The U.S. Fish and Wildlife Service (Service), has determined that the listing of the flat-tailed horned lizard (Phrynosoma mcallii) as a threatened species under the Endangered Species Act of 1973, as amended (Act), is not warranted, and therefore withdraw the November 29, 1993, proposed rule to list it under the Act. They made this determination in this withdrawal because threats to the species as identified in the 1993 proposed rule are not as significant as earlier believed, and available data do not indicate that the threats to the species and its habitat, as analyzed under the five listing factors described in section 4(a)(1) of the Act, are likely to endanger the species in the foreseeable future throughout all or a significant portion of its range.
The withdrawal is effective as of March 15, 2011.
The flat-tailed horned lizard (Phrynosoma mcallii) is a small, spiny lizard found in the Sonoran Desert of the southwestern United States and northwestern Mexico. All of the species of lizards in the genus Phrynosoma--the horned lizards--have dorso-ventrally flattened, "pancake-like" bodies; spiny scales; head spines or "horns''; cryptic coloration; and certain similar behavioral traits.
Among horned lizard species, the flat-tailed horned lizard has particularly long and sharp. Other characteristics that help distinguish flat-tailed horned lizards from other members of the genus include a dark line down the middle of the back (vertebral stripe), lack of external ear openings, two rows of fringe scales, an unspotted vent, and--as indicated by its common name--a long, broad, flattened tail. The flat-tailed horned lizard is average in size when compared to other horned lizard species. Flat-tailed horned lizards become adults when about 60 to 64 millimeters (mm) (2.4 to 2.5 inches (in)) long, not including the tail (snout-to-vent length), and may grow to be about 87 mm (3.4 in) long. The dorsal coloration of flat-tailed horned lizards varies and closely matches the colors of the desert soils on which they live, ranging from pale gray to light rust-brown, while their ventral coloration is white or cream-colored. First described by Hallowell in 1852, no subspecies have been described or are recognized for the flat-tailed horned lizard.
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