This is Mazon Monday post #330.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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While you were getting ready to celebrate our nation’s 250th birthday, a new fossil polychaete from Mazon Creek was being introduced to the world! It’s the first in annelid from Mazon Creek in over 20 years! Mazovermes magnaterminus was described in “The first new polychaete from the Mazon Creek fossil site in 20 years suggests cryptic annelid diversity in the Late Paleozoic“, which was published in the journal Geological Magazine. The paper is open access. The authors, Karma Nanglu, Jack Wittry, and Victoria McCoy, should be familiar to all of those interested in Mazon Creek fossils.

The etymology of the name, Mazovermes magnaterminus, is as follows:

Mazo from Mazon Creek, a common prefix for taxa from this site, and vermes, the Latin for worm. Magna is from the Latin for large, and terminus is from the Latin for end, describing the enlarged posterior margin of this new species.

All four known specimens of Mazovermes magnaterminus were collected at the “Jugtown Road Locality”, which led to the animal’s “Jugtown Worm” informal name. All the specimens are housed at the Field Museum. This locality is a relatively understudied marine locality. It and the surrounding areas were part of an “ancient embayment from the palaeocoastline into the nearshore coal swamp.”

The morphology of the worm is unique and unlike other Mazon Creek polychaetes. It has 32-42 body segments (chaetigers), a narrow anterior end, the posterior segments are greatly enlarged, simple bristles, a short pair of palps on its head, and backwards-pointing parapodia on the last several segments. Exceptional preservation allowed the researchers to identify remarkably fine details, including the chaetae (bristles), the digestive tract and pharynx, and possible circular muscle fibers preserved as tiny striations extending inward from the parapodia. Preservation of musculature in fossils is exceedingly rare, which makes this one of the best preserved Carboniferous annelids known.

The enlarged posterior and inward directed parapodia leads the researchers to conclude that Mazovermes magnaterminus anchored itself in sediment on the seafloor and lived a relatively sedentary existence compared to other Mazon Creek annelids.

One of the key takeaways from the paper is that lesser-known Mazon Creek localities deserve renewed attention. Reexamining these sites with modern techniques can produce important discoveries. Other undescribed soft-bodied invertebrates likely reside in museum and private collections. The actual extent of Mazon Creek’s annelid diverity is still largely unknown.

The first new polychaete from the Mazon Creek fossil site in 20 years suggests cryptic annelid diversity in the Late Paleozoic

Abstract
The Mazon Creek fossil site is famous for the preservation of diverse animal and plant assemblages. Among these fossils are representatives of animal groups that are rarely preserved with soft tissues. Annelida is one of these groups, and the Mazon Creek polychaete fauna comprises a disproportionately large sample of all known soft-bodied annelid fossils. Here, we describe the first new fossil annelid from Mazon Creek in over 20 years. This new species, Mazovermes magnaterminus, has a unique morphology with notably large posterior segments, giving an overall teardrop-shaped silhouette not found in any other fossil polychaete. It further preserves fine details such as the nanometre-sized chaetae, as well as muscular tissues. Fundamentally, this new species demonstrates not only a unique body organisation within this extremely morphologically and ecologically disparate phylum but also that the Mazon Creek Lagerstätte is likely to yield additional undescribed invertebrates with soft-tissue preservation upon re-investigation.

From the Discussion
Jugtown Road and the collecting localities along it roughly follow the path of an ancient embayment from the palaeocoastline into the nearshore coal swamp. The long narrow shape of the embayment and the surrounding freshwater in the coal swamp would likely have resulted in high variations in salinity within the embayment, which would be further enhanced by heavy rains and tides (Baird, personal communication, n.d.). M. magnaterminus was collected at the northernmost spot of the Jugtown embayment, at its narrowest, most terrestrially influenced end. At this point, salinity variations may have been at their highest, suggesting that this new polychaete may have been locally abundant due to being able to tolerate salinity variations.

These examples suggest that a high degree of environment specificity may obscure the total diversity of annelids at Mazon Creek. Further re-study of the Mazon Creek fossil annelids, with a particular focus on those collected from less well-studied localities, may yield critical insights into the evolution and extinct diversity of this important phylum.