Hot on the heels of the description of a juvenile Tarbosaurus bataar (MPC-D 107/7), Fowler et al. (2011) have produced a piece that seeks to reclaim some sanity when it comes to descriptions of apparently basal and/or juvenile tyrannosaurids as new taxa. They weigh in on Alioramus altai (along with Alioramus remotus by extension, being nearly the same size), MPC-D 107/7, and Raptorex kriegsteini. The latter in particular, which we’ve discussed before (and before that) in which even their use of scare quotes around the name in the paper’s title indicates their disdain for the nomenclature. In the end, they argue that LH PV18, the holotype of Raptorex kriegsteini, is a juvenile tyrannosaurid, and probably Tarbosaurus bataar and also from the Late Cretaceous of Mongolia. The case is interesting not because of what their conclusions are (which are predictable; Tsuihiji et al.  nearly said the same thing) but because of how they got to them.
Fowler et al. particularly take issue with three things about that tyrannosaurid specimen, it’s provenance, ontogenetic age, and its morphology. The first is used to argue that the material is from other tyrannosaurid (as opposed to tyrannosauroid) bearing strata, the second to affirm the material is that of a juvenile, and the third that the so-called autapomorphic features are not useful in this case.
First, they reject the utility of the presence of pelycepod (bivalves) and a fish vertebra used to imply the fossil is from the Early Cretaceous. The fish vertebra, they note, appears very similar to an ellimmichthyiform clupeomorph fish, which are relatively deep-bodied and ventrally armored oddities, though Sereno et al. (2009) had originally inferred the vertebra to be of a Lycoptera, a key denizen of the Jehol Group formations to which they suggested LH PV18 derived from. But while Fowler et al. are likely correct in dismissing the bivalve and fish data, they ignore Sereno et al.’s argument in regards to sedimentology, first by using the doubtful association of Sereno et al.’s suggestions and then by taking their own spin of the likely distribution of that material. Ellimmichthyiform fish, as noted by the authors themselves, span from the Early Cretaceous to the Eocene, and thus their presence would not be restrictive to the Upper Cretaceous, or to any particular formation.
But in ignoring the sedimentology of the material in which the specimen was recovered, which was available to Sereno et al. for study, they passed by an opportunity to test their assertions regarding which possible formations the specimen could be from. In their supporting online material, Sereno et al. note that the sediment of the matrix of the specimen is a green, tuffaceous sandstone, a mixture of fine sands and even finer ashes that form from volcanics. They also posited that many Upper Cretaceous formations that preserved large-bodied tyrannosaurs lack this type of sediment; specifically, they excluded the Cenomanian Iren Dabasu Formation, and this sedimentology prevents the material from being from the Nemegt Formation of Mongolia, or the Udurchukan, Subashi or Wulanshuhai Formations of China, all formations from which tyrannosaurids are known in Asia in the Campanian and Maastrichtian. This material is common in the lower Jehol Group, particularly the Yixian Formation, and especially the Lujiatun beds which are helpful in that they preserve material commonly as three-dimensional and articulated specimens. But this type of sediment is unknown from those formations, which are generally mudstones in fluviatile channel facies, not unlike that of the Dinosaur Park Formation.
So, while Fowler et al. make useful their doubt, they do not actually dismiss Sereno et al.’s argument for the possible locality from which LH PV18 was recovered. Their seemingly strongest data point, rather, is a note on a conversation one of the authors (Pete Larson) had with one of Sereno et al.’s authors (Henry Kriegstein) who bought the specimen from a private broker, and with that broker about how the material was originally offered for sale as being from Mongolia. Perhaps this type of exchange is useful anecdotally, but I have my doubts. Not because I distrust either of the people involved, but because I doubt any amount of word-of-mouth from retailers of fossils who chose not to collect detailed geographic and stratigraphic data upon collection, and I am surprised that Fowler et al. are not more critical of this than using it in support of their premise that LH PV18 cannot derive from the Early Cretaceous.
Second, and most problematically, Fowler et al. throw a monkey-wrench into the age-assessment criteria by noting several choice pieces of information. Sereno et al. over-estimate their age, the authors argue, by using as their baseline attachment of vertebral neural arches to their centra and partial closure of the sutures, but these vertebrae are described as “attached” but also in parts slightly dissarticulated. A similar, if not identical condition is present in IVPP V4878, holotype of Shanshanosaurus huoyanshanensis (Dong, 1977) where the vertebrae are sometimes disassociated or fused, with articulated but open sutures, etc.
The vertebrae are undescribed for MPC-D 107/7, resulting difficulty assessing the arguments of either Sereno et al. or Fowler et al. independently when comparing to a “confirmed” juvenile Tarbosaurus bataar. Yet MPC-D 107/7 remains particularly key here; without it, I doubt Fowler et al. would be so easily swayed to assuming LH PV18 derives from the Late Cretaceous of Mongolia, and is therefore a Tarbosaurus or whatever. Sereno et al. sectioned the limb bones of LH PV18, and with it argued that two apparent LAGs represented samples of age. LAG (lines of arrested growth) counting is actually more complicated than it seems. These are zones of bone formation that occur in the cortex of growing bone, and build around older layers; while the newer cortical bone beneath the surface layer (made of lamellar bone) is being placed down, bone in the medullary cavity is being eroded, or absorbed, and then rebuilt by secondary processes and refilling the space, thereby obliterating LAGs.
So as growth continues and new zones of bone tissue are placed down, older bone is eventually lost. The first ring of bone growth is usually being eroded by the time the animal hatches, and then this continues throughout life. Annual growth can increase, or decrease, and the relative thickness of the growth zones (LAGs) result in clues as to how fast the animal is growing, but even more so, concentrations of osteoblasts (the cells that actually grow bone) increase when bone is more vascular and sexual maturity is reached, at which point, bone growth typically slows (resulting in relatively thinner lines). Accounting for the lost rings, and thus lost years of measure, in bones can be tricky, and is based largely on having a relative sample of growth series to project a bsaeline from (usually a hatchling) and so forth through to the largest individuals; this technique, called back-calculation, requires not only estimating the rate of deposition of the innermost rings, but projecting this data onto an expectation of rapid versus slow growth. If the animal is precocial, say, rapid growth early on as the animal puts on the pounds should indicate thick lines initially, followed by thinner lines later on as maturity increases, and there is usually no sigmoid curve; when these lines are relatively the same, then growth is steady; when the lines increase in thickness, the growth is increasing.
Tyrannosaurs show a sigmoid curve during growth (Erickson et al., 2004), showing slow growth up to about 10yo, followed by steadily or rapidly increasing growth. As osteoblasts increase in the outer layers, the animal nears sexual maturity. Bone growth only stops when the periosteum forms around the cortex, capping off new bone growth. Both LH PV18 and MPC-D 107/7 have had their long bones sectioned for histological slides, producing valuable information. However, the authors in Fowler et al. and Sereno et al. appear to differ on some fundamentals of how old the same material is. Fowler et al. note that Sereno et al. had inferred thickness of the outermost LAG to the lamellar growth band was the result of rapid growth and deposition, and thus used the thickness between LAGs 2 and 3 to estimate internally missing LAGs. If Sereno et al.’s argument is true, LH PV18 would have undergone an unusual rapid spurt as it reached maturity, rather than slowing down, and this is highly unusual. The better explanation is that the thin band between LAG 2 and 3 is indicative of unusually slow growth during the early “slow growth” period, and thus may have been indicative of lean/hard times in the year following deposition of LAG 1. Thus Fowler et al. correctly infer that relative thickness of some LAGs need to be seen broadly, rather than narrowly.
Oddly enough, both papers propose the same maximum age of 6yo, while Fowler et al. err towards 3yo at the minimum, while Sereno et al. close in on 6yo. I’m tending to think Fowler et al. is correct on this one.
Third, there’s the issue of morphology. Sereno et al. distinguished LH PV18 from other tyrannosauroids by the following features:
Basal tyrannosauroid with a narrow accessory pneumatic fossa within the antorbital fossa dorsal to the maxillary fenestra, jugal suborbital ramus of particularly narrow depth (transverse width approximately 60% of vertical depth), and absence of a vertical crest on the iliac blade dorsal to the acetabulum.
[Sereno et al., 2009:pg.422:ref.15.]
However, Tsuihiji et al. (2011) revised this, noting in turn:
[T]wo of the characteristics that Sereno et al. (2009) proposed as diagnostic of R. kriegsteini, a shallow suborbital part of the jugal and presence of an accessory pneumatic fossa dorsal to the maxillary fenestra within the antorbital fossa, are also observed in MPC-D 107/7[.]
[Tsuihiji et al., 2011:pg.515.]
This left the issue of the absence of a vertical iliac crest. Fowler et al. note that Pete Larson observed a vertical iliac crest on the ilium, which would in turn remove the last morphological barrier cited by Sereno et al. for distinguishing Raptorex kriegsteini. They also refer to the cranial features noted by Tsuihiji et al. as “two minor skull characters,” and that has me perplexed. They must think that there is relative value to morphology when performing analysis, characters or suites of them that are “better” than others. If we were talking about features that are prone to convergence, or allometric scaling issues, or features relating to ecology that might crop up in different similarly habituated species, then we could, but no such assessment is made, just a casual dismissal (much as scare quotes around the name Raptorex peppers the paper, or the use of specimen numbers followed by “described as ” appellations when dealing with Alioramus altai and Shanshanosaurus huoyanshanensis. (I am actually surprised those names weren’t in scare quotes: Tsuihiji et al. all but nailed the latter as a juvenile Tarbosaurus bataar.)
The ilium deserves some note. If Pete Larson saw what I see in the photos above, then I suspect that while he feels he may be correct in arguing what appears to be a node on the surface of the ilium above the supracetabular crest is a vertical crest. However, that is not what I see; instead, it appears to be a divot of sorts, a defect owing to reconstruction of the material. The photo shows the original material, sans the skull, including clearly-colored plaster additions. This shows the ilium was reconstructed and broken, including around the area in which the “crest” lies. Tsuihiji et al. (2011) appear to affirm the lack of existence of this “crest,” so the identification seems ambiguous.
Tsuihiji et al. further note that while they think the material warrants further scrutiny on its age (which Fowler et al. appear to nail), some peculiarities prevent them from lumping the species in with Tarbosaurus bataar. This includes a large surangular foramen (see below), and the much taller anteroventral process of the vertical ramus of the lachrymal, nearly fully 1/3 the lachrymal height (as in Alioramus altai, below). A further differentiation is that unlike other juvenile tyrannosaurids, the maxillary fenestra is nearly as tall as it is long, and the promaxillary fenestra is obscured by the posterior extent of the anterior margin of the external antorbital fenestra; the former is quite usually much longer than tall, and the latter visible in smaller (IVPP V4878), equally-sized (MPC-D 107/7) and larger (MPC-D 100/1844) specimens. Partly due to this, as to uncertaintly regarding its age, Tsuihiji et al. do not think that LH PV18 is a juvenile Tarbosaurus bataar, although it may be a juvenile of another taxon.
Fowler et al. (2011) demonstrate with a strong series of arguments that LH PV18 is truly, actually a juvenile, rebutting Sereno et al. (2009)’s conclusion to the contrary. However, they seem more willing to also conclude that Raptorex kriegsteini is likely a juvenile with “affinities” to Tarbosaurus, despite the lack of any relation to Nemegt-equivalent beds in northeastern China. If Sereno et al. (2009) are correct (and they may not be) that LH PV18 derives from the Lower Cretaceous, this would extend what is essentially a Cenomanian-Maastrichtian clade clear through to the Barremian, and while such ghost lineages are not unexpected or necessarily wrong, they bear greater scrutiny for their extremes.
Despite this, Fowler et al. follow Tsuihiji et al. (2011) in their treatment of the specimen, albeit more along the lines of Tarbosaurus-y, but miss some of the finer details in the latter work. Certainly, the actual morphology is peculiar and not linear with older specimens attributed to Tarbosaurus bataar from the Nemegt-equivalent beds of China and Mongolia: The maxilla in youngest specimens (e.g., IVPP V4878) is shallow, triangular, and has a straight leading anterodorsal edge; in older specimens, the maxilla becomes more rounded on the anterodorsal edge, the depth of the maxilla below the antorbital fossa increases; tooth positions do not change number despite increasing size (unlike albertosaurine tyrannosaurids), and the maxillary fenestra changes from a shallow oval into a tall sub-triangular shape. In the specimens above, we see at the bottom the maxilla of MPC-D 100/1844, holotype of Alioramus altai (Brusatte et al., 2009), which despite its much larger size, does not resemble the maxillae of subadult tyrannosaurines, and is even relatively shallower than in juvenile tyrannosaurines, while the sub-antorbital depth of the maxilla is shallow, and teeth increase in number. LH PV18 agrees with all of these features as a juvenile Tarbosaurus, but this has me thinking that instead, it’s based more parsimoniously on juvenile tyrannosaurines. We simply lack the comparative material for Tyrannosaurus rex, and all this dismissal of taxa makes for applying relative juveniles of different taxa rather difficult.
I will post shortly on what I actually think of the taxonomic issues involved in the recent spate of juvenile-based tyrannosaur species being named of late. I’ve refrained in large part out of respect, though to be fair in my recent criticisms of some work, I will not hold back on others’. Does this mean I disagree with Sereno et al. on the subject of Raptorex, and agree with Fowler et al.? Well, yes and no.
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