This is Mazon Monday post #324. What’s your favorite Mazon Creek fossil? Tell us at email:esconi.info@gmail.com.

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Mazon Creek hagfish made headlines again last week. New research led by Victoria McCoy, Assistant Professor in the Department of Geosciences at the University of Wisconsin–Milwaukee, was published in the journal Biology Letters. In the paper, “Stepwise Loss of Complexity in Hagfish Eyes Prior to Deep-Sea Colonization,” McCoy collaborated with several familiar names… Arjan Mann and Jason Pardo of the Field Museum of Natural History, René and Bruce Lauer of the Lauer Foundation for Paleontology, Science and Education, Andrew Young of the David and Sandra Douglass Collection, and Tetsuto Miyashita of the University of Ottawa.

This marks the third newly published paper involving Bruce and René Lauer in just the last week, which is a testament to their continued contributions to paleontological research. Congratulations, Bruce and René!

This paper addresses a long-standing question in vertebrate evolution: how and when hagfish lost their eyes. By studying exceptionally preserved fossil hagfish from Mazon Creek and comparing them to living hagfish, the authors show that eye reduction occurred gradually over tens of millions of years and not as a rapid response to life in the deep sea.

Specimens of Gilpichthys greenei (see Mazon Monday #140), Squirmarius testai (see Mazon Monday #193), and Myxinikela siroka were used in the study. All of these fossils were from the Mazon Creek fossil deposit. These species are known for soft-bodied preservation, including their eyes. The researchers found that all three fossil hagfish species had small eyes covered by soft tissue (a condition found in modern hagfish), with retinal melanosomes preserved as spherical pigment bodies.

There were differences. Gilpichthys greenei and Squirmarius testai had fossilized lenses, while Myxinikela siroka did not. The authors think this points to Myxinikela siroka as representing an intermediate stage in the evolution of hagfish eyes.

The paper demonstrates that hagfish did not lose their eyes suddenly after moving into the deep sea. Instead, eye degeneration occurred gradually through multiple evolutionary stages, beginning in shallow-water Carboniferous environments. Fossils from Mazon Creek preserve these transitional forms and reveal that the distinctive blind or nearly blind condition of modern hagfish evolved through a long, stepwise process spanning hundreds of millions of years.

Stepwise loss of complexity in hagfish eyes prior to deep sea colonization

Abstract
The hagfish eye is highly reduced, lacking pigment and a lens, and covered by soft tissue. The timing and mode of this loss of complexity remain unknown. Here, we present high-resolution anatomical data on the fossilized eyes of three stem hagfishes that form a transitional series towards the highly vestigialized eye of modern hagfishes. All have small eyes covered in soft tissue and containing only spherical melanosomes. Lenses are present in the more stemward hagfishes but absent in the more crownward hagfish. Reduction of the eyes occurred gradually across the Palaeozoic, with an initial stage of size reduction and loss of cylindrical melanosomes from the retinal pigmented epithelium, an intermediate stage with loss of image-focus capability and finally a near-complete loss of vision (the hagfish crown group). The initial and intermediate stages of this process likely occurred in nearshore environments prior to the Permian colonization of the continental slope by hagfish.