Data from: 283,821 concretions, how do you measure the Mazon Creek? Assessing the paleoenvironmental and taphonomic nature of the Braidwood and Essex assemblages

The Mazon Creek Lagerstätte (late Carboniferous Period, Moscovian Stage;
Illinois, USA) captures a diverse view of ecosystems in delta-influenced
coastal settings through exceptional preservation of soft tissues in
siderite concretions. The generally accepted paradigm of the Mazon Creek
biota has been that of an inferred paleoenvironmental divide between what
have been termed the Braidwood and Essex assemblages, wherein the former
represents a freshwater ecosystem with terrestrial input and the latter a
marine-influenced prodelta setting with abundant cnidarians, bivalves,
worm phyla, and diverse arthropods. Here, we revisit the paleoecology of
the Mazon Creek biota by analyzing data from nearly 300,000 concretions
from 270 locations with complementary multivariate ordinations. Rather
than a homogenization of ecological communities, our results show the
Braidwood assemblage as a legitimate shoreward community and provide
evidence for further subdivision of the Essex assemblage into two distinct
subassemblages, termed here the Will-Essex and Kankakee-Essex. The
Will-Essex represents a benthos dominated by clams and trace fossils along
the transition between nearshore and offshore deposits. The Kankakee-Essex
is dominated by cnidarians, presenting an ecosystem approaching the
geographic margin of this taphonomic window. These new insights also allow
a refined taphonomic model, wherein recalcitrant tissues of Braidwood
organisms were subject to rapid burial rates while organisms of the Essex
assemblage typically had more labile tissues and were subject to slower
burial rates. Consequently, we hypothesize that the Braidwood fossils
should record more complete preservation than the Essex, which was exposed
for longer periods of aerobic decomposition. This is supported by a higher
proportion of non-fossiliferous concretions in the Essex than in the
Braidwood.