The Origin and Evolution of Tanystropheus

The King of the Bizarre Triassic Lizards
The Triassic is known for its many bizarre “experiments” in reptile morphology. It’s not commonly realized, but many of these oddballs nest together in one lizard clade, the Tritosauria. The largest of these Dr. Seussian reptiles was Tanystropheus, famous for its extremely long neck. You may wonder, as many have, how does an animal get such a long neck? The answer is: blame the parents. Here we’ll look at Tanystropheus and its ancestry, a subject that has never been accurately documented (that is, following a large phylogenetic analysis).

Key note: Tanystropheids were NOT prolacertiforms. Prolacerta and Protorosaurus nested with archosauriforms.

Tanystropheus and kin going back to Huehuecuetzpalli.

Figure 1. Tanystropheus and kin going back to the basal tritosaur lizard, Huehuecuetzpalli.

The Long Neck Has a Long History
The basal tritosaurian lizard, Huehuecuetzpalli, does not have a long neck. That honor goes to it sister taxon, Macrocnemus. There is a gradual size increase in various specimens of Macrocnemus that traditionally has been ascribed to levels of maturity. Here the morphological differences indicate phylogenetic variation. The smallest Macrocnemus is the most primitive, the one closest to Huehuecuetzpalli. The next largest one, T4822, gave rise to Tanystropheus and its kin, Tanytrachelos and Langobardisaurus. The largest Macrocnemus, T2472, gives rise to the underwater sit-and-wait predator, Dinocephalosaurus, which turns out to be not as closely related to Tanystropheus as everyone thought.

Next Step Langobardisaurus
The longer, more gracile neck vertebrae found in Tanystropheus are first seen in Langobardisaurus, a small, terrestrial tritosaur that likely experimented with a bipedal configuration. Note that Dinocephalosaurus had a longer neck made longer by adding vertebrae, a pattern unlike that of Tanystropheus and Langobardisaurus, which elongated its cervical vertebrae without increasing their number.

Teeth
The teeth of Langobardisaurus were unusual. The front teeth were rake-like. The posterior teeth were flattened and multi-cusped, like molars. The smaller Tanystropheus, wrongly considered a juvenile (MSNM BES SC 1018, Exemplar A) also had multi-cusped posterior teeth, though not quite the same shape. The large Tanystropheus had only simple conical teeth with a single sharp point. The anterior teeth were also longer and more rake-like. The change probably reflected a change in diet, perhaps from small insects to large vertebrates like fish.

Where Did Tanystropheus Live and Eat?
A diet of fish might involve a move from a terrestrial environment to an aquatic one, at least to the sea shore where a long neck could extend out over the surf to dip into deeper waters. Fish bones and squid jaws were found in the stomach area. There is not much else about the Tanystropheus skeleton that indicates any marine adaptations. However, such adaptations can be seen in Dinocephalosaurus. I also wonder if Tanystropheus raided tree limbs for the various climbing, gliding and flying animals that could be found there. That’s how the incremental changes employed by evolution could make a tiny, long-necked lizard reaching into bushes and trees into a very large long-necked creature reaching into taller bushes and tree — or capable of dipping into surf.

A purported “juvenile” Tanystropheus (Li 2007) was slightly larger than the A specimen and had fish bones in its gut area. The skull is unknown. Since juveniles were identical to adults, without similar specimens nearby, or eggshells, there is no way to tell if this specimen was indeed a juvenile or just small. The skull is unknown. The lack of ossification in the carpus is not a sign of immaturity, but a trait shared by other clade members, large and small, beginning with Huehuecuetzpalli.

The Pedal Digit 5 Problem
Careful observers will note that pedal phalanx 5.1 is elongated in most basal members of this clade and this extends into fenestrasaurs, like Cosesaurus and Sharovipteryx and basal pterosaurs. Three phylogenetic successors to Macrocnemus (Langobardisaurus, Tantrachelos and Tanystropheus) also had such a toe. Unfortunately no Macrocnemus, large or small, has this trait. Dinocephalosaurus, a fourth Macrocnemus successor, did not have an elongated p5.1 either. If Macrocnemus had a short p5.1 while its predecessors and successors had an elongated p5.1, this is a phylogenetic problem. Two solutions: 1) Pedal 5.1 re-elongated by convergence after Macrocnemus, and 2) there is yet another to-be-found Macrocnemus that retained an elongated p5.1.  Note that both Tantrachelos and Langobardisaurus are smaller than the BES SC specimen of Macrocnemus (Fig 1). If these taxa matured faster and smaller while retaining embryonic traits, they could have reverted to the elongated p5.1 of the ancestors of Macrocnemus, following solution #1. Let’s keep looking at the toes.

Post-Cloacal Bones – Traditional Hypothesis
Postcloacal (aka heterotopic bones) beneath the tail in some specimens is indicative of sexual dimorphism. Wild (1973) wondered if these were male copulatory organs, but they were too large and too complex. Another theory suggests they supported a brood pouch. They were not chevrons.

Post-Cloacal Bones – Heretical Hypothesis
Note that where postcloacal bones are present, chevrons are not. Smaller postcloacal bones on more primitive taxa more closely resemble typical chevrons. Therefore postcloacal bones were transformed chevrons. Dr. Silvio Renesto (2005) reported a great mass of flesh at the base of the tail, which would have helped balance (or in the heretical hypothesis, provide a base) for Tanystropheus. Postcloacal bones could have helped support this mass. The postcloacal bones doubled as tail skids (Fig. 1) wheneverTanystropheus was vertical, like the double-beam chevrons of the rearing sauropod Diplodocus.

Juvenile Tanystropheus from China

Figure 2. Juvenile Tanystropheus from China (Li 2007). It was slightly larger than the A exemplar of Wild (1973) and had slightly different neck vertebrae and limb proportions. The ribs were more robust. The head and teeth are unknown.

What Would a Tanystropheus Egg Look Like?
As a tritosaur, Tanystropheus hatchlings greatly resembled their parents, as in pterosaurs and Huehuecuetzpalli. The purported juvenile (Li 2007, Fig. 2) had a long neck that would have been flexed within the eggshell bringing the head down to the level of the groin or tail. That produces a very long egg. The examples of pterosaurs indicate that other tritosaurs may have retained eggs within the cloaca until just prior to hatching. If Tanystropheus was like the pterosaur Darwinopterus, only one egg at a time was produced. The giant chevrons/postcloacal bones of only some Tanystropheus may indicate an extension of the cloaca further beneath the tail, beyond the hips to carry hyperelongated eggs.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Bassani F 1886. Sui Fossili e sull’ età degli schisti bituminosi triasici di Besano in Lombardia. Atti della Società Italiana di Scienze Naturali 19:15–72.
Li C 2007. A juvenile Tanystropheus sp.(Protoro sauria: Tanystropheidae) from the Middle Triassic of Guizhou, China. Vertebrata PalAsiatica 45(1): 37-42.
Meyer H von 1847–55. Die saurier des Muschelkalkes mit rücksicht auf die saurier aus Buntem Sanstein und Keuper; pp. 1-167 in Zur fauna der Vorwelt, zweite Abteilung. Frankfurt.
Nosotti S 2007. Tanystropheus longobardicus (Reptilia, Protorosauria: Reinterpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, Vol. XXXV – Fascicolo III, pp. 1-88
Peyer B 1931. Tanystropheus longobardicus Bass sp. Die Triasfauna der Tessiner Kalkalpen. Abhandlungen Schweizerische Paläontologie Gesellschaft 50:5-110.
Renesto S 2005. A new specimen of Tanystropheus (Reptilia Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia vol. 111, no. 3, 377–394. online pdf
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus (Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-16.
wiki/Tanystropheus

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