Canadian Rockhound: Fossils of the Horton Bluff Assemblage

Fossils of the Horton Bluff Assemblage,
Nova Scotia
By Ronnie Van Dommelen and Dr. Barry Cameron,
Halifax, Nova Scotia

When the vertebrate paleontology of Nova Scotia is discussed, two localities usually come to mind: the dinosaurian sandstones of Parrsboro and the Late Carboniferous locality of Joggins with its skeleton containing tree stumps. Another site, not too distant from the first two, adds another period to the Nova Scotian vertebrate timeline. The site is the Early Carboniferous Horton Bluff formation near Windsor, Nova Scotia. This was a period in Earth's history, 365 million years ago, when amphibians were still exploring dry land and reptiles and mammals had not yet developed.

Beautifully situated at the mouth of the Avon River, this site produces a rich diversity of flora and fauna. Sir William Edmond Logan (for whom the mineral weloganite is named), discovered the first amphibian footprints here in 1841. At the time, they were the oldest tetrapod tracks known in the world. Sir John William Dawson (for whom the mineral dawsonite is named) reported in his text, Acadian Geology, several vertebrate body and trace fossils as well as a wide variety of flora. An ongoing study of the exposure by Dr. Barry Cameron at Acadia University, and several students and ameteur collectors has produced an exciting assemblage of fossils.

Trace Fossils

Several types of trace fossils can be found in various sections along the beach. The most important of these are the many amphibian trackways that have been discovered over the years. To our knowledge these are the oldest known amphibian footprints in North America and perhaps the best preserved amphibian footprint assemblage in the world. A wide variety of species have been reported including Baropezia sp., Hylopus logani, and Anticheiropus bishopi. Obviously, several tetrapod species were present in the area during the Early Carboniferous of Nova Scotia. Perhaps the most interesting trackway is one of a Baropezia individual with the largest amphibian footprints ever recorded from rocks over 330 million years old. The 7-m trackway shows approximately a 1.5-m stride and 0.94-m pace. The trackway was preserved as a fiberglass cast in 1979 by the Nova Scotia Museum of Natural History.

Fish fin and tail drag marks can also be found on the beach. Many of these form sinusoidal groupings in the fine grained rock. Acanthodes is one species attributed to these fin marks. This species is also represented by fossil fin spines.

Several years ago, a rockslide produced several sets of scorpion trackways. These tracks are very well preserved and closely resemble Paleohelcura tridactyla. The trackways are about 2-cm wide and consist of 'footprints' with three tiny ovals about 1 to 3-mm in diameter each. Between the footprints is a continuous drag mark possibly from the tail. Some of these trackways are accompanied by fish tail drags. This might suggest a partially aquatic lifestyle. Such an assumption is supported by scorpion body fossils from the Silurian and Devonian which have been reported as aquatic organisms. It should be noted that the Horton Bluff scorpion trackways are some of the oldest in the world.

Other trace fossils found on the beach include small fish coprolites, worm borrows, ripple marks, and rain imprints.

Flora

The Horton Bluff flora are not actively studied by Acadia University, but quite a variety exists. The most common is probably Lepidodendron for which several species are present. Lepidodendron was a very common tree during the Carboniferous, with its crosshatched exterior. Also found are Calamites, Cordaites and Stigmaria. The seed Triletes is most abundant along the beach, being found with almost all of the other fossils.

Fauna

Unfortunately, the fauna represented at Horton Bluff do not form complete or even partial skeletons. All of the bones collected to date have been individual, and at best only nearby other bones. Nonetheless, a wide variety of body parts has been found over the course of several years of collecting. Some of the more important finds made through Acadia University are listed below:

a brain case

jaw fragments

large clavicles

interclavicles

cleithra

several limb bones (ulna, radius, etc)

vertebrae fragments

hip bone

body plates

Most of the bones listed above are believed to be remains of Rhizodus, a large rhipidistan crossopterygian fish. The jaw fragments are definitely Rhizodus as they contain partial or whole teeth which are easily identified. The fossils listed above are accompanied by numerous scales, teeth, ribs and fin spines. The most common remains are fin spines from Acanthodes, scales and teeth from Rhizodus, and scales from Acrolepis. These smaller remains are often found in concentrations. The sedimentary layering of these concentrations suggests the fossils collected as sediments from small rivulets.

Other important finds also exist. One particularly interesting find was an anterior median dorsal plate of an Antiarch fish very similar to Bothriolepis. These were heavily armor plated Placoderm fish. Although the majority of them appear to have gone extinct at the end of the Devonian, rare reports of Early Carboniferous remains exist. Several years ago another find of two femora was made by Princeton University. These femora suggest an anthracosaurian labyrinthodont amphibian. To round out the listing of fauna, small bivalve crustaceans can be found in the fine grained rock containing the coprolites.

Figure 1. Above is a nearly complete limb bone. The scale is in centimeters.

Figure 2. Above is an example of a hip bone. The bone is nearly complete but encased in very hard matrix. A partial rib can also be seen in the lower left of the photograph. Each tick on the scale is one millimeter.

Figure 3. Above is a partial shoulder bone or cleithrum. This specimen was found totally encased in rock (except the broken cross section at the right). The left end of the bone can be seen as the black area in the upper left corner. Since this picture has been taken, this bone has been completely freed from the matrix. The scale is in centimeters.

It is evident that the Horton Bluff assemblage is most interesting in its diversity of life. This small beach may yet be the source of great scientific findings. The aim of ongoing research at Acadia University is to unearth these findings and put together a picture of what life was like 365 million years ago. If you know of any other collected fossil material from the Horton Bluff formation, we would be most interested to hear from you.

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