The 'Tully monster' is a vertebrate.

Area of Science:

• Paleontology
• Evolutionary Biology
• Vertebrate Zoology

Background:

• Problematic fossils with enigmatic morphology challenge paleontological classification.
• The "Tully monster" (Tullimonstrum gregarium) from the Carboniferous Mazon Creek biota has remained phylogenetically uncertain for over 50 years.
• Previous hypotheses suggested affinities with invertebrates like nemerteans, polychaetes, gastropods, conodonts, or stem arthropods.

Purpose of the Study:

• To resolve the phylogenetic position of Tullimonstrum gregarium.
• To analyze the morphology of Tullimonstrum using a large specimen dataset.
• To determine the evolutionary affinities of this enigmatic fossil.

Main Methods:

• Morphological analysis of over 1,200 Tullimonstrum specimens.
• Phylogenetic analysis incorporating new morphological evidence.
• Comparative anatomy to identify key vertebrate features.

Main Results:

• Tullimonstrum possesses a proboscis with teeth, laterally projecting eyes on a rigid bar, and a segmented body with a caudal fin.
• New evidence reveals a notochord, cartilaginous arcualia, gill pouches, and articulated proboscis structures.
• Phylogenetic analysis places Tullimonstrum as a vertebrate on the stem lineage to lampreys (Petromyzontida).

Conclusions:

• Tullimonstrum gregarium is definitively identified as a vertebrate, specifically an early lamprey relative.
• This classification resolves a long-standing paleontological enigma and expands the known morphological diversity of extinct lampreys.
• The study highlights the power of modern phylogenetic methods in deciphering the affinities of problematic fossils.