Skeletal completeness and phylogenetic information of the dinosaur fossil record in Mexico

To find records of dinosaurs in Mexico, the search engine Google Scholar was used using the terms “dinosaurs+Mexico”, “dinosaur+Mexico”, and “Mexican+dinosaurs”. Most of the papers collected this way referred to descriptions of new species and to reviews that were performed in the last 20 years. Of these reviews, Ramirez-Velasco and Hernandez (2015) include the most comprehensive tables documenting all the findings for the Late Cretaceous reported in academic and grey literature, such as unpublished theses, concerning dinosaurs in public collections. A second source of data was the paleobiological database (PBDB.org) that covered most of the records already retrieved through Google Scholar. Finally, the Mexican Geological Service (SGM in Spanish) has compiled the Léxico Estratigráfico Mexicano (LEM) over the last 15 years, a list of all the lithostratigraphic units in the country on a government-supported free access website (Arriaga 2025). Each unit has an information sheet as downloadable PDF files with the same entries all over and includes both fossiliferous and non-fossiliferous outcrops. A survey through the available files allowed finding reports of dinosaur material documented in passing in other publications. Only the material that had an identification as a specimen, even if uncatalogued, was collected. There is an extensive record of ichnofossils and eggs, but these entries were excluded as the focus was only on skeletal remains. The final compilation has a total of 673 entries. Each entry represents one specimen that has been registered into a collection and that has been mentioned in at least one publication. For instance, in the Huizachal Canyon, Tamaulipas, there are reports of heterodontosaurids (Clark et al. 1994) and a coelophysoid (Munter and Clark 2006) that have been given collection numbers, but there are also reports of large dinosaur bones that, given the age and size, may correspond to early-diverging sauropodomorphs (Munter and Clark 2006, 53–54). Similarly, for the Otlaltepec Formation, there are reports of 81 fossil fragments, but only three of them have been described as one specimen and identified as a diplodocoid remains (Rivera-Sylva and Espinosa-Arrubarrena 2020). However, the lack of identification as collection specimens excluded them from this analysis. Some entries only have the collection they are part of, indicating that the specimen has been accessioned, even if it has not been catalogued and given a name by the time of publication. Included in this list as well are reports of specimens accessioned into collections that have been documented through secondary sources, like a review reporting on a specimen in a dissertation, but the search did not extend to thesis repositories. Some entries represent documented specimens that, at the time of publication in a review, were still in situ. A final search was performed through the information sheets at the LEM for Mesozoic localities, which has a criterion of maximum information to work as a guide; however, reports of fossils from abstract conferences, proceedings, and working reports were excluded. The SCM was standardized to be applicable across the different dinosaur clades despite the widely variable relative size of the skull, axial skeleton, and limbs across the different taxa. The total score of 100 is distributed along the different regions of the body: a score of 10 is assigned to the skull, 51 to the axial skeleton, 35 to the appendicular skeleton, and 4 to non-skeletal bone. The SCM in the skull is distributed along the cranial regions: the upper arcade lateral series (SCM=1, comprising premaxillae and maxillae), the circumorbital series (SCM=2, lacrimal, prefrontal, postfrontal, jugal and postorbital), the median series (SCM=2, nasal, frontal, parietal, post-parietal and the bones of the palate), the cheek series (SCM=1, squamosal, quadrate, and quadratojugal), the braincase (SCM=0.5), the mandibular series (SCM=2.5, predentary, dentary, angular, surangular, articular, and splenial), and the teeth (SCM=1). The SCM of the axial skeleton is then subdivided into four regions (cervical vertebrae, SCM=15; dorsal vertebrae, SCM=15; sacral vertebrae, SCM=5; and caudal vertebrae, SCM=16), each considering vertebral bodies and ribs or chevrons as subdivisions. The SCM in the appendicular skeleton is distributed along the pectoral girdle (SCM=5), forelimbs (SCM=12), pelvic girdle (SCM=6), and hindlimb (SCM=12). When compiling the data, the SCM was assigned according to what was reported accounting penalization for “incomplete” bones, and for the number of bones. For instance, two complete femora account for a SCM=4; if a paper reports one distal half of a femur, then it is scored as SCM=1. In some detailed descriptions, e.g., the monographic description of a new species, the assessment is easy, but when a paper mentions in passing the completeness of a bone, this is done to best capture the SCM, even if overestimated. However, in other cases, such as compilations or lists in reviews, the description is given as “fragmentary femur” or “large bones”; in these cases, for instance, the penalization is to give values like “0.5” to indicate the presence of one femur. The CCM is a more dynamic metric because it relies on accessible phylogenetic data. The CCM is allocated as the proportion of characters for each region in the SCM. For instance, in Prieto-Marquez (2016), there are 189 cranial characters out of 273, i.e., 69.2% of the characters are cranial, or as another example, 19 characters (6.9%) deal exclusively with the ilia. To create a proxy that represents phylogenetic signal, it is assumed that having two ilia from the same individual is more informative to score a character than only one ilium, because it is possible to differentiate preservation or deformation by seeing the character twice, so the proportional value for one ilium is 3.45%. Only when there is a full description with illustrations, the characters are counted as to what the paper says can it be assessed. The CCM×SCM product works as an estimator of the phylogenetic information contained in the osteological record (potential CCM). For the CCM to be estimated, the phylogenetic dataset needs to include an appended character list, creating a disconnect between the new phylogenetic hypotheses and models and the CCM if this information is not included as supplementary material or in a data repository. Data available upon request was not considered, as this would mean having data not included in this paper. The CCM was organized into the same number of categories as in the SCM, and this score is simply stated as the number of characters in each region. To compensate for the difference in character distribution and number, the final product is normalized so that the most informative specimen gets a 100, and the least informative specimen gets a 0. To contrast the findings for the Mexican dinosaur fossil record, a similar analysis from the ETE Bonebed Database described above was performed using the Paleobiology Database (PBDB) to see if a similar pattern of preservation is identified. All occurrences of Archosauria (fossils, ootaxa, and ichnites) for the time span between the Ladinian (240 Mya) and the Late Maastrichtian (66 Mya) were selected. From the 27,070 records, 23,936 (around 88%) are registered in collections from terrestrial environments, of which 8,166 (nearly 60% of the occurrences in terrestrial environments) are associated with fluvial deposits. The number is likely higher, since most records (11,745 entries) are classified as “indeterminate terrestrial environments” or simply “terrestrial”.