CRINOIDS CRINOID [CALYX] × ½ CRINOID COLUMNALS × 1 HOLOTHURIAN SCLERITES (GREATLY ENLARGED) [CRETACEOUS] [FOSSIL] STARFISHES PENTAGONASTER × 1 PENTACEROS × 1
Plate 36
[CRETACEOUS] ECHINOIDS
SALENIA × 1 [ECHINOID] SPINES × 2 ECHINOID PLATE × 2 HEMIASTER × 1 HOLASTER × 1 HOLECTYPUS × 1
The [echinoid] [test] ([exoskeleton]) is composed of many intricately fitting [calcareous] plates ([Pl. 36]) which enclose the animal’s soft parts. The exterior of the test is typically covered with large numbers of movable spines ([Pl. 36]) which vary greatly in size. These spines are of some aid in locomotion, support the skeleton of the animal, and provide a measure of protection from enemies.
The oldest known echinoids have been recorded from rocks of [Ordovician] age, but it was not until the [Mesozoic] that the group began to flourish. They were especially abundant during the [Cretaceous] and have been abundant and varied from that time until the present.
Echinoids are particularly numerous in many of the Lower [Cretaceous] formations of Texas where they are commonly found in an excellent state of preservation. Heart urchins and biscuit urchins may be found in large numbers in many areas of the State, and especially in areas where there are good exposures of [fossiliferous] Lower Cretaceous rocks.
Subclass Holothuroidea.—
Members of this class, commonly called sea cucumbers, have a rather elongate, sac-like, cucumber-shaped body and bear little resemblance to other members of the [phylum] Echinodermata. The sea cucumbers do not have a well-defined skeleton; rather the body is supported by many small, disconnected, [calcareous] plates or rods called ossicles or sclerites ([Pl. 35]). These minute structures are embedded in the leathery skin which covers the body of the sea cucumber and may be preserved as fossils. Such remains are locally abundant in certain formations in Texas, but because of their small size, scattered occurrence, and problems in classification, this group is of little use to most paleontologists.