Recent Results: Our own little monster

I had never heard of the Tully monster (Tullimonstrum gregarium) before moving to Chicago for graduate school. But after spending several years studying at the Field Museum, I became a Tullimonstrumophile like everyone else in Illinois. (It was named the state fossil in 1989). However, neither I nor anyone else knew what type of monster it actually was: an early relative of backboned animals? something more closely related to snails and clams? or an evolutionary branch with no close living relatives?

A simplified cladogram of vertebrate evolutionary relationships showing the position of lampreys (in red), the closest living relatives of the Tully monster. Diagram by the author.

A study recently published in the journal Nature seems to have solved this question by carefully studying some 1,200 Tully monster specimens using a variety of techniques. To cut to the chase, the Tully monster is a close relative of lampreys, which are rather eel-like animals that latch onto larger fishes with their round, sucker-like mouth and gradually consume their tissues. So although the Tully monster looks like something from outer space - with eyes on long, stiff stalks and a two-jointed proboscis tipped by a pincer - it is a vertebrate, a member of our own evolutionary branch of the tree of life.

The reason the evolutionary affinities of the Tully monster remained a mystery for so long is because scientists were unsure how to interpret its anatomy. All Tully monster specimens come from a single site in northeastern Illinois called Mazon Creek that preserves animals in their entirety: soft tissues as well as hard ones. But the animals are flattened, and not all tissues are preserved equally well. Thus, the information is tantalizing in its completeness but frustrating in its ambiguity.

A reconstruction of the Tully monster (top), along with a well-preserved specimen from Mazon Creek (middle), and an illustration of its anatomy (below). In the bottom diagram, the notochord is in red and the gut tube is in green. From McCoy et al. 2016.

One trick the scientists used to figure out the anatomy of the Tully monster was to compare it to other, less controversial Mazon Creek species. For example, they noted that a structure commonly interpreted as the Tully monster’s gut tube was not preserved in the same way as the gut tube of Gilpichthys, a relative of hagfishes. Based on this and other information, they reinterpreted this structure as the notochord, a characteristic feature of vertebrates. They were able to test this reinterpretation by using variation in how Tully monster specimens were flattened (top to bottom versus side to side) to reconstruct its anatomy in three dimensions. They found that - in specimens that preserve both a notochord and a true gut tube - the gut is positioned below the notochord as in all other vertebrates. Other important vertebrate features confirmed by the study using these and other strategies included segmented musculature and teeth.

But what about those bizarrely positioned Tully monster eyes, which sit on opposite ends of a rigid bar? No chordate alive today has a closely similar arrangement, though it is vaguely similar to what is seen in hammerhead sharks. The scientists speculate that their position may have something to do with the Tully monster’s equally strange feeding apparatus. Perhaps both the eyestalks and the proboscis evolved and lengthened in concert to create a highly effective and fearsome predator. Although the Tully monster would have terrorized only the smaller members of the Mazon Creek ecological community (larger Tully monsters were about 35 cm long, just over one foot), one cannot help but marvel at this remarkable product of natural selection.

-Dr. Darin Croft is an Associate Professor at Case Western Reserve University in Cleveland, Ohio


McCoy, V. E., E. E. Saupe, J. C. Lamsdell, L. G. Tarhan, S. McMahon, S. Lidgard, P. Mayer, C. D. Whalen, C. Soriano, L. Finney, S. Vogt, E. G. Clark, R. P. Anderson, H. Petermann, E. R. Locatelli, and D. E. G. Briggs. 2016. The ‘Tully monster’ is a vertebrate. Nature advance online publication. doi:10.1038/nature16992

Posted: 3/24/2016 11:56:59 AM by croftdarinadmin | with 0 comments
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