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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.
Acanthoceras is an extinct cephalopod genus belonging to the subclass Ammonoidea and family Acanthoceratidae that lived from the Albian to early Coniacian stages of the Cretaceous. [ 1 ] Description
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 ...
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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).
Ammonites — members of the Ammonoidea subclass of molluscan prehistoric cephalopods. Pages in this category should be moved to subcategories where applicable. This category may require frequent maintenance to avoid becoming too large.
Dactylioceras was a widespread genus of ammonites from the Lower Jurassic period, [1] approximately 180 million years ago (). [2] and Like many other ammonites, the genus Dactylioceras is extremely important in biostratigraphy, being a key index fossil for identifying their region of the Jurassic.
Adult Baculites ranged in size from about 7 centimetres (2.8 in) (Baculites larsoni) up to 2 metres (6.6 ft) in length. As with other ammonites , the shell consisted of a series of camerae, or chambers, that were connected to the animal by a narrow tube called a siphuncle by which gas content and thereby buoyancy could be regulated in the same ...