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A primary difference between ammonites and nautiloids is the siphuncle of ammonites (excepting Clymeniina) runs along the ventral periphery of the septa and camerae (i.e., the inner surface of the outer axis of the shell), while the siphuncle of nautiloids runs more or less through the center of the septa and camerae.
This list of ammonites is a comprehensive listing of genera that are included in the subclass †Ammonoidea, excluding purely vernacular terms. The list includes genera that are commonly accepted as valid, as well as those that may be invalid or doubtful ( nomina dubia ), or were not formally published ( nomina nuda ), as well as junior ...
Parapuzosia is an extinct genus of desmoceratid ammonites from the Cenomanian to the Campanian of Africa, Europe, and North America. [2] They are typically very large ammonites, reaching diameters of 60 cm (2.0 ft) or more, with the largest species measuring around 2 m (6.6 ft). [3]
It was once estimated that, if complete, this specimen would have had a diameter of approximately 2.55 m (8.4 ft) [2] or even 3.5 m (11 ft). [3] However, a study in 2021 estimated the diameter of the largest specimens to be around 2 m (6.6 ft). [ 4 ]
Goniatite shells are small to medium in size, almost always less than 15 centimeters (5.9 inches) in diameter and often smaller than 5 centimeters (2.0 inches) in diameter. The shell is always planispirally coiled, unlike those of Mesozoic ammonites in which some are trochoidal and even aberrant (called heteromorphs).
Arietitidae is a family of true ammonites that make up part of the superfamily Psiloceratoidea, named after the type genus Arietites.They comprise medium-size to large or gigantic genera which in general are strongly ribbed, tuberculate in some, with keeled or grooved and keeled venters, and well differentiated ammonitic sutures.
Fontannesia is an ammonoid cephalopod genus with a small to medium size, evolute, discoidal shell that was extant during the Jurassic Period.The sides are ribbed, the venter has a single median keel, and tubercles are lacking.
Phricodoceratidae is a family in the Eoderoceratoidea, aberrant ammonites from the Lower Jurassic characterized by a large adult size and a marked change of shell form and ornament with growth. Shells are stoutly ribbed, early growth stage is round-whorled with spines, followed by a high-whorled late growth stage with smooth, modified ribbing.