This is Mazon Monday post #320.  What’s your favorite Mazon Creek fossil?  Tell us at email:esconi.info@gmail.com.

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Our friends at the Field Museum have been busy… this time it’s paleobotany. The paper “A shoot at the root? Unique development and evolution of the stigmarian apical meristem” was published in the journal Proceedings of the Royal Society B a few days ago. The authors should be familiar… Michael P. D’Antonio, C. Kevin Boyce, Michael P. Donovan, and Fabiany Herrera. Michael D’Antonio, Michael Donovan, and Fabiany Herrera work at the Field Museum, while C. Kevin Boyce is at Stanford University. All have been referenced in posts on our website.

This paper looked at Stigmaria casts, which have been thought to preserve little detail. These lycopod roots were scanned using modern imaging techniques, micro-computed tomography, inside the fossils researchers found three-dimensional preservation of the internal anatomy.

Looking at the rootlet traces, the researchers concluded that these structures did not develop like typical roots. The traces originate at the apex and grow basipetally, beginning parallel to the axis before curving outward toward the surface. Some traces even merge before connecting to the central stele. This pattern is unexpected for roots and instead closely resembles shoot-like developmental processes in modern plants, despite Stigmaria clearly functioning as a rooting organ.

The fossil specimens used were from the collections at the Field Museum. And, while not from Mazon Creek, the specimens were from a similar time, the early to middle Pennsylvanian Period.

Like other recent posts, this research shows how with modern imaging techniques like CT scanning, museum collections, including many Mazon Creek specimens, still hold untapped information.

Stigmaria is a familiar fossil to collectors as distinctive, radiating root systems. But this study reminds us that even the most well-known fossils can still surprise us. Sometimes, what looks like a root might actually be something entirely different.

Abstract

Stigmaria rooted late Palaeozoic lycopsid trees, and its development
and homologies have long been debated. Micro-computed tomography
of stigmarian apex and distal axis cast fossils reveals exceptional and
surprising three-dimensional preservation of the internal system of rootlet
anatomy, including immature traces that articulate to the apical plug
prior to integrating proximally with the stele, reorientation of traces
from longitudinal to radial behind the apex, and trace anastomoses.
This record of development indicates that the stigmarian apical meristem
has characteristic developmental features typical of vascular plant
shoots, consistent with apical production of the plant morphogenic
hormone auxin and its transport away from the apex. Importantly,
the stigmarian meristem structure differs fundamentally from that of
the small rooting stocks of related plants, inviting reconsideration of
homology between Stigmaria and other lycopsid rooting structures. The
shoot-like developmental pattern of Stigmaria challenges the foundational
assumption that roots develop by auxin transport towards the apex,
offering a rare example in the fossil record of a lineage experimenting
with developmental architecture in ways unseen in living plants. The
employed methods give potential new life to thousands of cast fossils that
may, too, preserve internal anatomy.