Alioramus Kurzanov, 1976
A. remotus Kurzanov, 1976
= Alioramus altai Brusatte, Carr, Erickson, Bever and Norell, 2009
Maastrichtian, Late Cretaceous
Nogonn Tsav Beds, Mongolia
Holotype- (GI 3141/1) (juvenile) incomplete skull (~700 mm), mandible, four cervical vertebrae, partial tibia, proximal fibula, pedal ungual I, distal metatarsal II, phalanx II-1, pedal ungual II, distal metatarsal III, phalanx III-1, pedal ungual III, distal metatarsal IV, phalanx IV-1, pedal ungual IV
Early Maastrichtian, Late Cretaceous
Nemegt Formation, Mongolia
Referred- (IGM 100/1844; holotype of Alioramus altai) (9 year old juvenile; 369 kg) incomplete skull (~635 mm), mandible, atlas, axis (36 mm), third cervical vertebra (42 mm), fourth cervical vertebra (42 mm), fifth cervical vertebra (65 mm), sixth cervical vertebra (75 mm), seventh cervical vertebra (57 mm), eighth cervical vertebra (60 mm), ninth cervical vertebra (67 mm), tenth cervical vertebra (51 mm), at least eight cervical ribs, fifth dorsal vertebra (55 mm), eighth dorsal vertebra, tenth dorsal vertebra (55 mm), thirteenth dorsal vertebra, dorsal rib, sacrum (?,?,75,79,97 mm), proximal caudal vertebra (87 mm), distal caudal vertebra (82 mm), distal caudal vertebra (84 mm), mid chevron, incomplete ilium, ischium (430 mm), femur (560 mm), distal tibia, distal fibula, proximal tarsal, metatarsal I, phalanx I-1, pedal ungual I, metatarsal II, phalanx II-1, phalanx II-2, pedal ungual II, metatarsal III, phalanx III-1, phalanx III-2, phalanx III-3, pedal ungual III, metatarsal IV, phalanx IV-1, phalanx IV-2, phalanx IV-3, phalanx IV-4, pedal ungual IV, metatarsal V
Diagnosis- (after Kurzanov, 1976) 16-17 maxillary teeth (ontogenetic?); 18-20 dentary teeth (ontogenetic?).
(after Brusatte et al., 2009 for A. altai) accessory pneumatic fenestra posterodorsal to promaxillary fenestra of maxilla (ontogenetic?); maxillary fenestra enlarged and 1.9 times longer than deep; laterally projecting jugal horn; thick ridge on dorsal surface of the ectopterygoid; anteroposteriorly elongate anterior mylohyoid foramen of splenial; thin epipophysis on atlantal neurapophysis that terminates at a sharp point; pneumatic pocket on anterior surface of cervical transverse processes (ontogenetic?); external pneumatic foramina on dorsal ribs (ontogenetic?); anterodorsally inclined midline ridge on the lateral surface of the ilium.
Other diagnoses- Kurzanov (1976) listed many additional characters, most of which are probably due to the Alioramus type's juvenile age- 'average' size; low skull; elongate snout; series of prominent nasal rugosities; small postorbital boss; labiolingually compressed teeth. Two rows of maxillary nutrient foramina are present in most tyrannosaurids (Currie, 2003), as are the laterosphenoid contacts noted by Kurzanov (forms part of the supratemporal cavity and contacts the postorbital). Currie also noted the trigeminal foramen near certainly contacted the laterosphenoid as opposed to being completely contained by the prootic. While he defended the prominence of the nasal rugostities as potentially diagnostic, they are lower in IGM 100/1844.
Brusatte et al. (2009) stated several characters united the Alioramus specimens in their analysis, most being previously used by Kurzanov except for the long posterior squamosal process. Yet Carr (2005) notes that juvenile Tyrannosaurus have long processes, making this potentially ontogenetic. Among characters listed in the diagnosis for A. altai which are unknown in the A. remotus holotype, the palatine pneumatic recess extends posteriorly beyond the posterior margin of the vomeropterygoid process in juvenile Daspletosaurus and Tyrannosaurus more than in adults.
Comments- Currie et al. (2003) found Alioramus to be the sister taxon of Tarbosaurus because they both lack a lacrimal process on the nasal, though this is present in Daspletosaurus as well. In addition, Hurum and Sabath (2003) note Alioramus and Tarbosaurus share a dentary-angular interlocking mechanism which makes the mandible rigid. Currie (2003) suggested the specimen could be a juvenile Tarbosaurus based on skull proportions and juvenile characters. He stated the prominent nasal rugosities and high tooth count argue against this, but juvenile Tyrannosaurus have high tooth counts and some juvenile Daspletosaurus and Tarbosaurus have rows of nasal rugosities, albeit lower ones as in the A. altai holotype. Holtz (2004) recovered Alioramus in two possible positions- just basal to Tyrannosauridae or sister to Tarbosaurus + Tyrannosaurus. The former position is due to the high tooth count, low snout and slender dentary, which are all possible juvenile characters. The latter position was due to the thick parietal nuchal crest, reduced basal tubera, and posteroventrally directed occipital region. Carr (2005) recovered Alioramus in an uncertain position basal to Tyrannosauridae, but this could be due to juvenile characters. However, the evidence cannot be examined as characters excluding the taxon from Tyrannosauridae were not given, nor was Alioramus included in the printed data matrix. Brusatte et al. (2009) found Alioramus to be a basal tyrannosaurine using an updated version of Carr's matrix, but importantly coded it as if it were an adult when both morphology and histology show known specimens are juveniles. Thus its position is suspect, as similarly aged Tyrannosaurus individuals also emerge as basal tyrannosaurines if run in a similar matrix (Carr, 2005). IGM 100/1844 also provides further evidence for a relationship to Tarbosaurus, as it has a subcutaneous flange on the maxilla and a deep pneumatic fossa on the dorsal surface of the posterior centrodiapophyseal lamina, both otherwise only known in that genus. However, they also noted additional characters which differ between Alioramus and Tarbosaurus of the same size (ZPAL MgD-I/29, 31 and 175)- shallow maxilla and dentary; maxilla less convex ventrally; smaller postorbital boss; postorbital lacks an orbital process; more dentary teeth; muscular fossa above surangular foramen faces mostly dorsally; laterally projecting jugal horn; deep pocket behind surangular fenestra; fibular facet of tibia faces strongly laterally; lateral malleolus of tibia projects less distolaterally. The first six characters are typical of juveniles and could potentially indicate Alioramus individuals are larger at a younger age than ZPAL MgD-I/29 and 31, or that different individuals acquire adult features at different ages. The jugal horn and surangular pocket are ornamental and pneumatic features respectively, which show a high amount of individual variation. Brusatte et al. even state that an ontogenetic decrease in pneumaticity is known in theropods and that Tarbosaurus itself is known to lose pneumatic vertebral features with age, potentially explaining the surangular pocket and some of A. altai's supposed diagnostic features (see diagnosis above). Ontogenetic variation in tyrannosaurid tibiae has not been examined yet. Whle Brusatte et al. claimed that ornamentation increases in ontogeny in dinosaurs, this is not always the case as shown by juvenile tyrannosaurines with larger nasal rugosities and the newly discovered ontogenetic changes in Triceratops (= Torosaurus) and Pachycephalosaurus (= Stygimoloch and Dracorex). The fact most differences could be explained by ontogeny, coupled with the unique similarities present in Alioramus and the contemporaneous Tarbosaurus strongly suggest the former is a juvenile of the latter. The alternative presented by Brusatte et al., where a distinct genus known only from juvenile specimens and based on characters largely found in juvenile tyrannosaurs is contemporaneous with a taxon it is not the sister group of yet shares autapomorphies with, is considered unlikely.
Brusatte et al. (2009) erected a new species Alioramus altai based on a partial skeleton discovered in 2001 from the contemporaneous Nemegt Formation. However, the listed diagnostic characters are problematic. Most are not determinable in A. remotus (accessory pneumatic fenestra posterodorsal to promaxillary fenestra of maxilla; maxillary fenestra enlarged and 1.9 times longer than deep; thick ridge on dorsal surface of the ectopterygoid; palatine pneumatic recess extending posteriorly beyond posterior margin of vomeropterygoid process; thin epipophysis on atlantal neurapophysis that terminates at a sharp point; external pneumatic foramina on dorsal ribs; anterodorsally inclined midline ridge on the lateral surface of the ilium) or potentially determinable but unreported (anteroposteriorly elongate anterior mylohyoid foramen of splenial; pneumatic pocket on anterior surface of cervical transverse processes). The laterally projecting jugal horn was also coded as present in A. remotus. Having 20 dentary teeth instead of 18 is within the range of variation in other tyrannosaurid species. The subcutaneous flange on the maxilla is known to vary in Tarbosaurus. The authors themselves note in the supplementary information that some of the characters they list as distinguishing A. altai from A. remotus vary within other tyrannosaurid species- anterior process of quadratojugal terminates posterior to anterior margin of lateral temporal fenestra; squamosal anterior process extends anterior to anterior margin of lateral temporal fenestra; epipterygoid not bifurcated ventrally. The number and prominence of nasal rugosities is highly variable in tyrannosaurids, so A. remotus having six large rugosities is not significant compared to A. altai's three low ones. Finally, Brusatte et al. list three characters which are size-related in other tyrannosaurid taxa- 17 maxillary teeth instead of 16; single dorsoventral groove between basal tubera; tapering anterior process of the parietals overlapping frontals on the midline. They considered these potentially diagnostic since the holotypes are similar in size, but at least the maxillary tooth count is variable in similar-sized specimens. Here the other two characters are considered individual variation as well.
References- Kurzanov, 1976. A new Late Cretaceous carnosaur from Nogon-Tsav Mongolia. Sovmestnaa Sovetsko-Mongolskaa Paleontologiceskaa Ekspeditcia, Trudy. 3, 93-104.
Currie, 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica. 48(2), 191-226.
Currie, Hurum and Sabath, 2003. Skull structure and evolution in tyrannosaurid dinosaurs. Acta Palaeontologica Polonica. 48(2), 227-234.
Hurum and Sabath, 2003. Giant theropod dinosaurs from Asia and North America: Skulls of Tarbosaurus bataar and Tyrannosaurus rex compared. Acta Palaeontologica Polonica. 48(2), 161-190.
Holtz, 2004. Tyrannosauroidea. In Weishampel, Dodson and Osmolska (eds). The Dinosauria Second Edition. University of California Press. 861 pp.
Carr, 2005. Phylogeny of Tyrannosauroidea (Dinosauria: Coelurosauria) with special reference to North American forms. Unpublished PhD dissertation. University of Toronto. 1170 pp.
Brusatte, Carr, Erickson, Bever and Norell, 2009. A long-snouted, multihorned tyrannosaurid from the Late Cretaceous of Mongolia. Proceedings of the National Academy of Sciences. 106(41), 17261-17266.
I would have figured at least some reasponse to this by know. I'd certainly think that trying to sink genera like this would raise someone's hackles.ReplyDelete
One must be cautious when relying on one ontogenetic series to explain others to the same degree, especially when there is contravening data (in this case, the apparent juvenile Tarbosaurus bataar maxilla that differs from that in either Alioramus remotus or Alioramus altai. Even when there is variation, much of this is limited, or the extent of the prevalence of variation is unknown. While it might exist, and is useful when being cautious, that it exists does not cause taxonomy to become invalid.
Well, the juvenile Tarbosaurus specimens that are the same size but seem older, and the fact Nanotyrannus isn't quite certainly a juvenile Tyrannosaurus led me to keep Alioramus as a provisionally valid genus. The undescribed complete juvenile Tarbosaurus and Jane should help clear these matters up.ReplyDelete