Nilssoniocladus in the Cretaceous Arctic: new species and biological insights

Abstract

Nilssoniocladus (Kimura and Sekido, 1975) is an important Mesozoic plant because it is widely regarded as representing a deciduous (leaf shedding), frost-resistant cycad. It has been used to argue that evergreenness was not obligatory in Mesozoic cycads and therefore reduced obliquity need not be invoked to explain the observed presence of cycads at high latitudes in the Mesozoic. However, until now the presence of Nilssoniocladus at high latitudes was conjectural. In this paper two new species of Nilssoniocladus are described, one from the latest Albian (?) Lower Killik Tongue of Northern Alaska (palaeolatitude 75°N) and one from the latest Albian-earliest Cenomanian Krivorechenskaya Formation of northeastern Russia (palaeolatitude 73°N) that demonstrate evidence for synchronous leaf shedding (deciduousness) and short shoot abscission. The Alaskan specimens exhibit thin woody stems bearing short shoots with persistent leaf scars and circular scars interpreted as short shoot abscission points. The specimens are associated with mats of Nilssonia leaves. The Russian example of Nilssoniocladus shows evidence of short shoot shedding with the leaves remaining attached to the shed short shoots. The two new species differ from the Japanese material in terms of leaf and leaf scar morphology. Short shoot scars are described for this plant for the first time. Nilssonia leaves are widespread in middle and high northern latitudes throughout the Jurassic and Cretaceous and many of these leaves were produced by the plants represented by the genus Nilssoniocladus. The genus was quite diverse, and comprised a minor, but often locally abundant to subdominant, component of the now extinct deciduous polar forests of the Mesozoic “greenhouse” world.