Picea critchfieldii

Picea critchfieldii is an extinct species of spruce tree formerly present on the landscape of North America, where it was once widely distributed throughout the southeastern United States. Plant macrofossil evidence reveals that this tree became extinct during the Late Quaternary period of Earth's history. At present, this is the only documented plant extinction from this geologic era. Hypotheses as to what specifically drove the extinction remain unresolved, but rapid and widespread climatic changes coincided with Picea critchfieldii's decline and ultimate extinction.

History and classification Picea critchfieldii was first described by Stephen T. Jackson and Chengyu Weng in a 1999 paper titled, "Late Quaternary Extinction of a Tree Species in Eastern North America" published in the journal Proceedings of the National Academy of Sciences of the United States of America. They coined the specific epithet critchfieldii as a patronym honoring botanist William B. Critchfield. The plant was named to honor Critchfield and his longstanding "advocacy of understanding the role of Quaternary history in shaping genetic structure of conifer populations."

Description To describe Picea critchfieldii as a new and distinct species, carefully analyzed plant macrofossil specimens of fossilized spruce needles and cones were assessed. After close examination, these specimens could not be assigned to any extant species of Picea given distinctive morphological and anatomical features of their needles and cones. Fossil evidence thus supports the former existence of a distinct species of spruce: Picea critchfieldii. Plant macrofossils are fossilized deposits that represent the multicellular sporophyte stage in the plant life cycle. Given significant variability among the sporophyte stage for different plant species, macrofossil specimens can exist as seeds, fruits, ovulate cones, needles, leaves, buds, and a host of other forms. The size of macrofossils and the depositional material in which they are preserved can similarly vary. The sporophyte phase of plants is morphologically distinct between different species, which permits species-level identification of macrofossil specimens and thus provides information about past vegetation with "high taxonomic resolution". To glean species-level data, macrofossil specimens must be carefully studied for distinct morphological and anatonomical features that permit their definitive assignment to a particular species, whether extant or extinct. The specificity and distinctiveness of macrofossil deposits of cones, seeds, and needles were paramount to the identification of the not-previously-known spruce Picea critchfieldii.

Cones Picea critchfieldii had cylindrical ovulate cones with “scales narrowly fan-shaped with rounded margins” that were somewhat irregular. The dimensions of a cone in its entirety varied between approximately 60–100 mm in length and 14–20 mm in diameter. The dimensions of the rounded, fan-shaped cone scales varied between 18–21 mm in length and 11-13.5 mm in width.

Seeds Picea critchfieldii had ovate, winged seeds. The seeds varied in size from about 3.5-4.5 mm in length and 2.6-2.8 mm in width with wings spanning about 8–11 mm in length.

Needles The needles of Picea critchfieldii were between 7–9 mm in length and 0.6-1.0 mm in diameter. These needles had a cross-section that was quadrangular, an acute apex, and two resin ducts.

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