Stegosaur Diets and Morphospace Exploration

In 2017, Romano wrote that Stegosauria saturated morphospace in the Jurassic. In his view, the stegosaur cranium explored the most morphospace in the Oxfordian. The postcrania did this in the next stage, the Kimmeridgian. Due to the paucity of Cretaceous stegosaur remains, Romano dumped all known material (or Cretaceous taxa) into a single time bin, even though it spanned 5-6 stages. He concluded morphospace occupation in Stegosauria was temporally symmetrical, peaking in the middle of stegosaur history (late Jurassic). This is in contrast to other, "bottom heavy" clades, experiencing saturation early in their history, or "top heavy" ones, with saturation near the end. 

Careful research challenges the claim of a "middle heavy" history for Stegosauria. Sparse as the early Cretaceous record is, it argues for significant evolutionary changes hence continued morphospace exploration after the Jurassic. Evidence from South Africa, China, Spain, Mongolia and Russia supports this view.

 Paranthodon africanus, from the Hauterivian of South Africa,  has three unusual features setting it apart from Jurassic relatives:

1. A relatively large cingulum.

2. A secondary palate, unknown in any Jurassic stegosaur.

3. A large extension of the posterior premaxillary process, which is longer and deeper than in predecessors.

A large cingulum protected the gums from hard food.  The secondary palate prevented tough plant matter from blocking the breathing pathways during mastication. The premaxillary extension resisted increased stress incurred while plucking or biting off tough vegetation.

Evidence for harder plant food, requiring evolutionary responses, is not limited to South Africa. A recent study of stegosaur teeth from Yakutia, Russia, reveals extensive wear and rapid rates of tooth replacement. By the Hauterivian, the teeth of plated taxa lasted only 95 days whereas in other dinosaurs they endured for 200 or more. The authors suspect local stegosaurs consumed conifer stems and branches--harder food than that of their ancestors. Further evidence for this is the more complex mastication of the Yakutia taxon (back and forth as well as up and down) indicated by wear. Chewing harder food required the evolution of additional musculature, and more area for its attachment. Inevitably, that would've altered the size and shape of the skull. Given the great distance separating South Africa and Yakutia, it is likely that new adaptations,  altering skull morphology, had become nearly universal among EK stegosaurs. Postcranial remains also testify to major changes in the Cretaceous.  

The ilia of Wuerhosaurus homheni diverge more from the midline than those of earlier stegosaurs,  indicating a wider body. The latter could accommodate a more extensive digestive mass capable of processing very fibrous plants.

A stegosaur centrum from the EK of Spain is more massive than that of its Jurassic ancestor Dacentrurus. This also hints at a broader, heavier body providing more support for voluminous intestines. The big Broome (Australia) stegosaur (if real) may have achieved the same result by evolving larger overall size.

Mongolostegus, the late surviving (Aptian-Albian) stegosaur, has relatively massive pubes. Again, this suggests a more massive gut capable of digesting tough vegetation.

Regardless of the functional significance of various features, it appears that morphospace exploration, in Stegosauria, did not cease after the Kimmeridgian. Plated dinosaurs continued to evolve, considerably, after the Jurassic.  Paranthodon and the Yakutia taxon attest to major changes in the cranium while the pelvis and other postcranial elements also underwent major transformations. Stegosauria may have been more "top heavy" than the aforementioned study concludes.

References

Marco Romano (2017) Disparity vs. Diversity in Stegosauria (Dinosauria, Ornithischia) cranial and post-cranial sub-dataset provide different signals. Historical Biology.

Pavel Skutschas et al. 2021 Wear Patterns and dental functioning in an Early Cretaceous stegosaur from Yakutia, Eastern Russia.

Xavier Suberbiola et al. 2003 First stegosaurian dinosaur from the early Cretaceous of Burgos (Spain)  with a review of Cretaceous stegosaurs.  Revista de Espanola de Paleontologia 18 (12) 145-150.

T. A. Tumanova and V.R. Alifanov First record of stegosaur (Dinosauria, Ornithischia) from the Aptian-Albian of Mongolia, Paleontological Journal 52 (14) 1771--1779.

McPhee et al. High Diversity of Sauropod Dinosaur fauna of the lower Cretaceous Kirkwood formation of South Africa: Implication for the Jurassic-Cretaceous transition.