Calling it "junk science," attorneys for a Baton Rouge man accused in the alleged beating death of his 8-year-old son in 2012 want a judge to exclude bite mark evidence from the first-degree murder case.

Coroner's officials classified Xzayvion Riley's death as a homicide and concluded he died of overwhelming infection from a ruptured bowel caused by blunt force trauma to his abdomen, but the boy's autopsy also revealed 60 external signs of recent and past trauma, including a human bite mark on his stomach.

The boy's sister told East Baton Rouge Parish sheriff's deputies she saw Michael Robertson bite her brother on a regular basis as a form of discipline.

A dentist who observed the bite mark on Xzayvion's body at the Coroner's Office and then made dental models of Robertson's upper and lower teeth impressions determined the models matched the bite mark.

Robertson doesn't have a trial date, but when that day comes, his attorneys say, any expert testimony about the bite mark evidence should be excluded as "scientifically unsound" because bite mark evidence is "grossly unreliable."

In their written motion to exclude the bite mark evidence, Robertson's lawyers argue that faulty bite mark evidence has been, at least in part, responsible for 28 wrongful convictions and indictments nationwide since 2000.

"So-called 'bite mark' evidence is used by prosecutors in many states to persuade juries that bruises or indentations on a human body are bite marks that can be connected to a particular person by comparing dental impressions to the bruising," Robertson's defense team of Jim Craig, Christine Lehmann and Maura Doherty wrote July 20. "In today's America, where `CSI' crime dramas have vastly overstated the accuracy of forensic science, the testimony of `forensic odontologists' is highly persuasive. But in reality, `forensic odontology' is junk science."

"The simple truth is that … human skin does not retain impressions in a consistent enough manner to allow for identification of a mark as a human bite mark, let alone as the bite mark of one particular person," the attorneys added. 

Robertson's lawyers also contend the probative value of the bite mark evidence is outweighed by the "substantial prejudicial effect" it would have on jurors.

"This effect is compounded by the fact that a bite mark demonstrates extreme violence in the commission of a crime, which on its own may prejudice the jury," they say.

East Baton Rouge Parish District Attorney Hillar Moore III, whose office is prosecuting both Robertson and the boy's mother, Lavaughn Riley, on first-degree murder charges in the death of Xzayvion, said Thursday his office only recently received the defense motion.

"We have not had an adequate opportunity to confer with our expert and file a response, but we will be doing so in the near future," Moore said.

The National Academy of Sciences stated in a 2009 report that there is "considerable dispute" within the scientific community about the value and reliability of bite mark evidence, Robertson's attorneys noted. This year, the Texas Forensic Science Commission concluded in an April report that bite mark evidence lacked sufficient reliability to be admitted in Texas courts, the lawyers added.   

Robertson, 50, and Lavaughn Riley, 36, have pleaded not guilty. Prosecutors have not said whether they will seek the death penalty against either one. Riley's attorney has said she intends to testify against Robertson, who was her boyfriend at the time of their son's death in June 2012.

Prosecutors allege Xzayvion's death was the culmination of an escalating pattern of physical abuse. The Louisiana Supreme Court ruled earlier this year in Robertson's case that a jury can hear about the abuse he allegedly inflicted on his son, including injuring the boy's mouth with a belt in late 2008, choking him and shoving his head into a toilet in August 2010 and breaking his leg in February 2012.

The state Department of Children and Family Services put a safety plan in place to limit Robertson's involvement with his son following the alleged 2010 incident, prosecutors have said. The plan was in place until April 2011.

Riley testified in 2014 at a hearing in Robertson's case that her son feared his father because he would hit and spank the boy to discipline him. She also recalled hearing Xzayvion's leg pop in 2012 when Robertson allegedly broke it while the boy tried to get away from him during what she described as horseplay.    

 Follow Joe Gyan Jr. on Twitter, @JoeGyanJr.