 June 2012

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