Tail of the Cock Traced on a Rock

On the side of a busy highway near the Ulster County city of Kingston is an series of outcrops quite well known to local geologists. They are along Route 199 just west of the Kingston-Rhinecliff Bridge. 

One part of these outcrops is composed of the Esopus Shale Formation and, in this location, it's dipping steeply to the east with a nice glacially-polished surface making for treacherous climbing onto it (especially since you'd slide down into a massive thornbush if you lost your footing).

Looking at this nice polished surface, you'll see some swirling patterns etched into the rock (there's a little iron staining from what looks like the weathering of small inclusions of pyrite - FeS2).

They reminded some of "rooster tails" leading to their common name.

A close-up shows the pattern a little better.

So what are these? Let's start with a little background first.

While paleontology is the study of ancient life (as preserved in fossils), ichnology is a branch of paleontology concerned with trace fossils - fossils that represent some trace of the ancient organism's activity. An example might be a dinosaur trackway (or the eurypterid trackway I wrote about a couple of weeks ago).

Sometimes all we have preserved from an organism is a trace fossil and we're not even sure exactly what species of animal made the trace. These trace fossils may be given taxonomic names and known as ichnospecies.

This swirly fossil in the Esopus Shale (it's found in some other units as well) is such an ichnofossil and was was given the ichnospecies name Zoophycos caudagalli where caudagalli means “tail of the cock” or "rooster tail".

The name Zoophycos was originally used to describe what was believed to be a new genus of algae (1855). It was later recognized that it wasn't algae at all but rather the trace of a marine worm - exactly what species of worm and exactly what this worm was doing is up for interpretation (although we can make some educated guesses).

Underlying the Esopus Shale, and just east of this particular outcrop on Route 199, are a series of limestone formations, chock full of marine invertebrate fossils, which formed in the early Devonian Period ( a hair over 400 million years ago). These limestones formed in the ancient Helderberg Sea - a shallow subtropical sea which once covered Ulster County (and beyond). Why subtropical? It's because this area was still in the Southern Hemisphere and closer to the equator than we are today.

This once clear sea began to get muddy, however, as mountain building started to occur to the east. This was the start of the formation of the Himalayan-scale Acadian Mountains. That's a story for a different day but suffice to say that the start of this mountain building brought a lot of sediments into the Helderberg Sea making it more muddy and leading to the deposition of the Esopus Shale. The previously-abundant marine invertebrates we see in the limestones disappeared as they were mostly filter feeders, straining organic material from the water, and those types of animals don't do well in muddy bottom waters.

Marine worms, however, do just fine in the bottom muds, as they churn through and extract organics from the sediments. While worms, and soft-squishy things in general, rarely preserve as fossils, they can leave traces of their activity. For Zoophycos, it's generally believed that the swirls were feeding traces of the worm (called spreiten) that lived in a vertical shaft, or burrow, beneath the traces.

Zoophycos fossil photo from the Kentucky Geological Survey

Zoophycos fossils have been found throughout the geologic record from 541 million years ago to modern times and even recent studies (Zhang, et al., 2015) acknowledge that we're still not sure of the exact group of marine worm (sipunculida, echiurida, or polychaeta) that may have made these fossils. Whichever one it was, it certainly hit upon a very successful strategy for existing in the muddy seafloor sediments for hundreds of millions of years.

Maybe my students don't feel the same, but I am always humbled when standing on such outcrops, ignoring the modern traffic whizzing by, and looking at fossils like this while imagining the seafloor that once existed here deep in the mists of time.