Computational protocol: A Diminutive New Tyrannosaur from the Top of the World

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Protocol publication

[…] There is currently only a limited amount of Nanuqsaurus hoglundi fossil material, but enough information is preserved to test the relationships of the taxon within the framework of two recent cladistic analyses of tyrannosauroid phylogeny , .. We first tested relationships of N. hoglundi in the context of a very tyrannosauroid-focused taxon-character matrix . In addition to adding N. hoglundi to the analysis, we altered the original dataset in three ways. First, we changed the coding of two characters for Dryptosaurus to match revisions made in a more recent analysis of tyrannosauroid phylogeny (character 178 changed from “?” to “0”; character 201 re-scored from “?” to “0”) . Second, we removed Raptorex kreigsteini from the analysis because of uncertainty over whether it represents a distinct tyrannosauroid taxon from the Lower Cretaceous, or is instead an immature individual of the Upper Cretaceous tyrannosaurine Tarbosaurus bataar . The presence or absence of Raptorex in the analysis had no impact on the resulting tree topology. Third, we added one new character to the dataset (character 308) to take into account the shared presence of a long rostral process of the frontal inserted between the articulations for the prefrontal and lacrimal in Nanuqsaurus and Teratophoneus. These changes, plus the coding of character 308 for the other taxa in the analysis are provided in .The original dataset was acquired as a Microsoft Word document, and was reentered in MacClade 4.08a for Macintosh OSX . All other characters remained numbered the same as in the original dataset, and we refer interested readers to the original study for the full character list and original dataset . Nanuqsaurus and the aforementioned new character were added to the taxon-character matrix, resulting in 23 taxa (4 outgroup taxa, 19 ingroup taxa) and 308 characters. Character coding for Nanuqsaurus is given in . The dataset was subjected to a parsimony analysis in PAUP* 4.0b10 , using TBR branch swapping. The analysis produced three equally most parsimonious trees with a length of 550, a consistency index of 0.6527, and a retention index of 0.8403 (). The three recovered trees differed only in the positions of Guanlong, Proceratosaurus, and Sinotyrannus relative to one another. Bremer support values were calculated using MacClade to create a PAUP Decay Commands file, which was then opened and applied in PAUP*.In all three hypotheses of phylogeny, Nanuqsaurus hoglundi was found to be a derived tyrannosaurine, the sister taxon to the Tarbosaurus+Tyrannosaurus clade (). This node was supported by a single unambiguous character, the presence of a dorsoventrally tall, paired sagittal crest on the frontal. The age of Nanuqsaurus hoglundi (70-69Ma) is consistent with its place in the recovered hypothesis of tyrannosauroid phylogeny, positioned in time between the more basal Daspletosaurus torosus (Middle to Late Campanian) and the more derived Tyrannosaurus rex (latest Maastrichtian) among other North American tyrannosaurines. Nanuqsaurus hoglundi may be contemporaneous with Tarbosaurus bataar (Late Campanian to Maastrichtian) of Asia, but the two taxa are easily distinguished from one another by the rostral process of the frontal separating the prefrontal and lacrimal in N. hoglundi, and by the two reduced mesial dentary teeth in N. hoglundi versus a single reduced first dentary tooth in juveniles and mature T. bataar , . The presence of a robust peg-in-socket articulation between the maxillae and nasals of mature individuals was a character shared by Daspletosaurus, Nanuqsaurus, and the Tarbosaurus+Tyrannosaurus clade, but the uncertain status of this character in the more basal tyrannosaurines Teratophoneus and Alioramus made the position of this character ambiguous in our results. It took no less than six additional steps to move Nanuqsaurus to a position on the albertosaurine lineage, highlighting the low likelihood that the Alaskan material might actually be referable to the contemporaneous taxon Albertosaurus sarcophagus.Although the Bremer support for the Nanuqsaurus+(Tarbosaurus+Tyrannosaurus) node is less (Bremer support value = 2) than that for some other nodes on the tyrannosaurine lineage (Bremer support values range from 2 to 5), given the high percentage of missing data for Nanuqsaurus (95.1 percent missing data) its location on the tree is an indication of the importance of the derived characters present in the material. Indeed, there is less support for the positions of the more basal tyrannosauroids Appalachiosaurus and Dryptosaurus, both of which are known from more material than Nanuqsaurus. Bremer support for other tyrannosaurine nodes was less in our analysis than was recovered in the study upon which it was based . In the original study, the Tarbosaurus+Tyrannosaurus clade had a very high Bremer support value of 9, and the Daspletosaurus+(Tarbosaurus+Tyrannosaurus) clade a very high Bremer value of 7. The dramatic reduction in Bremer values in our analysis likely reflects the intermediate position of Nanuqsaurus in the hypotheses of phylogeny, which serves to break up or redistribute potential diagnostic characters among the derived tyrannosaurines.The second cladistic test was based on that used in a more recent study that included a greater number of taxa from across a much wider range of theropod clades . The dataset from the original study also used a large number of new and altered characters compared to other recent studies , , . Not surprisingly then, the hypotheses of tyrannosauroid relationships generated by the newer work differed from previous studies. Most notably, the basal tyrannosaurine Alioramus was placed outside Tyrannosauridae, and Bistahieversor, Teratophoneus, and the newly described Lythronax argestes from the Middle Campanian of Utah were found to be members of the tyrannosaurine lineage more derived than Daspletosaurus torosus .We added Nanuqsaurus hoglundi to the second dataset, but made no other changes to the original taxon-character matrix. The matrix was copied directly from an electronic version of the paper into Microsoft Word, and then imported into Mesquite v.2.75 . Character coding for Nanuqsaurus in the second analysis is given in . The file was then opened in MacClade 4.08a for Macintosh OSX , and finally was subjected to a parsimony analysis in PAUP* 4.0b10 , using TBR branch swapping. The analysis recovered eight equally most parsimonious trees, each with a length of 2008 evolutionary steps, a consistency index of 0.3541, and a retention index of 0.7226. Bremer support values were calculated using MacClade to create a PAUP Decay Commands file that was opened and applied in PAUP*.In addition to greater tree length, the hypotheses of relationship in our second analysis differed from the original work in several other ways. These included the removal of Dryptosaurus from Tyrannosauroidea, additional altered relationships among basal tyrannosauroids, and non-tyrannosaurid taxa, and a sister-taxon relationship between Daspletosaurus torosus and the Two Medicine Formation taxon. Importantly, Nanuqsaurus hoglundi was again found to be a derived tyrannosaurine, and the sister-taxon to the (Tyrannosaurus+(Tarbosaurus+Zhuchengtyrannus)) clade in all eight recovered trees. The relationships within Tyrannosauridae generated by the second analysis are shown in . […]

Pipeline specifications

Software tools MacClade, PAUP*, Mesquite
Application Phylogenetics