Osteoderms are bony plates found in the skin of vertebrates, mostly commonly in reptiles where they have evolved independently multiple times, suggesting the presence of a gene regulatory network that is readily activated and inactivated. They are absent in birds and mammals except for the armadillo. However, researchers at the University of Florida have discovered that in four genera (Acomys, Lophuromys, Uranomys, and Deomys) of the rodent subfamily Deomyinae, there are osteoderms in the skin of their tails.
CT images of osteoderms (colorized by thickness) in (A) Acomys cahirinus, (B) Lophuromys flavopunctatus, (C) Uranomys ruddi, (D) Deomys ferruguineus, (E) Cabassous chacoensis, (F) Chlamyphorus truncatus, and (G) Egernia hosmeri. Scale bars – 10 mm. Image credit: Maden et al., doi: 10.1016/j.isci.2023.106779.
“Osteoderms are present in this sub-family of rodents and nowhere else in living mammals except armadillos,” said lead author Dr. Malcolm Maden, a researcher at the University of Florida.
“They are absent in birds, frequent in reptiles — think of dinosaurs and crocodiles — and infrequent in frogs.”
“This means that they can be lost and re-evolved time and time again in animals, and this has happened at least 19 times.”
“What distinguishes osteoderms from other appendages of the skin is that they’re made of bone. They’re also found deep in the lower dermis layer of the skin, not on the surface.”
“That’s in contrast to scales on the epidermis of many animals, including pangolins and birds, whose feet are made of keratin.”
The researchers first spotted Acomys osteoderms in CT scans of museum specimens made for the openVertebrate program.
The effort involved CT scanning 20,000 museum specimens from all around the United States to gather high-resolution anatomical data for more than 80% of vertebrate genera.
“I was scanning a mouse specimen from the Yale Peabody Museum, and the tails looked abnormally dark,” said senior author Dr. Edward Stanley, director of the Florida Museum of Natural History’s digital imaging laboratory.
“I initially assumed the discoloration was caused by an imperfection introduced during the specimen’s preservation.”
“But when I analyzed the X-rays several days later, I observed an unmistakable feature was intimately familiar with.”
“Spiny mice osteoderms have been observed at least once before and were noted by the German biologist Jochen Niethammer, who compared their architecture to medieval stonework in an article published in 1975.”
“Niethammer correctly interpreted the plates as being a type of bone but never followed up on his initial observations, and the group was largely overlooked for several decades.”
The CT scans revealed osteoderms in the other three genera — Lophuromys, Uranomys, and Deomys — of the Deomyinae subfamily as well and not in any other relatives, such as gerbils.
Further study found that osteoderms develop in spiny mouse tails starting in the proximal tail skin. The bony plates finish developing six weeks after birth.
The authors used RNA sequencing to identify the underlying genes and gene networks involved in their formation.
They found a widespread down regulation of keratin genes as osteoblast genes switch on.
“Beyond the existence of osteoderms, we also were surprised by how similar they are in shape and structure to extinct sloths, whose osteoderms are fossilized, so they have ‘been here before’ in mammals,” Dr. Maden said.
According to the team, the osteoderms in Deomyinae rodents as well as in fish-tale geckos possibly function like a type of escape mechanism.
“If a predator bites down on the tail, the armor might keep the teeth from sinking into the tissue beneath, which doesn’t detach,” Dr. Stanley said.
“The outer skin and its complement of bone plating pull away from the tail when attacked, affording the mouse a quick escape.”
The findings were published in the journal iScience.
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Malcolm Maden et al. Osteoderms in a mammal the spiny mouse Acomys and the independent evolution of dermal armor. iScience, published online May 24, 2023; doi: 10.1016/j.isci.2023.106779
