An international team of scientists has completely sequenced the mitochondrial genome of the Hispaniolan solenodon (a venomous, insectivorous mammal that diverged from other living mammals 78 million years ago), filling in the last major branch of placental mammals on the tree of life.

The Hispaniolan solenodon (Solenodon paradoxus), also known as the Dominican solenodon, or agouta, is one of only a few mammal species capable of producing toxic saliva, which it uses to immobilize its invertebrate prey.

The species is endemic to Hispaniola, the island shared by Haiti and the Dominican Republic. It is found in forests and brush country, as well as around plantations.

It looks much like a shrew. However, it is considerably larger. It measures between 11 and 13 inches (28–33 cm), with the tail adding an additional 10 inches (25 cm). It weighs 600 g – 1 kg.

A black to reddish-brown pelage covers the majority of the body, with the exception of the tail, feet, nose, and tips of the ears.

The Hispaniolan solenodon is nocturnal, and it finds shelter during the day by burrowing, or finding cover in hollowed logs, trees, or cracks in rocks.

It is relatively social, and up to eight individuals may inhabit the same burrow. Litter size is one or two young.

The young are born in a nesting burrow. Young solenodons remain with their mother for several months, which is exceptionally long for insectivores.

The species’ diet includes insects and spiders found in soil and leaf litter.

The new study, published in the journal Mitochondrial DNA Part A: DNA Mapping, Sequencing, and Analysis, confirmed that the Hispaniolan solenodon diverged from all other living mammals 78 million years ago, long before an asteroid wiped out the dinosaurs.

“It’s just impressive it’s survived this long. It survived the asteroid; it survived human colonization and the rats and mice humans brought with them that wiped out the solenodon’s closest relatives,” said lead author Dr. Adam Brandt, from the University of Illinois at Urbana-Champaign.

The study also supports recent findings that the island of Hispaniola contains genetically distinct northern and southern populations that should be conserved as separate sub-species: Solenodon paradoxus paradoxus and Solenodon paradoxus woodi.

Furthermore, the study found that the southern population has little diversity, whereas the northern population is much more diverse.

A previous study used a different set of genes to estimate that solenodons diverged from mammals during the Cretaceous period 76 million years ago. This study used a very different method but still established a similar estimate: 78 million years.

Interestingly, these two estimates align with a hypothesis regarding how the solenodon came to inhabit Hispaniola.

Some researchers speculate that the island was part of a volcanic arc connected to Mexico 75 million years ago and over time the arc has moved eastward.

“Whether they got on the island when the West Indies ran into Mexico 75 million years ago, or whether they floated over on driftwood or whatever else much later is not very clear,” said study senior author Prof. Alfred Roca, also from the University of Illinois at Urbana-Champaign.

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Adam L. Brandt et al. Mitogenomic sequences support a north–south subspecies subdivision within Solenodon paradoxus. Mitochondrial DNA Part A: DNA Mapping, Sequencing, and Analysis, published online April 20, 2016; doi: 10.3109/24701394.2016.1167891