Comparative biogeography: innovations and the rise to dominance of the North Pacific biota

Early Oligocene kelp holdfasts and stepwise evolution of the kelp ecosystem in the North Pacific

Kelp forests are highly productive and economically important ecosystems worldwide, especially in the North Pacific Ocean. However, current hypotheses for their evolutionary origins are reliant on a scant fossil record. Here, we report fossil hapteral kelp holdfasts from western Washington State, USA, indicating that kelp has existed in the northeastern Pacific Ocean since the earliest Oligocene. This is consistent with the proposed North Pacific origin of kelp associated with global cooling around the Eocene-Oligocene transition. These fossils also support the hypotheses that a hapteral holdfast, rather than a discoid holdfast, is the ancestral state in complex kelps and suggest that early kelps likely had a flexible rather than a stiff stipe. Early kelps were possibly grazed upon by mammals like desmostylians, but fossil evidence of the complex ecological interactions known from extant kelp forests is lacking. The fossil record further indicates that the present-day, multi-story kelp forest had developed at latest after the mid-Miocene climate optimum. In summary, the fossils signify a stepwise evolution of the kelp ecosystem in the North Pacific, likely enabled by changes in the ocean-climate system.

Evolutionary norm-breaking and extinction in the marine tropics

Evolutionary innovations, defined as character states that transcend clade norms, are often studied in an exclusively phylogenetic context, but their distribution in time and space indicates that geography also influences the evolution of new ecological, morphological, and physiological traits. In an analysis of 99 fossillzable, norm-breaking innovations in tropical marine Neogene molluscan clades that arose uniquely in either (but not in both) the Indo-West Pacific (IWP) or Atlantic-East Pacific (AEP) realms, I show that there are far more innovations in the IWP (79%) than those in the AEP (21%). Most of the innovations are interpretable as defensive or competitive adaptations or as indicators of extreme habitat specialization. Although the innovations arose in taxonomically rich biotas, only 9% are associated with subclades comprising 10 or more species each, indicating that they contributed little to overall taxonomic richness. Compilations of extant species in 30 pantropical molluscan clades show that the IWP accounts for 71% of tropical shallow-water species, implying that the per-species incidence of norm-breaking innovations is higher there than in the AEP. Only 5% of innovations became extinct in the IWP as compared with 38% in the AEP, mirroring a similar difference in the magnitudes of Late Miocene and later taxonomic extinction in the two realms. These data imply that large-scale disruption strongly limits norm-breaking innovation. Opportunities for adaptive innovation are therefore likely to be few in today's heavily overexploited and disturbed biosphere.

History's legacy: Why future progress in ecology demands a view of the past

History has profoundly affected the composition, distribution, and abundances of species in contemporary ecosystems. A full understanding of how ecosystems work and change must therefore take history into account. We offer four well-studied examples illustrating how a knowledge of history has strengthened interpretations of modern systems: the development of molluscan antipredatory defenses in relation to shell-breaking predators; the North Pacific kelp ecosystem with sea otters, smaller predators, sea urchins, and large herbivores; estuarine ecosystems affected by the decline in oysters and other suspension feeders; and the legacy of extinct large herbivores and frugivores in tropical American forests. Many current ecological problems would greatly benefit from a historical perspective. We highlight four of these: soil depletion and tree stunting in forests related to the disappearance of large consumers; the spread of anoxic dead zones in the ocean, which we argue could be mitigated by restoring predator and suspension-feeding guilds; ocean acidification, which would be alleviated by more nutrient recycling by consumers in the aerobic ecosystem; and the relation between species diversity and keystone predators, a foundational concept that is complicated by simplified trophic relationships in modern ecosystems.

Oldest record of monk seals from the North Pacific and biogeographic implications

True seals (crown Phocidae) originated during the late Oligocene-early Miocene (approx. 27-20 Ma) in the North Atlantic/Mediterranean region, with later (middle Miocene, approx. 16-11 Ma) dispersal events to the South Atlantic and South Pacific. Contrasting with other pinnipeds, the fossil record of phocids from the North Pacific region is scarce and restricted to the Pleistocene. Here we present the oldest fossil record of crown phocids, monachines (monk seals), from the North Pacific region. The specimens were collected from the upper Monterey Formation in Southern California and are dated to 8.5-7.1 Ma, predating the previously oldest known record by at least 7 Ma. This record provides new insights into the early biogeographic history of phocids in the North Pacific and is consistent with a northward dispersal of monk seals (monachines), which has been recognized for other groups of marine mammals. Alternatively, this finding may correspond with a westward dispersal through the Central American Seaway of some ancestor of the Hawaiian monk seal. This record increases the taxonomic richness of the Monterey pinniped assemblage to five taxa, making it a fairly diverse fossil assemblage, but also constitutes the oldest record of sympatry among all three extant pinniped crown clades.