Introducing Daspletosaurus horneri, the Two Medicine Tyrannosaurine: Ontogeny

In our recent publication (Carr et al., 2017), we presented a cladistic growth series for Daspletosaurus horneri in Figure 3. The complete list of growth changes that we recovered are listed, in code form, in the supplementary materials. In the interests of saving you the trouble of scrolling up and down multiple pages at a time, I have put together an illustrated summary of the growth stages here. For convenience, the codes are listed in each heading. 

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.

GROWTH STAGE 1 (exemplar:  MOR 553S/7.19.0.97)

This growth stage is represented by the juvenile paratype dentary. No mature features were recovered for this growth stage since all of its characters were scored as "0". Regardless, this specimen reveals many juvenile features for the dentary, including small body size, as inferred from the length of the tooth row.

GROWTH STAGE  2 (exemplar: AMNH FARB 5477): 1 (1)

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.

GROWTH STAGE 3 (exemplar: MOR 590): 2 (1), 6 (1), 8 (1), 11 (1), 14 (1), 19 (1), 20 (1), 21 (1), 22 (1), 28 (1), 30 (1), 33 (1), 34 (1), 37 (1), 38 (1), 40 (1), 42 (1), 43 (1), 45 (1), 49 (1), 50 (1), 52 (1), 53 (1), 164 (1).

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.

Several features cannot be seen from the side, and including them in the diagram would unnecessarily crowd the image. All of these descriptions pertain to the mature state, which is coded as a "1", the numbers correspond to the character number in the supplementary information for Carr et al. (2017); they include:

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.

GROWTH STAGE 4 (exemplar: MOR 3068): 135 (1), 136 (1), 141 (1), 144 (1), 145 (1), 147 (1), 148 (1), 153 (1), 155 (1), 156 (1), 158 (1), 161 (1).

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.

161. Massive and rostrally extending medial ridge on the caudal process of the splenial.

GROWTH STAGE 5 (exemplar: MOR 1130): 134 (1), 137 (1), 138 (1), 142 (1), 143 (1), 146 (1), 150 (1), 151 (1), 152 (1), 157 (1), 159 (1), 162 (1), 163 (1).

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.

Two characters of the splenial cannot be seen in this view:

162. Bar below the rostral mylohyoid foramen of the splenial is dorsoventrally deep.

163. Fossa along the dorsal margin of the rostral mylohyoid foramen is absent, where the corresponding surface is convex.

Concluding thoughts

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).

References cited

Carr, T. D., D. J. Varricchio, J. C. Sedlmayr, E. M. Roberts, and J. R. Moore. 2017. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports. Doi: 10.1038/srep44942.