A recent open access paper not yet mentioned:
Walter W. Stein (2019)
TAKING COUNT: A Census of Dinosaur Fossils Recovered From the Hell Creek and Lance Formations (Maastrichtian).
The Journal of Paleontological Sciences 8: JPS.C.2019.01 1
A census of Hell Creek and Lance Formation dinosaur remains was conducted from April, 2017 through February of 2018. Online databases were reviewed and curators and collections managers interviewed in an effort to determine how much material had been collected over the past 130+ years of exploration. The results of this new census has led to numerous observations regarding the quantity, quality, and locations of the total collection, as well as ancillary data on the faunal diversity and density of Late Cretaceous dinosaur populations. By reviewing the available data, it was also possible to make general observations regarding the current state of certain exploration programs, the nature of collection bias present in those collections and the availability of today's online databases.Â
A total of 653 distinct, associated and/or articulated remains (skulls and partial skeletons) were located. Ceratopsid skulls and partial skeletons (mostly identified as Triceratops) were the most numerous, tallying over 335+ specimens. Hadrosaurids (Edmontosaurus) were second with at least 149 associated and/or articulated remains. Tyrannosaurids (Tyrannosaurus and Nanotyrannus) were third with a total of 71 associated and/or articulated specimens currently known to exist. Basal ornithopods (Thescelosaurus) were also well represented by at least 42 known associated and/or articulated remains. The remaining associated and/or articulated specimens, included pachycephalosaurids (18), ankylosaurids (6) nodosaurids (6), ornithomimids (13), oviraptorosaurids (9), dromaeosaurids (1) and troodontids (1).Â
Over 41,800 isolated bones and teeth, were also located. This number represents only a small fraction of the actual total collection as many of the museums and institutions surveyed were unable to provide complete numbers on isolated elements. Over 46% of these isolated bones and teeth were identified as hadrosauridae, usually identified as Edmontosaurus. Isolated elements identified as ceratopsids made up just over 21% of the total. These were generally identified as Triceratops. Isolated bones and teeth of tyrannosaurids were significantly less, at only 4.6%. The large difference between the associated and/or articulated remains and the isolated bones and teeth of tyrannosaurids (10.9% down to 4.6%) and ceratopsids (51.3% to 21.5%) is likely due to both a preservational and collection bias towards the larger, more likely to be fossilized, and more likely to be collected, Tyrannosaurus and Triceratops skeletons and skulls.Â
Even though small theropods accounted for less than 0.6% of the total recovered associated and/or articulated remains, their teeth and isolated elements were encountered quite frequently. Isolated bones and teeth of dromaeosaurids, troodontids and âunidentified small theropodsâ accounted for as much as 16% of the total number of isolated remains. This data suggests that there is a tremendous level of collection and preservational bias in the current Hell Creek and Lance sample set Actual small theropod diversity and populations in the Late Cretaceous were most likely much higher than previously considered.Â
It is highly likely that the fluvial and geochemical environment that dominated the Late Cretaceous of this region was simply too rough and tumble for bones of most genera under the 400 kg live weight threshold to be preserved. Both preservational bias and collection bias appear to play significant roles in how we currently view the diversity of the Hell Creek and the Lance. This has directly influenced our perception of the end Cretaceous extinction event. It is entirely possible that many genera will never be known from more than fragmentary remains.