Officials have not been able to find the stone since it was removed Feb. 17. Prosecutors believe Ehlers later dumped the print into the Colorado River but declined to comment on a motive.
His attorney, Tara Isaacson, said her client was interested in the footprint and hoped to keep it.
"He feels terrible. He made a mistake," Isaacson told The Associated Press. "He didn't realize how serious it was."
She said Ehlers had no intention of selling the footprint but declined to offer further details.
After Ehlers entered his plea in U.S. District Court in Salt Lake City, Federal Magistrate Judge Brooke Wells scheduled a four-day trial to start July 7.
Ehlers, who works in construction, did not speak much during the hearing and declined afterward to comment to reporters. He faces up to 45 years in prison on charges of theft of government property, damage to government property, destroying evidence and removal of paleontological resources. The latter charge stems from a federal law passed in 2009 to protect paleontological artifacts and carries a maximum penalty of 20 years.
Assistant U.S. Attorney Richard McKelvie said he didn't believe prosecutors would recommend the maximum sentence in the theft of the footprint that's considered "invaluable."
"We're talking about a footprint that was left on the Earth 190 million years ago," McKelvie said.
The stone, estimated to weigh between 100 and 150 pounds, was loosened and likely would not have required any special tools to pry away, McKelvie said.
He said investigators don't suspect Ehlers had any accomplices.
Prosecutors say Ehlers pried up the fossil Feb. 17 and got rid of it March 3, according to court documents. McKelvie declined to comment on what prosecutors believe happened during that time or what may have drove Ehlers to dump it.
Authorities have said the track likely was tossed into the Colorado River about 32 miles northeast of Moab.
Divers searched the river for a day in March but didn't recover the fossil amid limited visibility, Grand County sheriff's Lt. Kim Neal said Wednesday.
"I don't know what the possibility is of it ever being recovered," Neal said.