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.
Fig. 20. Types of typical annelid worms. (a) Serpula (×1) (b) Hamulus (×2). (c) Spirobis (×15).