This is Mazon Monday post #106. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
Back in November 2020, Arjan Mann spoke at the ESCONI General Meeting about Mazon Creek tetrapods. Due to some research that had yet to be published, we couldn't post the video of his excellent presentation, which was entitled "Revisiting the exceptional tetrapod fauna of Mazon Creek, Illinois".
Arjan visited us back in early March 2022 and I had the pleasure of taking him on a tour of Pit 11, Braceville, and the surrounding area. He is planning to return for the trip to Danville in late April.
We were able to catch Torino Hill looking mysterious out in the middle of Braidwood Lake.
Arjan wrote this summary article about his interesting presentation.
Revisiting the exceptional tetrapod fauna of Mazon Creek, Illinois, leads to new discoveries in the origin of amniotes.
The Carboniferous is a critical time period for the evolution and diversification of tetrapods (i.e., land dwelling vertebrates). During this period, all major tetrapod lineages appear in the fossil record including various groups of non-amniotes (e.g. temnospondyl amphibians), as well as the two major lineages of amniotes, the synapsids (mammal lineage) and reptiles (reptilian lineage). The late Carboniferous-aged (309-307 Ma) Mazon Creek lagerstätte produces some of the earliest tetrapod fossils, including those of major Paleozoic lineages such as the second oldest reptile (Mann et al., 2019b). Despite this, the Mazon Creek lagerstätte has remained a difficult and unproductive vertebrate locality for researchers to utilize in tetrapod diversity studies due to the scarcity of fossils of this kind. Over the past decades, several new terrestrial tetrapod fossils collected from Mazon Creek have come to light. These include several new virtually-complete fossils of the earliest fossorially adapted recumbirostrans–a group of fossorially adapted reptilian tetrapods (Mann and Maddin, 2019; Mann et al., 2019a).
In my talk I provided a revised systematic study of the Mazon Creek pan-amniote fauna, in an attempt to reassess the terrestrial ecosystem diversity present at the late Carboniferous lagerstätte. The results accumulate to systematic descriptions of four new and unique recumbirostran taxa (Diabloroter bolti, Infernovenator steenae, FMNH 1309, and MPM VP359229.2) and a re-description of the basal eureptile Cephalerpeton ventriarmatum leading to the anointment of the oldest parareptile Carbonodraco lundi (formerly Cephalerpeton aff. C. ventriarmatum from Linton, Ohio).
Figure 1. Image of the Diabloroter concretion. (photo credit: Susan Kornreich-Wolf)
The research presented was broken into chapters, that followed the chapters used in my recently published doctoral dissertation. In order of appearance: Chapter 2, the new short bodied (17 presacrals), highly terrestrial, early-diverging, brachystelechid recumbirostran, Diabloroter bolti gen. et sp. nov. (Fig 1-2), was described as the first tetrapod from the Fulton County tetrapod locality. The early diverging position of Diabloroter contextualizes the unique cranio-dental specializations of later Permian-aged brachystelechids. I put forward the hypothesis that the unique tricuspate tooth morphology of these later brachystelechids may have been used for algivorous feeding–filling a previously unrecognized niche among the Permo-Carboniferous tetrapods. In Chapter 3, the serpentine molgophid recumbirostran, Infernovenator steenaegen. et sp. nov. (Fig 3), was described revealing it possessed extremely diminutive forelimbs. The unique cranial morphology of Infernovenator, including a near complete circumorbital series and shovel-headed snout morphology, allows for both the clarified position of lysorophians (molgophids) within Recumbirostra, and reveals the presence of mosaic evolution with respect to cheek emargination and snout ecomorphotypes (i.e. round, shovel, and spade-headed) (Fig 4). In Chapter 4, a reappraisal of fossils attributed to the basal eureptile Cephalerpeton ventriarmatum, led to the discovery of the oldest parareptile, Carbonodraco lundi gen. et sp. nov. (formerly Cephalerpeton aff. C. ventriarmatum from Linton, Ohio). The anatomy of Cephalerpeton reveals a gracile, highly terrestrial form, possibly occupying a scansorial niche. Chapter 5 provided a description of the new recumbirostran, FMNH 1309, yet another fossorially adapted, long-bodied ecomorph that uniquely retains well-developed limbs. The exceptional preservation of soft tissues in FMNH 1309, including body outlines and scales, reveal ultra-structural integumentary adaptations to a fossorial lifestyle, comparable to those of modern squamates. Last but not least, in Chapter 6, a new molgophid recumbirostran was described (MPM VP359229.2), introducing the first occurrence of complete loss of the forelimb, including girdle elements, in a Paleozoic amniote. The pattern of limb-loss in the new molgophid (i.e. forelimbs lost first) is similar to that of modern snakes, suggesting a common mechanism of limb-reduction may occur more broadly across the amniote tree. Furthermore, the description of this forelimbless taxon completes the picture of reptilian amniote bauplan diversity at Mazon Creek, showing a diverse assemblage of long-bodied fossorial ecomorphs (Brachydectes, Infernovenator, MPM VP359229.2) and diverse short-bodied, highly terrestrial ecomorphs (Diabloroter, Cephalerpeton).
In totality, the Mazon Creek amniote assemblage shows considerable taxonomic diversity with at least six reptilian taxa (at present), four of which were described in my doctoral dissertation. Although the inclusion of Recumbirostra into Amniota remains controversial, the phylogenetic analyses performed in this thesis continue to support this phylogenetic hypothesis. With this result in mind, the amniotes of Mazon Creek appear equally as taxonomically diverse as those of Joggins and Florence Nova Scotia, and even more diverse than those of Linton, Ohio and other Moscovian-aged cannel-coal localities (Reisz, 1972; Hook and Baird, 1986; 1993; Mann et al., 2020). Overall, the tetrapod fauna of Mazon Creek appears compositionally similar to that of most Permo-Carboniferous tetrapod localities (see taxonomic list below), revealing that even in lowland ecosystems amniotes were present and in greater diversity than previously recognized.
Across the newly described terrestrial fauna there is an unexpected ecomorphological diversity of body plans present. These range from the short-bodied Diabloroter to the serpentine, long bodied, and limb-reduced MPM VP359229.2. This diversity in combination with an array of unique cranial specializations present in the new taxa highlight the early experimentation with fossoriality in terrestrial vertebrate ecosystems.
Exceptional preservation of soft tissues in FMNH 1309 even reveal integumentary adaptations to a fossorial lifestyle. Collectively, I am able to provide a revised taxonomic list for the Mazon Creek Tetrapoda that reveals the fauna is compositionally similar to that of most Permo-Carboniferous tetrapod localities. Given recent studies that suggest that recumbirostrans are crown-group reptiles, the results of my work imply a more rapid mid-Carboniferous diversification of amniotes into a variety of terrestrial niches and consequently an earlier establishment of terrestrial vertebrate ecosystems.
Figure 2. Image of a latex peel of Diabloroter and interpretive drawing of the anatomy (Mann and Maddin, 2019).
Figure 3. Image and interpretive illustration of Infernovenator steenae (Mann et al., 2019a)
Figure 4. A phylogeny revealing the interrelationships of Recumbirostra. Mosaic evolution of the snout and cheek complexes have been highlighted (Mann et al., 2019a).
References:
Hook, R. W., & Baird, D. (1986). The diamond coal mine of Linton, Ohio, and its Pennsylvanian-age vertebrates. Journal of Vertebrate Paleontology, 6(2), 174–190.
Hook, R. W., & Baird, D. (1993). A new fish and tetrapod assemblage from the Allegheny Group (Late Westphalian, Upper Carboniferous) of eastern Ohio, USA. New research on Permo-Carboniferous faunas. Compiled by U. Heidtke. Pollichia-Buch, 29, 143–154.
Mann, A., Gee, B. M., Pardo, J. D., Marjanović, D., Adams, G. R., Calthorpe, A. S., & Anderson, J. S. (2020). Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem. Papers in Palaeontology (early view).
Mann, A., & Maddin, H. C. (2019). Diabloroter bolti, a short-bodied recumbirostran ‘microsaur’from the Francis Creek Shale, Mazon Creek, Illinois. Zoological Journal of the Linnean Society, 187(2), 494-505.
Mann, A., Pardo, J. D., & Maddin, H. C. (2019a). Infernovenator steenae, a new serpentine recumbirostran from the ‘Mazon Creek’Lagerstätte further clarifies lysorophian origins. Zoological Journal of the Linnean Society, 187(2), 506-517.
Mann, A., McDaniel, E. J., McColville, E. R., & Maddin, H. C. (2019b). Carbonodraco lundi gen et sp. nov., the oldest parareptile, from Linton, Ohio, and new insights into the early radiation of reptiles. Royal Society open science, 6(11), 191191. Reisz, R. (1972). Pelycosaurian reptiles from the middle Pennsylvanian of North America. Harvard University, 144(2), 1–61.
