Part 17

Although the shell of the only type of living nautiloid is coiled, many of the early forms had straight cone-shaped shells (Orthoceras, Pl. 32), and these are common in some of the Pennsylvanian formations of Texas. Fossil coiled nautiloids may be collected in certain of the Cretaceous and Tertiary strata of the State, but their remains are not common. Cymatoceras (Pl. 32) is a coiled fossil nautiloid from the Cretaceous of north Texas.

SCHIZODUS × 1 MYALINA × ½ ASTARTELLA × 1 NUCULOPSIS × 1 ALLORISMA × 1 NUCULANA × 1 YOLDIA × 1 PINNA × ½

PROTOCARDIA × 1 ALECTRYONIA LUGUBRIS × 1 PLICATULA × 1 PECTEN × 1

GRYPHAEA WASHITAENSIS × 1 GRYPHAEA GRAYSONANA × 1 INOCERAMUS × 1 TRIGONIA × 1

EXOGYRA ARIETINA × 1 EXOGYRA LAEVISCULA × 1 NEITHEA × 1 EXOGYRA PONDEROSA × 1 EXOGYRA TEXANA × 1

PACHYMYA × ½ OSTREA CARINATA × 1 OSTREA QUADRIPLICATA × 1 PHOLADOMYA × 1

LIMA × 1 OSTREA LISBONENSIS × 1 PITAR × 1 VENERICARDIA BULLA × 1 PACHECOA × 2 PHOLADOMYA × 2 OSTREA SELLAEFORMIS × 1 CRASSATELLA × 1

ORTHOYOLDIA × 1 TELLINA × 1 VOKESULA × 2 NUCULA × 2 VENERICARDIA × ½

PLICATULA × 1 PECTEN × 1 ANOMIA × 1 GLYCYMERIS × 2 CARYOCORBULA × 2 BARBATIA × 1

b Living chamber Aperture Septa Siphuncle Protoconch Chamber

Subclass Ammonoidea.—

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.

PHANEROCERAS × 1 ORTHOCERAS × 1

METOICOCERAS × ½ CYMATOCERAS × ½

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.

Subclass Coleoidea.—

These are squid-like cephalopods characterized by an internal shell or no shell at all. Included in this group are the squids, cuttlefish, octopuses, and the extinct belemnoids, but of these only the belemnoids are useful fossils. Members of this subclass range from Mississippian to Recent in age.

Order Belemnoidea.—

The belemnoids appear to be the oldest and most primitive of the coleoid cephalopods. Their earliest known occurrence is in rocks of Mississippian age, and they were particularly abundant during the Mesozoic. They became extinct at the end of Cretaceous time but have left considerable evidence of their existence in the Mesozoic strata of many parts of the world. Certain forms, because of their abundance and relatively short geologic range, are excellent guide fossils. Belemnoids have been found in the Upper Cretaceous of Texas (Pl. 33) but in general are rare or unknown in most Texas formations.

Phylum Annelida

Members of the phylum Annelida include the segmented worms such as the common earthworm. Annelids are marine, fresh water, or terrestrial and have apparently been common through much of geologic time. Because of their lack of hard parts, most of these worms have left little direct fossil evidence of their activities in the geologic past. Some annelids secrete straight or coiled calcareous tubes, and fossil worm tubes of this sort (fig. 20) are commonly found attached to brachiopods, mollusks, and other objects. Tubes of this nature have been reported from Paleozoic, Mesozoic, and Cenozoic rocks in Texas.

Some annelids have small chitinous jaws and teeth which also may be preserved as fossils. These dental structures are called scolecodonts and are microfossils.

Phylum Arthropoda

The arthropods are one of the more advanced groups of invertebrates, and they are known from the Cambrian to the Recent (Pl. 34). Modern representatives of this group include the crabs, shrimp, crayfish, insects, and spiders. Arthropods vary greatly in size and shape and are among the most abundant of all animals. They have become successfully adapted to a wide variety of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a chitinous exoskeleton which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages.

Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein.

Subphylum Trilobitomorpha