Tullimonstrum, colloquially known as the Tully Monster, is an extinct genus of soft-bodied bilaterian that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago."

Taxonomists cannot yet agree on where it belongs in the animal taxonomy, or even as to whether it's a vertebrate or not. See: Wikipedia

Its fossils are an example of a large group of as yet unidentified remains collectively known as 'probematica' or Incertae sedis (Latin for "of uncertain placement").

References:

The ‘Tully Monster’ is a vertebrate. Nature 532, pages 496–499

The ‘Tully Monster’ is not a vertebrate: characters, convergence and taphonomy in Palaeozoic problematic animals. Palaeontology Volume 60, Issue 2.

Update Apr. 2023

A new paper describes research which used 3-D scanning tech to compare the structure(s) of 153 fossil specimens :

Our investigation of the resulting comprehensive 3D morphological dataset revealed that structures previously regarded as myomeres, tri-lobed brain, tectal cartilages and fin rays are not comparable with those of vertebrates. These results raise further doubts about its vertebrate affinities, and suggest that Tullimonstrum may have been either a non-vertebrate chordate or a protostome.

Source : Palentology, ^{[ Paywalled ]} Volume 66, Issue 2

Also see: Number of speciesplugin-autotooltip__plain plugin-autotooltip_bigNumber of species

"We demonstrate that after more than six decades, estimates of global species richness have failed to converge, remain highly uncertain, and in many cases, are logically inconsistent."

Source : Global species richness estimates have not converged for Trends in Ecology & Evolution.