An article published in the journal “Palaeontology” opens a dispute on the classification of Tullimonstrum gregarium, a strange marine animal that lived about 300 million years ago, commonly known as the Tully monster (photo ©Ghedoghedo). During 2016, two research brought arguments to support the thesis that this animal was a vertebrate but now a group of paleobiologists led by Lauren Sallan of the University of Pennsylvania brought counter-arguments.
According to Lauren Sallan’s team considering all the evidence regarding the Tully monster leads to a conclusion contrary to the thesis that this is a vertebrate. They believe that the set of evidence that include anatomy, preservation and comparison with other species lead to very different conclusions once again putting into question the classification of this strange animal in the tree of life.
An objection concerns the conclusions of the analysis of the various fossil Tully monsters’ internal parts in which the researchers identified gills and a rudimentary spinal cord, described in the article published in March 2016. This is some of the anatomical elements that convinced them that it was a vertebrate related to modern lampreys. According to Lauren Sallan’s team these conclusions are based on really little preserved tissues and lampreys were found in the area where the Tully monster lived but they don’t look like it.
The exam of the Tully monster’s eye, described in the article published in April 2016, is disputed as well. The researchers discovered two forms of melanosomes and pointed out that only vertebrates have them in two different forms. According to Lauren Sallan’s team that doesn’t constitute evidence because the eyes evolved several times, even in complex structures such as in arthropods and cephalopods such as octopuses, so it’s conceivable that the eyes of Tullimonstrum gregarium had similarities with those of vertebrates.
The researchers even offer an opposite argument claiming that the Tully monster had eyes with a simpler structure than that of all known vertebrates. Molluscs and annelids are among the animals that have structures similar to those of Tullimonstrum gregarium, as well as some primitive chordates. In essence, perhaps this animal was related to vertebrates but was not part of that group.
Overall, according to Lauren Sallan team none of the more than 1,000 Tully monster specimens examined shows structures that can be considered universal in aquatic vertebrates. If in at least a portion of those specimens such structures were preserved the conclusions of previous research would be accepted but this animal doesn’t appear to have any of those.
The various conclusions concerning Tullimonstrum gregarium derive from complex tests performed on fossils and various analyzes of the collected data. This shows once again the difficulties that paleontologists can face in classifying creatures that lived hundreds of millions of years ago that have only vague similarities with the current ones.
Honestly after two different research that claimed that Tullimonstrum gregarium is a vertebrate based on different reasons I thought the matter was closed. The objections raised by Lauren Sallan’s team might restart the studies, also because no alternative classification was provided.
