Xenophidion: The Snake with the Mystery Penis - Reptilenesia

Xenophidion schaeferi From Das 2010, painted by Szabolcs Kókay

For a combination of phylogenetically distinct, taxonomically confusing, and poorly known, you simply cannot beat the spinejaw snakes, genus Xenophidion. Described in 1995, there are two species, each known from a single specimen¹. That makes even dwarf pipesnakes (family Anomochilidae), of which we've obtained several new color photos recently, seem relatively well-represented. Putting together this article strained my research powers—xenophidiids don't even have an English language Wikipedia page (yet). Google the name of the family and it asks whether you meant Xenophilius, the first name of a minor character from Harry Potter, who has more than twice as many results. Xenophidion means "small strange snakes" in Greek, and indeed we have barely scratched the surface of how strange these snakes probably are. And, to top it all off, no one has ever seen its penis. Read on to find out why.

Collection locations of the only specimens of Xenophidion acanthognathus (red) and Xenophidion schaeferi (green)

The story of Xenophidion begins on the morning of November 20th, 1987. It was 8:15 AM when Chicago Field Museum Herpetologist Robert F. Inger found a snake beneath some moss on rock during field work in a selectively-logged forest near Mendolong, in Sabah's Sipitang District on the island of Borneo. Inger, an expert in the herpetology of southeast Asia who by that time in his life had "made thorough searches of thousands of square meters of forest floor litter with the help of very sharp-sighted local men", had never seen a snake like this before, and he brought it back to Chicago and placed it in the Field Museum collection.

Almost a year later, at 10:00 PM on November 5th, 1988, German amateur herpetologist Christian Schäfer collected and photographed a snake at the edge of a trail near Templer Park, about 12 miles north of Kuala Lumpur in peninsular Malaysia. Schäfer donated his specimen to the Zoological Museum in Berlin in the spring of 1993. Curators Rainer Günther and Ulrich Manthey recognized it as unique and asked esteemed herpetologists Van Wallach and Bob Inger to compare it to specimens at Harvard and Chicago. Inger recognized similarity between Schäfer's specimen and his own, and sent both specimens back to Berlin to be described as new species. The dissimilarity between the two new specimens and all other known snakes was so great that they chose to establish a new genus, which they tentatively placed into the family Colubridae (which at the time was much more inclusive). The genus was elevated into a new family after the dissection of the X. acanthognathus specimen by Wallach and Günther in 1998 failed to reveal an obvious affinity with any existing family.

Drawing and photograph of the jaw spine of X. schaeferi (labeled 'Pp') From Günther & Manthey 1995

The two specimens share a number of unique features that distinguish them from all other living snakes. Their head scales, especially those along their lips, bear numerous sensory papillae. Their prefrontal scales are much larger than those of other snakes, taking up most of the top of the head in front of the eyes, and the space between their eyes is slightly concave. Their upper jaw bears a long, spiny palatine process, after which X. acanthognathus ("spine jaw" in Greek) is named. Their small eyes, short tail, and wedge-shaped head all suggest a mostly fossorial lifestyle. Like many "henophidian" snakes, their ventral scales are only slightly wider than their dorsal scales. But, unlike so many henophidians, both species of Xenophidion lack any vestiges of a pelvic girdle, left lung, or coronoid bone, suggesting that they are more closely related to caenophidian snakes. Wallach and Günther noted several similarities among the visceral characteristics of Xenophidion and tropidophiids, including a tracheal lung and unlobed kidneys., although we now know that tropidophiids are most closely related to aniliids. They also suggested that Xenophidion and another enigmatic snake family, bolyeriids, might be related.

The only photograph of a living Xenophidion schaeferi (FMNH 235170), taken by W. Grossmann. From Günther & Manthey 1995

In 2004, the sequence of the cytochrome b gene of X. schaeferi was sequenced. This is still the only gene we have from either species of Xenophidion, and it has suggested a sister relationship between Xenophidion and Bolyeriidae and a distant relationship between Xenophidion and Tropidophiidae in several studies. Evidently, unpublished CT scans of the skull of Xenophidion show that these snakes also have a joint in the maxilla, a characteristic unique to bolyeriids. We know almost nothing about the diet of Xenophidion, but thankfully the stomach of the X. acanthognathus specimen contains a Sphenomorphus skink. Skinks are also eaten by bolyeriids, which use their hinged upper jaws to grasp their hard-bodied,  relatively non-deformable prey. It's not inconceivable that Xenophidion might do this as well. The current geographic distribution of Bolyeriidae is limited to Round Island in the Indian Ocean, which suggests that the common ancestor of these two families was probably ancient and widespread across Gondwanaland.

Ventral view of the sole specimen of Xenophidion acanthognathus (ZMB 50534) From Günther & Manthey 1995

There are numerous differences between the two species of Xenophidion. Both have 23 dorsal scale rows at midbody, but the dorsal scales of X. acanthognathus are more heavily keeled than those of X. schaeferi. They have a similar number of ventral scales (181 vs, 178), but X. acanthognathus has 51 subcaudals, 8 more than X. schaeferi. Xenophidion schaeferi also has more teeth on the palatine (10 vs. 8), pterygoid (16 vs. 13), and especially the dentary bone (19 vs. 12) than X. acanthognathus. Finally, X. acanthognathus has a large yellow-white patch on its neck. Because both of the specimens are females, the hemipenes, which contain many taxonomically useful characters, have not been described. But, conveniently, the oviduct of the X. acanthognathus specimen contains two eggs, so at least we know the reproductive mode of these snakes.

Snake family tree from Figueroa et al. 2016, showing Xenophidiidae + Bolyeriidae as sister to Caenophidia Click for a larger version

Some phylogenetic studies suggest that Xenophidiidae and Bolyeriidae might be sister to Caenophidia, leading some to call these two families "proto-colubroids". However, other genetic analyses group them with boas, pythons, and other "henophidian" snakes instead. Hopefully further gene sequencing will sort this out, and of course fresh Xenophidion specimens wouldn't hurt. The forestry station where Inger collected X. acanthognathus is still operational and researchers continue to work there—I hope they know to keep their eyes open for small, strange snakes. Unfortunately, the primary forest where X. schaeferi was collected was cleared two years later and is now a banana plantation. Both peninsular Malaysia and Borneo are losing their forests to timber harvesting and oil palm plantations at an alarming rate. People get upset when they learn that deforestation endangers charismatic species such as orangutans, leading to efforts to make palm oil production more sustainable. This is really challenging because palm oil is used in all kinds of delicious things, such as Girl Scout Cookies, and high-profile controversy over its sustainability has been fueled by people's love for orangutans. I'm here to suggest that the many mysteries of Xenophidion—including what its penis looks like—may never be solved if the rain forests of southeast Asia are lost, and that Xenophidion is at least as valuable and interesting as orangutans.

UPDATE: Additional specimens have since been discovered in Malaysia, Sumatra, and Borneo including at least one male, bringing the total number of individuals known to six, potentially representing three species if the Sumatra specimen proves to be distinct. Some photos of one of the new specimens here.

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