Friday, July 14, 2017
Monday, April 3, 2017
The following convention is followed for the labels in all of the images: a solid leader line extends to a feature that can be seen in lateral view, whereas a dashed line points toward a feature that is out of the plane of view. At this stage, all of the images that I can offer are in lateral view. Therefore, some features are simply not labeled, but they are listed following the illustration. Be forewarned that some features, although they are labeled, may not be clearly seen or not seen at all; i.e., some labels indicate simply where to look and I apologize in advance for that limitation.
This growth stage is represented by a juvenile maxilla. One unambiguously optimized feature was recovered for this growth stage,large size, as given by the length of the tooth row. This maxilla has many features that are representative of the juvenile condition, and they are labeled in the figure below.
This growth stage is represented by the imposing skull and jaws of the type specimen, MOR 590. Given the limitations of the data set, all 24 of the unambiguously optimized features pertain to the maxilla.
40. Base of the medial interfenestral strut is positioned at or behind the midlength of the lateral strut.
42. Caudal antromaxillary fenestra is close to the anteroventral margin of the internal antorbital fenestra.
43. Epiantral recess is deeply excavated.
45. Palatal process of the maxilla is sigmoid in shape.
49. Margin of the choana on the maxilla is positioned caudally, where it extends along the level of alveoli 7-10.
50. Distinct depressions for dentary teeth below the rostral end of the palatal process.
52. Interdental plates are positioned close to the alveolar margin of the bone.
This growth stage is represented by a referred partial mandibular ramus, and all 12 of the unambiguously optimized characters pertain to the lower jaw.
Two additional characters cannot be seen in this view:
156. Dorsoventrally shallow medial bar.
This growth stage is represented by the majestic paratype adult; all 13 unambiguously optimized characters pertain to the mandibular ramus. The limited data set results in only a batch of characters unambiguously optimized for the caudal- and mid-regions of the mandibular ramus.
Although we successfully obtained a growth series for D. horneri, the incomplete nature of the majority of the specimens results in a limited account of the changes across the entire skull from stage to stage. Instead, we obtained a patchy picture of the sequential changes that happened during growth, which will hopefully be filled by future discoveries of complete juveniles.
Also, at this point we did not provide any explanation for the changes that occur; that will be the subject of a future work (by me) that will make comparisons across derived tyrannosauroids (Bistahieversor + Tyrannosauridae).
Saturday, April 1, 2017
In our article (Carr et al., 2017), we argued that Daspletosaurus horneri was the end point of an anagenetic lineage of tyrannosaurines. Anagenesis is the mode (process) of speciation (production of a descendant species from an ancestral species) where an ancestral species, under the action Natural Selection, is modified into a different descendant species.
Both species of Daspletosaurus are from the northern Rocky Mountain Region, from what is now Alberta (D. torosus) and Montana (D. horneri), and so they satisfy the requirement for occurrence in the same land area (at the regional level).
Levels of Inference
- ► April (2)
- ► 2015 (21)