Conjoined, or Siamese, twins are an extremely rare phenomenon in nature, and reptiles are no exception. Reports in the literature documenting cases among this class of vertebrates are scarce, but those published so far include a few examples of conjoined turtles, crocodiles and lizards such as geckos.

Until recently, scientists only knew of conjoined twinning in monitor lizards from one single report, which described a pair of emerald tree monitors. Now, researchers have documented the second known case for monitor lizards, an impressive and diverse group which includes the largest lizard in the world—the Komodo Dragon. Detailed descriptions of the animals, which are the first known conjoined twins in the Pacific monitor lizard group, can be found in the journal Herpetology Notes.

The animals, which were quince monitors (Varanus melinus), were actually first discovered back in 2009 in Cologne Zoo, Germany. This particular species was only described back in 1997 and very little is known about them in general. They’re bright yellow and black lizards found exclusively in Indonesia, and they can reach lengths of up to 120 centimeters (4 ft).

The newly described twins were part of a clutch consisting of nine eggs which were deposited in June 2009. Six of these hatched, but two didn’t show any development and one contained the dead twins. Interestingly, another clutch of five was laid a few months later from the same parents, two of which didn’t develop, and one contained a dead, deformed lizard.

“Interesting for us was that both clutches of the same pair comprised malformed offspring, which indicates that this probably did not happen coincidentally,” said reptile expert and report author Mona van Schingen.

The twins were joined at both the head and the abdomen, making them so called “cephalothoracopagus” conjoined twins. Although their bodies were generally well-developed, the twins and the malformed hatchling were smaller than their clutch mates and all shared a few distinct features. For example, all of them had curved spinal columns and were missing skin on their bellies, meaning their internal organs were spilling out.

Although the researchers can't be certain of what caused these abnormalities, the fact that three malformed individuals all came from the same parents but different clutches suggests there may have been some genetic predisposition. The parents were siblings, so it’s possible that low genetic variation could have played a role. However, it’s also possible that a non-genetic factor contributed as scientists have previously linked reptile malformations to environmental causes. For example, back in 2010, a crocodile hatchling was found with eight legs in a National Park in Venezuela that is known to be under the influence of agricultural chemicals.

As pointed out by Live Science, although reports of conjoined reptiles are few and far between, it’s likely that they occur more often than scientists are aware since they usually die before hatching and thus go unreported.

[Via Herpetology Notes and Live Science]