The following Supplementary Materials are available for download as a zipped file.

They are also available at https://figshare.com/account/home#/projects/131276

#### Figure and Table Captions

**FIGURE S1.** STL model of the left premaxilla of TATE-099.**FIGURE S2.** STL model of the right premaxilla of TATE-099.**FIGURE S3.** STL model of the left maxilla of TATE-099.**FIGURE S4.** STL model of the right maxilla of TATE-099.**FIGURE S5.** STL model of the left pterygoid and quadrate of TATE-099.**FIGURE S6.** STL model of the left quadratojugal of TATE-099.**FIGURE S7.** STL model of the left palatine of TATE-099.**FIGURE S8.** STL model of the left ectopterygoid of TATE-099.**FIGURE S9.** STL model of the right ectopterygoid of TATE-099.**FIGURE S10.** STL model of the right dentary of TATE-099.**FIGURE S11.** STL model of the right surangular of TATE-099.**FIGURE S12.** STL model of the braincase (cast) of TATE-099.**FIGURE S13**. Comparison of the basioccipital-basisphenoid complex of TATE-099 to three other diplodocids (modified after Tschopp et al., 2015).**FIGURE S14.** Tschopp et al. 2015 matrix with TATE-099 with replica-based braincase.**FIGURE S15.** Whitlock and Wilson, 2020 - matrix with characters of TATE-099 added.**FIGURE S16.** Bremmer support and Bootstrap values.

**TABLE S1.** Replacement tooth measurements of the right premaxilla of TATE-099.**TABLE S2.** Replacement tooth measurements of the right maxilla of TATE-099.**TABLE S3.** Replacement tooth measurements of the right dentary of TATE-099.**TABLE S4.** Measurements of the crown heights of the erupted maxillary teeth of TATE-099.**TABLE S5.** Specimen numbers and taxa used for comparisons with TATE-099.

#### Dataset files

TATE-099 Right Dentary CT Data - Dataset

TATE-099 Right Maxilla CT Data - Dataset

TATE-099 Right Premaxilla CT Data - Dataset