Happy National Fossil Day – Earth Science Week 2023

To celebrate, we are highlighting one of the more rare fossils in the state! This ammonoid fossil measures about three inches across and is beautifully preserved in Pennsylvanian aged sandstone in northwest Arkansas. Ammonoids are coiled cephalopods that have been extinct since the end of the Cretaceous Period but are present in the Mississippian and Pennsylvanian-age rocks of the Ozark Plateaus and in the Cretaceous-aged rocks of the West Gulf Coastal Plain.  These animals were swimmers and floaters in shallow to deep water. Because particular species only existed for a relatively brief time in the geologic record, paleontologists can use their fossilized remains to determine the age of the rocks in which they are found and correlate them to similar-aged rocks worldwide.

The left photo shows an ammonoid from the Mississippian Fayetteville Shale and the right photo an ammonoid from the Prairie Grove Member of the Hale Formation. Notice the suture pattern or, wavy lines, on the fossil.

Ammonoids are classified according to their suture pattern: goniatitic (fairly simple), ceratitic (more decorative), or ammonitic (highly decorative).  Ammonoids with a goniatitic suture are called goniatites, those with a ceratitic suture, ceratites, and those with an ammonitic suture, ammonites. Mississippian and Pennsylvanian ammonoids are classified as goniatites because they have a goniatitic suture pattern.

Ammonoids in Cretaceous rocks in the southwest part of the state have highly decorative or ammonitic suture pattern, therefore, they are called ammonites. The photo below shows the complex suture of an ammonite found in southwest Arkansas.

For more information about fossils visit our website at https://www.geology.arkansas.gov/geology/fossils.html

Trilobites from the Powell Dolomite and Imo interval of northern Arkansas

Happy National Fossil Day! To celebrate, we’d like to tell you a little about one of the many varieties of fossils preserved in the rocks of Arkansas. One of the most recognizable and perhaps popular invertebrate fossils anywhere is the trilobite. The photograph above shows the external mold of the middle (thorax) and tail (pygidium) portions of a trilobite that lived in the Ordovician period. It was preserved in the Powell Dolomite in north Arkansas and only recently made its way back to the surface after millions of years of erosion removed all the rock that had covered it. It measures about 3 inches in length. This specimen is the largest trilobite ever found in the rocks of Arkansas. Preserved portions of smaller trilobites and trilobite parts are commonly found in several formations located in various parts of the state depending on whether conditions were favorable for them to have lived there and to be buried after death.

Trilobites belong to a class of extinct Arthropods that lived in ancient, warm, shallow seas for millions of years from the Cambrian to the Permian Periods (the Paleozoic Era), however, they were most abundant in the Cambrian and Ordovician Periods (541 to 444 million years ago). The class went extinct near the end of the Permian Period, meaning their very existence is based entirely on the fossil record they left behind. They were small, averaging about 5 to 8 centimeters (2 to 3 inches) in length, but giant specimens could reach lengths of nearly 75 centimeters (30 inches). They lived their life crawling about on the floor of ancient seas, occasionally making a mark or depression in the sediment. Where these are preserved, they are known as trace fossils and can be used to help interpret trilobite behavior. To see a photo of a trilobite trace fossil, check out this link to another blog post on our page:

https://arkansasgeological.wordpress.com/2019/02/14/geo-pic-of-the-week-trilobite-romance/

Occasionally, a few specimens became buried in the sediment, and the hard parts of some became preserved over time as the sediment became rock. Much more commonly preserved, however, were various fragments of trilobite exoskeleton. In order to grow, trilobites actually needed to cast off pieces of their carapace during a molting process. This process, which could happen 2 to 3 times per year, resulted in a buildup of disarticulated fragments on the sea floor. Because of their sheer numbers, these parts are much more likely to be preserved than whole specimens. Many species of trilobite are known only by the study of these partial remains by dedicated paleontologists.

During deposition of the Powell Dolomite, the area recently known as Arkansas was much closer to the equator and was covered by a shallow sea. These warm waters provided a perfect environment for these critters and others to survive and thrive here. Preservation, though rare, gives us insight into the diversity of marine organisms, including trilobites, that lived here during that time.

The trilobites pictured below lived at a much later time but are found in a similar environment. At the very end of the Mississippian, an influx of clay, silt, and sand shed from the newly formed Appalachian Mountains to the east began to be deposited in the shallow sea that still covered northern Arkansas. Many species of trilobite lived, grew, and molted in the shallow estuaries common in the area at that time. The Late-Mississippian Imo interval has preserved many trilobite parts along with numerous other marine species in shaly sections deposited in those estuaries. Following are photos of some of the Imo-aged trilobite parts:

Middle and tail portion of a trilobite from the Mississippian Imo interval. Penny for scale.

Eyes in the head portion of a rolled-up trilobite from the Mississippian Imo interval. Penny for scale.

To find out more about Arkansas Trilobites, check out Plate IX on page 54 in the Arkansas Geological Survey’s Bulletin 22-Fossils of Arkansas. This is a great little book that also discusses other common fossil species found in our state:

https://www.geology.arkansas.gov/docs/pdf/publication/bulletins/bulletin%20-22-fossils-of-ar.pdf