Friday 30 November 2018

Helveticosaurus: the small-headed, long-armed Triassic marine reptile that just wants to be your friend :(

Helveticosaurus zollingeri, one of those strange Triassic marine reptiles that no-one ever talks about, wrestling in a coastal swamp. Not everything about being a marine tetrapod takes place in the sea.
The fossil record is full of fascinating, relatively well-represented species that, on paper, seem like they should be widely known and featured in all sorts of palaeontological media, and yet in reality are almost entirely overlooked in popular literature, documentaries and games. Triassic marine reptiles are definitely among these animals. Many are distinctive, unusual and well-researched species that are just as interesting (if not more so) than many more familiar Triassic animals, and yet their popular coverage is frequently dire: even their Wikipedia pages are little more than footnotes.

In interests of trying to correct this injustice even a little, it's time to talk about a Triassic marine reptile with a criminally poor popular coverage/deserved interest ratio: Helveticosaurus zollingeri. Discovered in Middle Triassic rocks of Switzerland in 1933 and described some years later (Peyer 1955), this small-headed, long-armed marine reptile represents a unique anatomical experiment among aquatic tetrapods: a sort of lizard-seal thing with a skull from an '80s supernatural horror film. Its basic bauplan is well demonstrated thanks to a mostly complete and reasonably preserved holotype, missing only the end of the tail and some parts of the limbs. Alas, some especially informative aspects of its anatomy are poorly represented, including the skull, distal limbs and pelvis. Though all are present, they are disarticulated and difficult to interpret. Additional Helveticosaurus specimens are known (Kuhn-Schnyder 1974), but are not as well preserved or complete as the holotype and don't add much to our knowledge of this species (Rieppel 1989). Though attracting reasonable scientific interest in the last half century, much about its lifestyle and evolutionary relationships remain unexplored or contentious.

The Helveticosaurus zollingeri holotype, as illustrated by Kuhn-Schnyder (1974). Although a little jumbled, a good portion of the skeleton is preserved. It's unfortunate the skull is such a mess. Check out Wikipedia for a photo of the actual specimen.
Much of our modern take on this animal has been informed by Olivier Rieppel's 1989 paper on its anatomy and function, and the following overview is largely based on this assessment. Helveticosaurus was a small-headed creature with a short neck, long body and a tail of unknown length. The preserved portion of the tail comprises large, well developed vertebrae and it doesn't seem unreasonable to assume it was much longer when complete. If we had a more secure idea of the phylogenetic position of Helveticosaurus we might take a stab at estimating the tail length, but this doesn't seem possible at the moment.

Tail proportions are not the only issue confusing predictions of the overall body length of this animal. When preparing this post I found that the total length estimates of Helveticosaurus provided in modern papers are at odds with measurements of skeletal elements within the holotype, to the effect that we might be significantly underestimating its overall size. Recent papers give a total predicted length of c. 2 m for the holotype animal (e.g. Rieppel 1989; Cheng et al. 2014), while also reporting that the lower jaw of the same specimen is 250 mm long, and the humerus as 205 mm (Rieppel 1989; Cheng et al. 2014). Even just eyeballing images of the holotype suggests some sort of miscalculation here: there's no way the entire animal - including the missing tail - is just 10 times the length of these bones. Using a line drawing of the holotype from Khun-Schnyder (1974) and the reported mandible and humerus measurements, I found that 2.1 - 2.8 m better describes the length of the preserved skeleton (see calculations in the image below, note that the reported 45 mm difference between the humerus and mandible length is not obvious in the drawing I used, resulting in two different body length estimates. Scaling from photos or illustrations is not a substitute for measuring actual specimens). This is back-of-the-envelope stuff, but it's enough to convince me that Helveticosaurus wasn't a 2 m long animal. I wonder if the figures reported by Khun-Schnyder (1974) are more plausible: he reported a 2.5 m length for the preserved holotype skeleton, and an estimated total length of 3.6 m. That would add another metre onto the holotype, which seems quite plausible - maybe even conservative - to me.

Just how big was Helveticosaurus? It's hard to say without a complete specimen, but the individual represented by the holotype skeleton clearly exceeded the oft-cited 2 m body length. Perhaps other published estimates of 3.6 m are more reasonable?
One of the most interesting features of Helveticosaurus is its short, c. 25 cm long skull. Alas, the best Helveticosaurus skull remains we have look as if they were hit by a truck before fossilisation: scattered, broken, and with many unidentifiable parts (Rieppel 1989). Enough is known to allow for a tentative reconstruction but a confident picture of the face of Helveticosaurus awaits better preserved material. The front of the upper jaw was abbreviated, blunt and tall, creating a skull profile that might have been somewhat box-like in lateral aspect. The orbital and temporal regions are poorly known, but they seem to hint at the presence of an upper and lower temporal fenestrae and a large eye socket. A number of oversize conical teeth line each jaw. The exact number of teeth is unknown, but a notable feature is the large 'canine' in the upper jaw. Neither the size of the temporal region or the lower jaw (the latter being one of the best preserved cranial elements) imply an especially large set of jaw muscles, though the mandible has an expanded retroarticular process - a prong of bone at the back of the jaw associated with opening the mouth. This likely has implications for the feeding style of Helveticosaurus, although I'm unaware of any studies into its function. The aberrant size of the Helveticosaurus skull is peculiar for a marine reptile lacking a long neck, and perhaps only challenged in proportion by the Triassic marine vacuum cleaner Atopodentatus (Cheng et al. 2014). Distinct anatomy make it clear that these animals were very different ecologically however, and it's possible that their diminutive skulls reflect very different adaptive regimes.

Tentative Helveticosaurus skull reconstruction, from Rieppel (1989). The jaws remain the best known elements, and some question exists over the arrangement of the rest of the skull. Scale bar represents 50 mm.
The body of Helveticosaurus is similar, at least superficially, to many other Triassic marine reptiles, especially early sauropterygians. It's torso was long, with well-developed and high-spined vertebrae, stout ribs and an extensive set of gastralia (belly ribs). Differentiation between the vertebral spines at the front and back of the body hint at some functional distinction, perhaps related to larger muscles associated with the shoulder region (Rieppel 1989). As is assumed for plesiosaurs, the combination of stout ribs and gastralia likely reduced the flexibility of the torso and may have improved swimming efficiency. The tail, so much as it is known, bears the same high neural spines as the trunk vertebrae, as well as caudal ribs. These features indicate it was likely well-muscled for use in sculling propulsion, although the chevrons are not particularly large. Assuming these anchored the caudofemoralis muscle, as they do in most reptiles, I wonder if this indicates diminished musculature associated with hindlimb retraction.

After the peculiar head, the forelimbs of Helveticosaurus are perhaps its most unusual feature. They anchored to an atypically well-developed pectoral girdle which - unlike most marine reptiles - has a long, robust scapula. Marine reptile shoulder blades are often extremely reduced, little more than bony nubbins that create a shoulder joint. But here, the scapulae are long enough to create a deep, U-shaped shoulder girdle that would not look out of place on a terrestrial animal (Rieppel 1989). The forelimb itself is proportionally elongate, both with respect to the body and in comparison to the hindlimb. It's exact length remains uncertain because the bones of the hand are scattered, but the major limb bones are each 10% longer than their counterparts in the hindlimb. The humerus in particular is very long for a marine reptile, and maintains hallmarks of functionality beyond just being the top of an stiffened flipper (Rieppel 1989). The fingers are hyperphalangic (i.e. they have an enhanced number of finger bones) in a fashion typical of marine tetrapods, and - in contrast to several Helveticosaurus palaeoartistic reconstructions (all five of them that exist) - they lack claws. The arrangement of the fingers requires some reconstruction but their slender bones and arrangement in the holotype implies more of a broad, rounded paddle than a narrow ichthyosaur or plesiosaur-like flipper.

Helveticosaurus forelimb, as illustrated by Rieppel (1989). Some ribs and gastralia have been removed for clarity. Note the elongate scapulae and long forelimb elements - this is not a typical marine reptile arm. Scale bar represents 100 mm.
The hindlimb shares some general characteristics with the forelimb - relatively elongate limb bones for a marine form, hyperphalangy, spreading, unclawed digits - but is shorter, noticeably more gracile and probably more cartilaginous than the forelimb. The pelvis is poorly known, but it also appears to have been at least partly cartilaginous, the joints of the pelvic bones being insufficient to contact one another around the hip joint without some additional skeletal material (Rieppel 1989). These features imply that the hindlimb was structurally weaker than the forelimb.

How might this mix of anatomies have functioned? A qualified assessment by Rieppel (1989) makes some sensible interpretations of Helveticosaurus locomotion. On the whole, the animal is mostly adapted for life in water, with aquatic adaptations being especially obvious on the limbs, pelvis and tail. Although the tail is missing, its robust, high-spined and complex vertebrae are consistent with features of sculling animals and we might envisage Helveticosaurus propelling itself with powerful motions of its tail when swimming, akin to marine iguanas or crocodylians. The weak pelvis and hindlimb indicate the rear limbs contributed less to propulsion. Rieppel proposes that, like swimming lizards, they may have been pulled against the body when swimming save for the occasional action to help with steering or thrust. The forelimbs were evidently strong and likely useful in swimming, though the configuration of the shoulder girdle does not imply any rigid kinematics for underwater flight in the manner of a penguin or turtle. They might have functioned more like the foreflippers of otariid seals (the eared seal group: sealions, fur seals etc.) in providing some thrust, but also playing important roles in steering and breaking (Rieppel 1989). While the shoulder girdle does not seem optimised for powerful downstrokes, the large size of the arm, and implied articulation of at least some parts of the limb (see below), suggest it was a dynamic steering aid. Helveticosaurus may have been quite an agile swimmer.

But where Helvetiosaurus becomes especially interesting is out of the water. Even in the Middle Triassic many marine reptiles had wholly committed themselves to aquatic lifestyles, but Helveticosaurus appears to have remained some terrestrial capabilities. Why it did this remains uncertain: did it still lay eggs? Did it have a complex life history involving both land and sea phases? Did it live in settings where periodic escapes from the sea were advantageous? We don't have insights into any of this yet, but we can predict how Helveticosaurus might have moved around on land. Supporting limbs during terrestrial gaits is not simply a matter of having strong limb bones, it's also necessary to have a robust and stable limb girdle. For shoulders, this requires support and control exerted by muscles attached to the torso and neck, as well as having a big enough scapula for these to act on. The robust shoulder girdle of Helveticosaurus seems to meet these criteria. It not only provides space for the necessary muscle to support and move the forelimb on land but also - with particular reference to the relatively big scapula - is sufficiently developed to brace the shoulder against the body skeleton (Rieppel 1989). The length and robustness of the forelimb is also notable, as are the retention of humeral features associated with flexing the lower limb. Marine reptile limbs are often immobile south of the shoulder or hip, and readers with good memories might recall that this makes terrestrial locomotion difficult. The articulations of the Helveticosaurus limb are not well preserved - they seem to have been highly cartilaginous - so we don't know the full extent of its forelimb mobility, but muscle attachment scars hint at abilities to flex the wrist and fingers (Rieppel 1989). Any flexible jointing would enhance its terrestrial potential, so this is another tick in the box for relatively proficient land locomotion. The hindlimb, in being less developed and more cartilaginous, probably contributed little to terrestrial locomotion. Helveticosaurus may have therefore crawled and flopped around more like a seal than a lizard, using its arms to drag and push itself around, maybe occassionally assisted by its legs and thrashing motions of the tail to propel itself faster. It must have been pretty neat to see a reptile move like this: a sort of creeping, lolopping reptile-mermaid topped off with the face of the Engineer from Hellraiser.

When Helveticosaurus collide. In the image illustrating this article, I've assumed that the terrestrial capabilities of Helveticosaurus were sufficient to bring them into terrestrial coastal habitats, perhaps for mating, nesting or some other reason. We have no evidence of this happening, but analogous behaviours are seen today in turtles and seals, some of which travel kilometres inland despite their limited terrestrial abilities. Maybe some Mesozoic marine reptiles did the same.
We can't go this far into discussion of Helveticosaurus without questioning its ecology. I'm not aware of any analyses that address this issue, so this paragraph is shot from the hip based on what others have said about its functional morphology and a basic form-function reading of Helveticosaurus anatomy - take it with an appropriate pinch of salt. As already noted, the skull of Helveticosaurus is too poorly preserved to say much about specifics of foraging, but its long, slender teeth clearly betray a predatory lifestyle. Worn tooth tips indicate that it did not eat entirely soft, fleshy prey, but the teeth are not robust enough to suggest a tough diet. I'm aware that a similar suite of dental features occur in pterosaurs that are assumed to small fish, squid and other diminutive swimming creatures (Ősi 2010), and I wonder if a similar diet might apply here. The skull of Helveticosaurus is also too small to suggest it routinely ate large prey, though I guess scavenging carcasses is difficult to rule out. The enlarged retroarticular process is of interest because such features are often seen in suction feeders - aquatic animals which rapidly open their mouths to suck up prey within a pressure gradient. Short faces often characterise suction feeders too, but we need knowledge of other anatomies - such as the bones of the throat - to reliably infer such foraging strategies (Motani et al. 2014). We also have to acknowledge that a short jaw and specifics of the posterior mandible can be related to other functions. A small head capable of fitting between rocks and other obstacles would be useful if Helveticosaurus sought benthic or demersal prey, for instance. The combination of a swimming tail and large limbs may have made Helveticosaurus relatively agile, a useful trait when chasing small prey. In all, I wonder if the seal analogy applied to some aspects of Helveticosaurus anatomy and locomotion might extend to its lifestyle. It would be great to see this looked into with a dedicated study.

Bringing this post back to firmer scientific ground, it's finally time to ask: just what the heck is Helveticosaurus? Initially interpreted as a placodont (Peyer 1955), Helveticosaurus has since jumped all over the reptile tree as different teams use different approaches to resolve its placement. There are probably several reasons for our inability to pin down the evolutionary home of Helveticosaurus. Firstly, the anatomy of Helveticosaurus confuses character distribution in phylogenetic trees, it having features of enough groups to scramble easy reading of homologies and convergences (Ezcurra et al. 2014). This makes Helveticosaurus very sensitive to taxon and character choices used in our evolutionary calculations, and prone to shifting in position dramatically from one cladogram to the next (e.g. Chen et al. 2014). Helveticosaurus is far from the only marine reptile to present such a problem, and there are debates among researchers about how to deal with what some regard as a problematic amount of convergence between aquatic Mesozoic reptiles (see, for recent takes, Chen et al. 2014 vs. Scheyer et al. 2017). A third issue concerns the ongoing controversy over the origins of marine reptiles generally. The relationships of even well-supported groups like ichthyosauromorphs, turtles and sauropterygians to other reptiles remain contested, and these clades have major 'pull' in phylogenies when they move about, hauling possible relatives like Helveticosaurus around as tree topologies change.

We don't know of any species quite like Helveticosaurus, but the Triassic diapsid Eusaurosphargis dalsassoi - here represented by an excellent fossil of a juvenile skeleton - has been recovered as a near relative in several recent analyses. Intriguingly, it also seems well adapted for terrestrial locomotion, implying that such abilities may have been common to their branch of marine reptile evolution. Image from Scheyer et al. (2017).
Perhaps for this reason, it's not uncommon to see many authors sidestepping classifying Helveticosaurus altogether, instead simply labelling it an 'enigmatic diapsid' and moving on. But others have tackled the issue more head on and, while it would be premature to say we know what Helveticosaurus is, some clarity is emerging about which branch of reptile evolution it belongs to (even if the position of that branch is a more open question). The placodont affinity for Helveticosaurus has been questioned on grounds of very limited shared anatomies (Sues 1987; Rieppel 1989) and this identification has not been supported in recent analyses. Other ideas - a tentative interpretation as some sort of archosauromorph (Rieppel 1989; Naish 2004) or a near relative of lepidosaurs (Chen et al. 2014) - have also not found much traction. But a large number of authors have recovered Helveticosaurus as a close relative of Sauropterygia (Müller 2004; Bickelmann et al. 2009; Li et al. 2011, 2014; Neenan et al. 2013; Chen et al. 2014; Scheyer et al. 2017), and it's looking like this is the best horse to back concerning the phylogenetic position of this historically enigmatic animal.

Alas, this is not the neat end of the story we might think it is, as the origins of Sauropterygia itself remain poorly understood. In at least some analyses Helveticosaurus and Sauropterygia is part of a marine reptile 'superclade', a huge, unnamed group containing ichthyosaurs, sauropterygians and a number of Triassic lineages that have long struggled to find homes. Another Swiss Triassic reptile, the possibly mostly terrestrial Eusaurosphargis dalsassoi (above), has been postulated as a close relative of Helveticosaurus several times (e.g. Scheyer et al. 2017). Sauropterygians are deeply nested in this 'superclade' and the position of the terrestrially-enabled Helveticosaurus and Eusaurosphargis is interesting with respect to the evolution of aquatic lifestyles in Triassic marine reptiles. Given that more rootward lineages in the 'superclade' are entirely aquatic forms, might genera like Helveticosaurus and Eusaurosphargis represent animals that returned to land from swimming ancestors, or are they representatives of a more basic semiaquatic ancestral bauplan that remains underrepresented in other lineages? At the risk of ending on an old palaeontological cliche, we need more specimens, more data and more investigations to answer these questions.

It turns out that marine reptiles are a pretty fun group, I think you'll be seeing more art and reading more about them here in the coming months. If all goes to plan, we'll be walking (or not) with plesiosaurs and meeting some giant ichthyosaurs before too long.

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