Reanalysis of “Raptorex kriegsteini”: A Juvenile Tyrannosaurid Dinosaur from Mongolia

Abstract

The carnivorous Tyrannosauridae are among the most iconic dinosaurs: typified by large body size, tiny forelimbs, and massive robust skulls with laterally thickened teeth. The recently described small-bodied tyrannosaurid Raptorex kreigsteini is exceptional as its discovery proposes that many of the distinctive anatomical traits of derived tyrannosaurids were acquired in the Early Cretaceous, before the evolution of large body size. This inference depends on two core interpretations: that the holotype (LH PV18) derives from the Lower Cretaceous of China, and that despite its small size, it is a subadult or young adult. Here we show that the published data is equivocal regarding stratigraphic position and that ontogenetic reanalysis shows there is no reason to conclude that LH PV18 has reached this level of maturity. The probable juvenile status of LH PV18 makes its use as a holotype unreliable, since diagnostic features of Raptorex may be symptomatic of its immature status, rather than its actual phylogenetic position. These findings are consistent with the original sale description of LH PV18 as a juvenile Tarbosaurus from the Upper Cretaceous of Mongolia. Consequently, we suggest that there is currently no evidence to support the conclusion that tyrannosaurid skeletal design first evolved in the Early Cretaceous at small body size.

A new multi-faceted framework for deciphering diplodocid ontogeny

“…While this methodology is convenient, straightforward, and simplistic, this retro-calculation method implies uniform growth rates throughout ontogeny. Whereas some studies suggest that sauropods had a slower initial growth rate (e.g., Erickson et al, 2001;Rogers and Erickson, 2005;Griebeler et al, 2013), others indicate that immature dinosaurs exhibited faster growth early in ontogeny (Curry, 1999;Horner et al, 1999Horner et al, , 2000Sander, 2000;Erickson et al, 2001;Padian et al, 2001;Horner and Padian, 2004;Sander et al, 2004;Erickson, 2005;Lee and Werning, 2008;Lehman and Woodward, 2008;Woodward and Lehman, 2009;Sander and Tückmantel, 2003;Fowler et al, 2011;Tsuihiji et al, 2011;Campione et al, 2013). If so, the distances between LAGs in immature diplodocids should be greater during early development.…”

“…While the characters used in a phylogenetic analysis are derived from morphologic attributes, this analysis would suggest that many of the characters are ontogenetically dependent. As demonstrated with tyrannosaurs (Carr and Williamson, 2004;Fowler et al, 2011), ceratopsians (Scannella and Horner, 2010;Scannella et al, 2014;Frederickson and Tumarkin-Deratzian, 2014), and hadrosaurs (Campione et al, 2013;Fowler and Horner, 2015), ontogeny (and stratigraphy) does affect taxonomy (likewise echoed in Hone et al, 2016). In animals that undergo an order of magnitude in size change, one could predict that many phylogenetic characters are simultaneously size determinant (ontogenetic in the sense they change as an animal gets larger with age) characters.…”

The Braincase Anatomy of the Late Cretaceous DinosaurAlioramus(Theropoda: Tyrannosauroidea)

“…This software was also employed to construct a variety of 3D movies viewable at the Digital Morphology website (www.digimorph.org/ specimens/Alioramus_altai). Any uncited comparative observations are based on our examination of the following specimens (see (CMN 8506;FMNH PR308;TMP 1985TMP .062.0001, 1994TMP .143.0001, 2001.036.0001), Dilong (IVPP V14243), Gorgosaurus (AMNH FARB 5336, 5664;ROM 1247ROM , 1422TMP 1986.144.0001), Guanlong (IVPP V14531), Raptorex (LH PV18, a young specimen described as a distinct taxon by Sereno et al [2009], but that may be a juvenile of a previously known taxon [Fowler et al, 2011]), Tarbosaurus (ZPAL MgD-I/3, MgD-I/4, MgD-I/38, MgD-107/7), Teratophoneus (BYU 8120/9396), and Tyrannosaurus (AMNH FARB 5027, 5029, 5117;CMNH 7541;DDM 35;FMNH PR2081;RSM 2523.8).…”

A new clade of Asian Late Cretaceous long-snouted tyrannosaurids

“…twice the body mass) and more mature than the holotypes of both species of Alioramus, thereby giving the first clear insight into the size and morphology of a larger-bodied, non-juvenile alioramin. There has been some concern that the holotypes of A. altai and A. remotus may represent juveniles of previously known tyrannosaurids (possibly the coeval Tarbosaurus) rather than distinct taxa 44,45 . Both Alioramus specimens are juvenile individuals, as confirmed by bone histology in the case of the A. altai holotype 10 , and juvenile tyrannosaurids are known to have proportionally longer and shallower skulls, more gracile bones and more teeth than conspecific adults 27,30 .…”