Thursday, February 26, 2015

A Cretaceous bivalve that looks like a feather

Inoperna bellarugosa Popenoe, 1937 (length 7.3 cm = 73 mm).
The bivalve genus Inoperna, which belongs to the mytilid family (includes mussels), lived during the Triassic (latest part), and the Jurassic to Cretaceous (a total geologic time range of just over 60 million years). It was globally widespread in warm seas, and it had its widest distribution during the Jurassic. It showed up on California during the Cretaceous.

The genus is characterized by a narrow elongate shell, with nearly parallel dorsal and ventral margins. Its surface is divided by a diagonal ridge, with numerous strong ribs above the ridge and smooth shell beneath the ridge. The shape and ribbing of this genus resembles that of a feather and is very memorable. Individuals of this genus apparently lived on the sea floor and were stationary.

The species shown above is from Turonian strata of Late Cretaceous age in the Santa Ana Mountains, Orange County. As you might remember from my last post, the Turonian (93.5–93 million years ago) was the warmest time of the Cretaceous Period.

The etymology (name derivation) of I. bellarugosa is from bella, Latin for beautiful, and rugosa, Latin for wrinkles. I think that you will agree that it is aptly named.

Sunday, February 15, 2015

An even more unusual gastropod

In my last post, I showed pictures of an unusual Cretaceous gastropod belonging to genus Trochactaeon, which belongs to the actaeonellid family. It is unusual because of the structure of the interior of its shell. In this post, I deal with yet another "unusual" gastropod, and its internal-shell structure is even more "unusual," some might say "bizarre." This "bizarre" snail belongs to a group known as the nerineids. This group lived during the Jurassic and Cretaceous (see diagram above). Like Trochactaeon, nerineid gastropods are indicative of tropical to subtropical conditions and were confined to an area roughly parallel to the equatorial region.

The above photograph is the exterior of the shell of one of several species of nerineids found in the Alisitos Formation of Albian age. The Albian correlates to a specific interval of the  Early Cretaceous (see above diagram). The exterior of this shell (height  4.5 cm, width 2.7 cm) of Eunerinea sp. is very poorly preserved. 

This photograph shows the interior of the same specimen shown above. As in most nerineids, the body cavity has the development of spiral folds (reason[s] unknown). This specimen helps prove the old saying (i.e., changed slightly, herein): you cannot judge a shell by its exterior. 

Tuesday, February 3, 2015

An unusual Late Cretaceous gastropod from California

This post concerns an interesting gastropod (snail) that belonged to the extinct actaeonellid family. Actaeonellids lived only during the Cretaceous, and during this time, they were one of the most common members of shallow-marine communities that were widely distributed in a belt of tropical (and to a lesser degree subtropical) waters that paralleled the equator. Common associates are rudistid bivalves (see one of  my previous posts for October, 2014). Rudistids and actaeonellids went extinct at the end of the Cretaceous.

Over the years, I have accumulated several specimens of the actaeonellid Trochactaeon packardi (Anderson, 1958). This species is found in northern and southern California, and the species lived during Turonian time (approximately 92 million years ago--see above), which was the warmest time of the Cretaceous. The specimens shown below are from the Turonian-age Baker Canyon Member of the Ladd Formation in the Santa Ana Mountains, Orange County, southern California.

Like many actaeonellids, Trochactaeon packardi is characterized by a cylindrical shell with a low spire at the top. The specimen shown above is 4.6 cm in height and 2 cm in diameter. This view (front) is of its aperture, where the soft-bodied part of the gastropod would have been able to extend itself somewhat out of its shell.

This is the abapertural (back) view of the same specimen shown above. The shell is smooth, which would have allowed to shell to be streamlined (perhaps for burrowing); a useful feature for a carnivorous animal. 

This is a second specimen (height 6 cm, diameter 3 cm). Its anterior end is well exposed and shows two prominent plicae = spiral features near the anterior end (a third one, positioned nearer the anterior end, is much weaker). These plicae are very important in the identification of this gastropod.

This third specimen (height 5 cm, diameter 2.8 cm) was cut in half (from top to bottom) in order to expose its very characteristic internal structure. There has been some crushing of some of the thinner internal shell walls. 

This last view is of a specimen (3.7 cm in diameter) that was cut in half (from side to side). Actaeonellids have more revolutions of the shell wall than are normally found inside of most gastropod shells.