The Poundstone Clypeus ponti

The fossil Clypeus ponti is the so-called "poundstone." This term was used for this species because during Medieval times in western Europe, people would use these fossils as counterweights for measuring one pound of butter or of cheese (i.e., a single specimen = one pound). 

The species is of Middle Jurassic age (Bajocian Stage = 170 million years old). This specimen shown above is 7 cm (2.75 in.) in diameter, and the photograph shows its top (dorsal = aboral) side. The specimen is from France.

There are several very informative articles on the web that discuss the history of the usage of this fossil, but, so far, all the ones I have read refer incorrectly to this species as a "sea urchin." Clypeus ponti does belong to phylum Echinodermata, as do sea urchins, which are regular echinoids. Clypeus ponti, however, belongs to the irregular echinoids. Unlike sea urchins, which live on the ocean floor and have long spine, irregular echinoids (like sand dollars, etc.) burrow into the sea floor and have very short spines.

This is the same specimen illustrated above but shows the bottom (ventral or oral) side. Notice the five-ray symmetry of its feeding grooves

Again, this is the same specimen, but the view is of the front end with its depressed central area. I thank the Natural History Museum of Los Angeles County for allowing me to photograph this specimen of C. ponti.  

Cambrian mystery trace fossil

I have been puzzled by this Early Cambrian trace fossil ever since I collected it about 35 years ago while on a field trip with my paleontology class. The trace consists of two parallel grooves that show up on the bottom of a bed of slightly metamorphosed sandy siltstone. What you see is the sediment that infilled the grooves (the sediment constitutes a cast). The trace is from the Inyo Mountains, California. I wonder what kind of animal could have made this trace. Did a single animal make it while crawling over the surface of the muddy sediment? Or, did two animals crawling alongside each other make the grooves?

I have tentatively identified this trace fossil (i.e., technically speaking, an ichnofossil) as Scolicia sp.

If anyone has a better identification, please let me know via the comments option on this blog site.

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.

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. 

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.

Early Cretaceous belemnite from California

Belemnites are fossil cephalopods (include nautiloids, ammonoids, squids, octopus) that were abundant during the Mesozoic. Belemnites had an internal shell enclosed by the soft body. The shell consisted of a straight, tapering chambered part (phragmocone) that was embedded into the most massive part of the skeleton, the cigar-shaped rostrum or guard.

Some well preserved specimens have been found that show that belemnites had 10 arms (tentacles). Belemnites were fast-moving nektonic (swimmers) carnivores (meat eaters) that resembled squids. The largest belemnites had a total length of approximately 18 cm. Although some hypothetical reconstructions show flattened "hands" at the end of two of their tentacles (as shown above), some well preserved specimens that actually have impressions of the arms (click HERE) show no "hands" (as shown in the commercially available model below).

The geologic time range of belemnites is shown below:

Most belemnite specimens are like the one shown above; only the posterior part (rostrum) of the shell is present.  This is an incomplete rostrum (length 9.3 cm, diameter 2.7 cm) of the belemnite Acroteuthis kernensis Anderson, 1938 from the Hex Formation in the Devils Den District of central California. This formation is of late Early Cretaceous age (i.e., approximately 128 million years old and corresponding to what geologists refer to as the late Barremian Stage of the Early Cretaceous). The rostrum consists of solid, dense calcite (calcium carbonate). It acted as a counterbalance for the heavy head and tentacle areas of the belemnite shell; otherwise, the animal would have swam head down (not a desirable trait for an otherwise streamlined creature).

This is the ventral view of the specimen shown above. The ventral groove (function unknown) is well developed. Not all belemnites shells have a ventral groove.

This sketch shows the alveole, which is where the phragmocone (the chambered part of the shell) fits into the rostrum.  

Alveoles are rare at the Devils Den District locale, but I was able to find this interesting specimen (3.9 cm in total length) that shows the interior of the rostrum with an impression (a negative feature that can hold water) of the alveoli.

This is a cross-section view of the specimen shown above in the first two pictures of Acroteuthis kernensis. You can readily see how the calcite that comprises the rostrum was secreted in successive concentric layers (like tree rings) by the animal. The genus Acroteuthis is restricted to the Early Cretaceous between 140 and 125 million years ago. It has been found mostly in Canada (British Columbia, Yukon, and Northwest Territories), as well as in Greenland, Germany, and Russia. 

Helicoplacus, an Early Cambrian echinoderm

Helicoplacus gilberti Durham & Caster, 1963

This post concerns a group of very rare fossils called helicoplacoids. They belong to the phylum Echinodermata (i.e., includes starfish, sea urchins, sand dollars, sea cucumbers, etc.). Helicoplacoids, however, do not remotely look like echinoderms. Instead of having fivefold symmetry (= pentaradial or pentameral), they all look like the above-sketched specimen of Helicoplacus gilberti. They are characterized by an oblong body (most are about 3 cm long). Near one end of the body is the spiral food groove that acted like a mouth. Their "skin" was covered in spirals of overlapping calcareous plates, which were not sutured together tightly like on most echinoderms. Many "specimens," therefore, consist of small concentrations of easily disarticulated (scattered) calcareous plates.

Helicoplacoids are the earliest well-studied fossil echinoderm. They are only known from Lower Cambrian strata, around 525 million years ago, and they apparently lasted for 15 m. y.

A nearly complete specimen of Helicoplacus gilberti, 2.8 cm long, from Lower Cambrian strata, White Mountains, California. The food groove area is poorly preserved.

It is currently believed that Helicoplacus was a suspension feeder that lived in burrows in muddy substrate of shallow-marine waters. They might have extended their flexible bodies outward to feed. All the specimens found in the White Mountains of California are in siltstone that has been slightly metamorphosed. Their calcareous plates have been either weathered away or dissolved because only molds of the plates are now present.

Although some fragmental specimens of Helicoplacus are found in a few areas other than the White Mountains of California, the only complete specimens of this genus are found in these mountains.

The above picture shows a small cluster of H. gilberti from Lower Cambrian strata, White Mountains, California. The length of this slab is 8 cm.

This picture shows a large cluster of H. gilberti from the White Mountains. The longest dimension that incorporates all the specimens is 15.5 cm. This is an astonishing slab of specimens, the likes of which have never been shown before. These fragile-looking specimens must have undergone a short distance of post-mortem transport before being concentrated and deposited together.