The plant material may fill in information gaps about life during a transitional period between the mass extinction of the late Triassic period and the rise of dinosaurs as a dominant species on the landscape, he said.
"We're really excited and we've got institutions from all over the country interested in material from here," Kirkland said in a telephone interview from St. George.
About 15 volunteers were at the site where excavation began last week to clear the way for an office complex with restaurants, shops and office space. The spot is in a bare lot near the Virgin River, not far from the city's Dinosaur Discovery Site at Johnson Farm, where dinosaur tracks were found eight years ago.
Andrew Milner, the city's paleontologist, said the property's developers have agreed to excavate the privately owned land slowly so crews have time to pick through the dirt in search of hidden fossils.
"We've collected about 150 specimens in the last few days," Milner said.
The first plants in the area were found in 2002 when dirt was peeled away to make way for large retail stores. A 2006 study identified them as conifers, ferns and horsetails, which are slender hollow-stemmed plants.
Kirkland said he's been struck by how many conifer remnants there are, including seeds and some hardened branches with cones still attached.
Milner said different plants from the early Jurassic have been found elsewhere, including along the East Coast.
The fossils are tantalizing clues about what life may have been like near the early Jurassic lake known as Lake Dixie, which once covered stretches of what is now southwestern Utah.
"We really hit the jackpot in finding this plant site," Milner said.
Researchers are still trying to understand what happened there after cataclysmic extinctions that wiped out scores of plants, reptiles, insects and amphibians.
Species that returned in the early Jurassic had to eat something, and plants were likely part of that diet, Kirkland said. It's still unclear how the plants being excavated in St. George fit into that puzzle.
"We've got a lot to learn," Kirkland said.
With the topsoil peeled back, crews have been going through the dirt by hand, looking for hard-to-find fossils and cutting out slabs with good material.
"You've got to really look hard to spot these things," Milner said.
Two plant slabs will be preserved and presented at the Dinosaur Discovery Site, he said.
Specimens have been requested by researchers elsewhere, including those at the University of Utah, Brigham Young University, the Smithsonian and the Museum of Natural History in New York.