Reasons for this include:

1. The fact that herbivory is an ecologically broad tent, includes a wide range of specialized diets
2. Complex mapping of form to function: “...a complex interplay of one-to-many, many-to-one, and many-to-many relationships taking place between ecology, biomechanics, and morphology” (Tamagnini et al 2021; Zelditch et al 2017).
3. Variation between species of developmental constraints that limit evolutionary possibilities  

Morphological changes needed to herbivorize could to some degree be idiosyncratic to each species. This is consistent with the work of Radinsky (1981a, b).

“...independent evolution of similar diets can either produce convergence but can also, and probably more commonly among carnivorans, push species toward different directions in the craniomandibular multivariate trait space (Boessenecker 2012, 2017; Timm-Davis et al. 2015; Radinsky 1981b)” (Tamagnini et al 2021).

“We used multiple pattern-based metrics designed for de-tecting the occurrence of retained and/or evolved similarity, each with a slightly different biological meaning. This allowed us not only to test for the presence of convergence, but also to distinguish between episodes of convergent evolution and conservatism, that are the most common processes leading to trait similarity (Moen et al. 2013). Our results support the exis- tence of a complex relationship taking place between ecology, biomechanics, and morphology that makes convergent evolu- tion a rare phenomenon. Ecological equivalence was the only condition that produced convergence in more than one occasion for carnivorans. Further studies about the interaction between ecological equivalence and convergence could be extremely interesting for further clarifying the mechanisms leading to a condition of evolved trait similarity. Increasing the number of studies that disprove the occurrence of convergent evolution in a specific clade, like ours, is a pivotal need in evolutionary biology and might ease the quest for previously unknown episodes of convergence” (Tamagnini et al 2021).

Tamagnini, D., C. Meloro, P. Raia, and L. Maiorano. 2021. Testing the occurrence of convergence in the craniomandibular shape evolution of living carnivorans. Evolution. Forthcoming.  

Radinsky, L. B. 1981a. Evolution of skull shape in carnivores: 1. Representa- tive modern carnivores. Biol. J. Linn. Soc. 15:369–388.

Radinsky, L.B. 1981b. Evolution of skull shape in carnivores: 2. Additional modern carnivores. Biol. J. Linn. Soc. 16:337–355.

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