Maclurites, an early gastropod with a confusing shell

The gastropod genus Maclurites Le Sueur, 1818 is characterized by large flat-spiral shells. It lived in shallow, subtidal, warm-marine waters and was widespread where carbonates were deposited. Locally, it can be abundant and associated with algal fossils.   
Maclurites is restricted to Ordovician time, thus it is a guide [or index] fossil for the Ordovician Period. This genus belongs to family Macluritidae, which includes 10 genera.

Maclurites has a type of shell coiling called hyperstrophic, in which the animal is anatomically dextral (its genitalia are on right), but its shell is falsely sinistral, being actually ultradextral. If there is an associated operculum (the lid-like, partial or complete covering of the aperture), the operculum exhibits counter-clockwise coiling.  

Most gastropods have dextral (right hand) clockwise coiling of the shell, and any associated operculum exhibits counter-clockwise coiling (see image immediately below). A few gastropods are mirror opposites and have sinistral (left hand) counter-clockwise coiling of the shell, and any associated operculum exhibits clockwise coiling.

In my Nov. 20, 2014 post, entitled "Gastropod operculum," I provided views of the dextrally coiled, gastropod shell Megastrea undosa. In this present post, I provide another view of the exterior side of its operculum (38.7 mm hight), which shows counter-clockwise coiling.

Although hyperstrophy is best determined using soft parts for anatomical study, in the case of extinct gastropods, like Maclurites, molluscan paleontologists have to rely on the coiling direction of its calcareous operculum. As shown below, the operculum of Maclurites is coiled counter-clockwise, thus it cannot be a sinistral gastropod.

The above image is a sketch of the apertural view of a complete specimen of Maclurites logani [86 mm diameter], of Middle Ordovician age from Quebec, Canada. [Fascimile of fig. 105 in The Treatise on Invertebrate Paleontology, Part 1, Mollusca (1964), p. I188].

The following four views are of a specimen of Maclurites sp. [68.4 mm diameter, 21 mm height] from the Lower Ordovician Lebanon Limestone, near Nolensville, Tennessee. 

   front or apertural view, aperture (poorly preserved) is to the right

                                  back or abapertural view

                                            basal view

                            dorsal view (the "tip" is missing)

The shell rested on its very flat base, which provided great stability for the shell, thereby resisting being flipped over by waves or currents. The flatness of one side of the shell is inferred to be related to its sessile (unmoving), filter-feeding mode of life. 

Enrolled Trilobites

Trilobites are common fossils of the Paleozoic Era. They are easily recognized and treasured among collectors. Trilobites are an extinct group of marine arthropods in which the tri-lobed body is divided into a distinct head (cephalon, with compound eyes), segmented body (thorax), and, in some cases, a distinct "tail" (pygidium). The exoskeleton is divided also length-wise into three parts. See my previous post ["Amazing eyes of the trilobite Phacops dana from Ohio; July 2, 2015] for more information.

Three views (top, bottom, and side) of a plastic model of a Cambrian trilobite (12 cm length). The mineralized "shell" is  green, the numerous legs and antennae are red, and the gills (located on top of the legs) are white.

An actual specimen of a representative trilobite (28 mm length) is shown above. The left eye is better preserved that the right eye, which was "smoothed over" by weathering. This specimen, which is slightly curved, appears to show the initial stage of enrollment (curling up).

Some trilobites are found enrolled. That is to  say, their mineralized exoskeletons rolled up (partially or completely), much like modern pill bugs do. Enrollment was the way for
trilobites to protect their legs, soft undersides, and antennae from predators. 


The ability to enroll developed during early, as a few Cambrian forms have been found preserved this way. It became common in the Ordovician, Silurian, and Devonian but was not acquired by all genera.

Calymene meeki (enrolled state), Ordovician, Ohio. 15 mm width.

Some trilobites could "tuck" the end of their pygidium under the edge of the cephalon, thereby allowing more resistance to being pried apart by a predator. On the image on the right, you can see how the edge of the cephalon is indented to accommodate the projected end of the pygidium.

Other Gastropods that Carry Shells

My previous post concerned the "carrier" gastropod Xenophora, the most common example of a marine gastropod that cements foreign objects onto/into its shell.


In this new post, I present a very broad overview of the other main examples of "carrier" gastropods." 

A high-spired Straparollus, approximately 1 cm height, with fragments of shells attached to its whorls.

One of these other "carrier" gastropods is Straparollus, of Ordovician to Devonian age in North America. Whether this gastropod crawled or was stationary (sessile) is unknown. Straparollus has an uncertain classification and evolution. I used the following reference for obtaining information about Straparollus and as the basis for my own sketch of its shell:

Linsley, R.M. and E.L. Yochelson. 1973. Devonian carrier shells (Euomphalidae) from North America and Germany. United States Geological Survey Professional Paper 824, pp. 1–26.
_________

Scaliola bella, 1.2 mm height, Indonesia.

Another "carrier gastropod," is the minute Scaliola, which ranges from the Cretaceous to modern day. There are only a few species, and specimens are generally less than 1 mm in height. They live predominantly in the Indonesia region of the Indo-Pacific ocean region. This gastropod cements sand grains onto the adult part of its shell. The family placement of Scaliola is quite different from that of Xenophora.

The following reference was used for obtaining information about Scaliola and as the basis for my own sketch of its shell.

Bandel, K. and H. A. El-Nakhal. 1993. The history and relationship of Scaliola, a gastropod that cements particles to its shell. Mittelungen aus dem Geologisch Palaontologische Institut der Universität Hamburg, 73:171–191.

The following two groups of "carriers" are not true "carriers" of shells upon their own shell, buy they do collect foreign objects and glue them onto themselves. Thus, they are of interest, in a comparative way. The first group is the microscopic agglutinated foraminifers, which are amoeba-like, one-cell protists. Their geologic record ranges from the Early Paleozoic (Middle Ordovician).

Agglutinated (also referred to as arenaceous) foraminifers cement sand, mica, or calcareous particles, as well as tests of other micro-organisms (especially sponge spicules), onto themselves. The cement is calcium carbonate, silica, or iron-bearing compounds. The type of material they attach to can be dependent upon the temperature of the surrounding ocean water (e.g., quartz grains in cold water versus calcium-carbonate particles in warm water). Two examples are: 

Sand-grain agglutinated foraminifer Rhadbdammina, 0.1 mm height, Ordovician, Oklahoma. 

Sand-grain aggluinated foraminifer Saccammina, 0.05 mm height, Silurian, Oklahoma.
_______

The second group of debatable carriers are some amphipods, which are microscopic shrimp-like crustaceans having no carapace (hard "shell-like covering). This type of amphipod "collects" pieces of shells, cuts them, and then cements them onto its sides (thereby forming blisters or tubes). I could not fine any images of them, and I do not believe that they have a known fossil record.