The ammonoids are a group of extinct cephalopods which are related to
the nautiloids but are characterized by more complex suture patterns.
Members of this subclass have an external partitioned shell which is
straight, curved, or spirally coiled (Pl. 33). This group of cephalopods
first appeared in Devonian time, became extremely abundant and varied
during the Mesozoic, and was extinct by the end of the Cretaceous
period.
Most Paleozoic ammonoids are characterized by a combination curved and
angular suture pattern, and this type of suture pattern is referred to
as _goniatitic_ (fig. 19b). Sutures that are curved and crenulated
(marked in places by a series of tooth-like indentations) are known as
_ceratitic_ (fig. 19c). Ceratitic sutures first appeared in the
Mississippian, became increasingly abundant during the Triassic but were
much less abundant during the Cretaceous. The _ammonitic_ suture has a
very complexly subdivided pattern (fig. 19d). Cephalopods with ammonitic
sutures range from Pennsylvanian to Cretaceous in age and were the most
abundant cephalopods of the Mesozoic.
[Illustration: Plate 32]
[Illustration: PENNSYLVANIAN CEPHALOPODS]
PHANEROCERAS × 1
ORTHOCERAS × 1
[Illustration: CRETACEOUS CEPHALOPODS]
METOICOCERAS × ½
CYMATOCERAS × ½
[Illustration: Plate 33
CRETACEOUS CEPHALOPODS]
TEXANITES × ½
ACANTHOCERAS × ½
TURRILITES × ½
DUFRENOYIA × ½
OXYTROPIDOCERAS × ½
BACULITES × ½
BELEMNITE × ½
Ammonoids are locally abundant in many of the fossiliferous rocks of
Texas and are among the more useful Mesozoic guide fossils. Goniatites
may be found in the Pennsylvanian of north-central and Trans-Pecos
Texas, and ammonoids with the ceratitic suture pattern can be collected
from the Lower Cretaceous of many parts of the State. Cephalopods
exhibiting the typical ammonitic suture pattern are abundant in many of
the Cretaceous rocks of Texas, and these fossils have contributed much
toward an understanding of the Cretaceous strata of this State.