The Arboretum also hosts two Taxodiaceae that may be considered, like ginkgo and dawn redwood, two living fossils: California redwood [Sequoia sempervirens (D. Don) Endl.] and big tree [Sequoiadendron giganteum (Lindl.) Buchholz].

California redwood produces a valuable reddish timber and is a typical species in woodland mixed with deciduous broadleaf trees of the western coast of the United States, from California to Oregon, where it is protected from excessively cold temperatures by the nearby ocean and is provided with water from fog, which is frequent even in dry summers. It forms spectacular woods, many of which are protected (National Park north of San Francisco), and is particularly impressive when its trees grow in dense, columnar shapes, like true vegetal monuments, the tops of which soar up in the sky. It is a long-lived species (the oldest tree is over 2200 years old) and, together with eucalyptus, may grow to the maximum height of the vegetal kingdom (110-115, up to 137 metres).

Big trees form gigantic woods in parks (Yosemite National Park and Sequoia National Park) in the western Sierra Nevada in California. They are very tall and slightly less long-lived than redwood (although many specimens exceed 1500 years in age). They are never as tall as redwoods (maximum 100 metres), but their buttressed trunks may have a circumference of 12 metres.

The two species are clearly different, owing to the characteristics of their leaves. Redwoods have linear, flat leaves, yellowish-green on the underside, and arranged in two rows. In big trees, leaves are arranged in three rows and are smaller, oval and blue-green, turning to brown after two or three years. Female cones in redwoods have a small number of scales, are erect and grow each year, followed by seed dispersion. Big trees, instead, have larger cones that mature every two years (they start hanging from the second year, but do not grow any larger) and, unlike any other conifer, their cones never open to release the seeds, which may be preserved for even 20 years before germinating.

In the Arboretum, near and outside the circular wall, between the south and west gates, is an ultra-centenarian redwood, easily identified by the two large cables supporting it: in the late 1900s, it was struck by lightning twice, which reduced its stability and damaged a large portion of its canopy. However, it is still quite tall and keeps its towering shape, because its branches are not very long and the canopy is therefore dense. The Arboretum also houses one specimen of big tree, between the south and east gates. It is young, and not very large yet, but its presence is important for teaching purposes.