Kelp evolution: a comment

Abstract

The very enlightening article by Estes and Steinberg (1988) presents a model of kelp evolution which leads them to suggest (p. 20) (1) that the kelps had a North Pacific origin; (2) that they radiated extensively within the North Pacific during the late Cenozoic; and (3) that they evolved in an environment in which the intensity of herbivory was low because of the limiting predatory influences of marine mammals that consumed kelp-eating sea urchins. Although this model has many attractive features, I believe that parts of it are open to question. Specifically, I wish to suggest that (a) kelps radiated before (perhaps long before) the Middle Miocene and (b) they were subject to intense marine-mammal herbivory from the Late Oligocene on, at least in shallow, protected waters. However, it is not clear that any marine mammals were really important to the evolutionary history of kelps. The entire area of kelp-marine mammal interactions needs further study. The Antiquity of Kelp. -Estes and Steinberg seem to have in mind a Middle or Late Miocene date for the actual origin of kelps (Laminariales) and a Pliocene date for their diversification. This would appear to account for the single, late Middle Miocene fossil record of a kelp (Julescraneia) as well as the considerably later appearances of the Late Miocene kelp-associated sirenians Dusisiren dewana at 9.0-10.4 Ma and Hydrodamalis at 8 Ma or less (Takahashi et al. 1986) and of Pleistocene kelpassociated limpets. However, I strongly suspect that kelps not only arose but diversified well before the Middle Miocene, for the following reasons. A. Julescraneia is not a generalized ancestral kelp but a derived form well within the Lessoniaceae, ancestral at most to Pelagophycus and Nereocystis, if we accept Parker and Dawson's (1965) interpretation of the admittedly few and unsatisfactory fossils. This implies