Wednesday, June 18, 2008

Albertaceratops nesmoi MK II

Albertaceratops nesmoi MK II! It looks a lot better than my previous one, which is way too embarassing to link to now... You may have noticed but I am having fun with all these ceratopsians. It seems like I'm stuck in a ceratopsian phase right now...

I can't really comment much on Albertaceratops other than what's already been covered elsewhere. I don't think there's really anything done with this dinosaur other than the original description. But then again, what is there to do with a dinosaur known from a single skull and some fragments? Ah - someone should do functional morphology, like FEA - provided they have access to a huge scanner, that is... Not just in Albertaceratops but someone should look into stress distribution patterns in ceratopsian crania using FEA. Better yet, someone should look at pachycephalosaurs...

Anyway, I think it's the new paper but the pencil just came out too dark...and it started getting smudged so I couldn't really add any more to this. I am not liking this new paper at all...

Sunday, June 15, 2008

Triceratops vs tyrannosaur

This is my newest addition to the practice series. This time, it's not just a single dinosaur, but two dinosaurs interacting! Wow! It's incomplete as you can tell immediately...I might flesh them out and make them look nicer if I get around to it...

Anyway, its supposed to be a Triceratops flicking away a young tyrannosaur. In doing so, the Triceratops is almost rearing up...I initially wanted to draw the Triceratops standing firmly with both its forelimbs contacting the ground but ended up with this construct. So I guess the Triceratops locked its horns onto the tyrannosaur then reared upwards and flicked its head sideways, essentially pushing the tyrannosaur up off its feet then tossing it aside.

I doubt this would have happened between adult individuals. This tyrannosaur is probably inexperienced and got too close the Triceratops. Or perhaps it was starving and was willing to take the risk...who knows, perhaps we'll never know....

Centrosaurus apertus - initially Monoclonius crassus

My attempt at another ceratopsian; this time, Monoclonius crassus - but I realised after reading up on this genus that my drawing is probably more like Centrosaurus. Damn! I really wanted to draw Monoclonius but I guess the picture I found on the Wikipedia that was labeled "Monoclonius" is probably a Centrosaurus...

According to Sampson et al. (1997), "Monoclonius specimens are generally defined on the presence of a thin, scalloped parietal and on the absence of hooks, spikes and horns seen on the posterior transverse ramus of other genera". The only complete skull specimen of Monoclonius, a specimen previously attributed to M. lowei, has a short nasal horn core and a pair of low rounded supraorbital horns (Sampson et al. 1997).

The absence of elaborate cranial ornamentations in Monoclonius and its occurrences in slightly older strata can be indicative of a primitive condition in Monoclonius. On the other hand, these features are also commonly associated with juvenile and subadult centrosaurine specimens, indicating a possible paedomorphosis in Monoclonius. However, it seems more likely that Monoclonius is based on subadult specimens rather than being adults with primitive/juvenile characteristics. Sampson et al. (1997) regard Monoclonius as a numen dubium because 1, diagnostic characters of Monoclonius are present in subadults of other centrosaurines; 2, suture closure and bone surface texture supports subadult status of Monoclonius specimens; and 3, Monoclonius-type cranial elements have been found in bonebeds of other centrosaurines spanning a large time period.

The third point is the most convincing argument against the validity of Monoclonius. Monoclonius-type parietals have been found in association with Centrosaurus, Einiosaurus, and Pachyrhinosaurus bonebeds in Montana and Alberta. These are indistinguishable to isolated parietals usually assigned to Monoclonius. Perhaps, Monoclonius lived alongside other centrosaurines in several different locations over a large time period. Sampson et al. (1997) however think this unlikely and proposes that Monoclonius is likely made up of subadult specimens of Styracosaurus, Centrosaurus, Einiosaurus, and/or Achelousaurus and that all centrosaurines go through a 'Monoclonius stage' through ontogeny (Sampson et al. 1997).

On the other hand, Dodson (1990) argued that the type specimen of Monoclonius is diagnostic and that this genus is valid based on biometric results. Dodson et al. (2004 in The Dinosauria, 2nd edition) also seem to follow this argument as they treat Monoclonius as a valid monotypic genus with the complete skull, M. lowei, included in the type species. Though not discussed in detail, Dodson et al. (2004) citing Tumarkin and Dodson (1998) mention the possibility of paedomorphosis in Monoclonius based on the large adult-sized 'M. lowei' - i.e. retaining juvenile characteristics while attaining adult size.

As much as I love the name Monoclonius, it seems more study is in need in order to either invalidate or validate the genus. As Dodson et al. (2004) mention, aging techniques may perhaps provide more evidences. Bone histology comes to mind...

References:

Dodson, P. 1990. On the status of the ceratopsids Monoclonius and Centrosaurus. In: Carpenter, K. and Currie, P. J. (eds.). Dinosaur Systematics: Approaches and Perspectives. Cambridge University Press, Cambridge. Pp. 231-243.

Dodson, P., Forster, C. A., and Sampson, S. D. 2004. Ceratopsidae. In: Weishampel, D. B., Dodson, P., and Osmóslka, H. (eds.). The Dinosauria (Second Edition). California University Press, London. Pp. 494-513.

Sampson, S. D., Ryan, M. J., and Tanke, D. H. 1997. Craniofacial ontogeny in centrosaurine dinosaurs (Ornithischia: Ceratopsidae): taxonomic and behavioral implications. Zoological Journal of the Linnean Society, 121: 293–337.

Tumarkin, A. R. and Dodson, P. 1998. A heterochronic analysis of enigmatic ceratopsids. Journal of Vertebrate Paleontology, 18(suppl.): 83A.

Monolophosaurus jiangi

Monolophosaurus jiangi is a theropod from the Middle Jurassic of Xinjiang, China. It's phylogenetic position is rather uncertain - though there will be a monograph coming out soon but I can't remember if there was a phylogenetic analysis associated with this redescription.

Of course the most distinctive feature of Monolophosaurus is the large sagittal crest on the skull. The crest is quite thin and has several openings so is probably not a functional structure.

There was some speculation that the basal tyrannosauroid Guanlong wucaii, also from Xinjiang, represents a juvenile morphology of Monolophosaurus. However intriguing this claim may seem, at least one Guanlong specimen shows signs of arrested growth histologically so is quite likely to be an adult.

Here, you see a fine specimen of Monolophosaurus scoping out a carcass.

Tuesday, June 10, 2008

Thecodontosaurus antiquus

This is my attempt at a reconstruction of Thecodontosaurus antiquus. No one really knows what the skull looked like except for a juvenile of a closely-related southern Welsh species Pantydraco caducus (formerly known as Thecodontosaurus caducus) so I pretty much made it up - loosely basing it on Benton et al. (2000). The interesting thing about Theco is that there are so many postcranial jumbled together that no one has any idea what the forelimb to hindlimb ratio is. I attempted a lower fore-hind limb ratio - so as to make it look like Theco is actually not obligatory bipedally nor quadrupedally, basically facultative.

Thecodontosaurus was discovered in 1834 at the Durdham Down in Clifton, Bristol, UK and is the fourth dinosaur to be named. It is also the oldest dinosaur from Britain at 203-215 million years old. The original Clifton materials were destroyed in the second world war when a bomb hit the Bristol City Museum. However, some elements, including a braincase, had fortunately escaped destruction as they had been taken back to the United States by Othniel Marsh as part of a collections exchange with the Yale University Museum in the late 19th century.

Most of the more recent studies on Theco have been conducted on materials collected from a cave deposit in a quarry in Tytherington, just north of Bristol, in the 1970s. Over the course of the next few decades, numerous specimens (at least from 30 or so individuals) were prepared out, but the University of Bristol has just under 6 tons of bone-bearing rocks still awaiting preparation.

Recent work suggests that Bristol and its surrounding areas, including south Wales where Pantydraco lived, were a series of tropical islands - in which case, Pantydraco may just be another island species of Thecodontosaurus.

Allosaurus fragilis 3

So it has been a while...and for my rehabilitation, I give you, tah-dah... Allosaurus again! Well, it could be any non-descript tetanuran theropod...from this angle anyway. Again, this is one of my series(?) of practice drawings, and for that, who else but Allosaurus? I have attempted this angle before but never as "good" as this one turned out to be - at least I think it looks good enough.


Anyway, I had a bit of a problem with my scanner when trying to upload this - it always came up with some weird faint line across the middle. I tested all sorts of different papers and book covers to see what the cause was. It came out fine on really strong colours and backgrounds with multiple colours so I determined it was something to do with white so I thought it might need some adjustments or something. And I found the callibration button! Woohoo! Such a simple thing and it takes a postgrad student nearly half an hour to figure out...