Plesiosaur necks: not so flexible after all

With a neck WAAAYYY longer than half the total length
elasmosaurs, like Albertonectes (Figs. 1, 2), have been traditionally referred to as ‘a snake threaded through a sea turtle’ (going back to the Buckland lectures 1832, full story online here). Snakes have no trouble swimming, but so far, paleontologists have not considered the long, minimally flexible neck of elasmosaurs a propulsive organ, as in sea snakes. That might change a little today.

Figure 1. A weak attempt at making sine waves in the neck of Albertonectes.

Figure 1. A weak attempt at making sea snake-like sine waves in the neck of Albertonectes. Note the minimum of bending through effort. Relaxation realigned the neck.

Earlier a vertical configuration was suggested
to explain the weird and extreme morphology of elasmosaurs, entering fish and squid schools from below, distinct from all other oceanic predators. While the flippers were powerful propulsive organs for long distance, when it came to fine tuning while hovering, perhaps the increasingly longer (Fig. 2), snake-like necks helped some. It also moved the bulky flapping torso further from the mouth, so the school of fish would be less and less  likely to notice the intruder in the middle.

Figure 3. Click to enlarge. Albertonectes reconstructed. This 11 m elasmosaur is the longest thusfar recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus.

Figure 2. Click to enlarge. Albertonectes reconstructed. This 11 m elasmosaur is the longest thusfar recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus.

By contrast, Noe, Taylor and Gomez-Perez 2017 reported,
Based on the anatomy of the articular faces of contiguous cervical vertebral centra, neural arches, and cervical ribs, the plesiosaur neck was mainly adapted for ventral bending, with dorsal, lateral and rotational movements all relatively restricted. A new model is proposed for the plesiosaur bauplan, comprising the head as a filter, straining, sieve feeding or sediment raking apparatus, mounted on a neck which acted as a stiff but ventrally flexible feeding tube, attached to the body which acted as a highly mobile feeding platform.”

“The neck increased drag due to its form and large surface area, but was also potentially part of an integrated locomotor system, for instance affecting steering (as it lies in front of the locomotor apparatus) and because the rear of the neck acted as anchorage for musculature from the anterior limb girdles. Hence, any explanation of neck function should consider both slow speed locomotion and more rapid movement during respiration, feeding and predator avoidance.”

Their study looked at
Muraenosaurus (Figs. 3, 4), Cryptoclidus and Tricleidus (none if these yet in the LRT) as exemplars of long-necked plesiosaurians. All are related to one another, not to elasmosaurs. Noe, Taylor and Gomez-Perez presented a history of plesiosaur neck interpretation and presented their own interpretation (ventral flexion, Fig. 5). Given that comprehensive review, apparently no prior workers envisioned a sea-snake analog for the long neck of elasmosaurs, nor have any envisioned a vertical feeding orientation.

Figure 2. Muraenosaurus in dorsal and lateral views. Compare to figure 1.

Figure 2. Muraenosaurus in dorsal and lateral views. Compare to figure 1.

Rather than a flexible ball-and-socket joint
between cervicals, each plesiosaur vertebra consisted of a spool-shaped centrum with flat or slightly concave articular surfaces (Fig. 4). Most cervical centra are wider than deep. according to Noe, Taylor and Gomez-Perez, but that is largely due to a dorsal indentation for the neural spine. Cervicals preserved in situ indicate no intervening cartilage between centra. So, think of plesiosaur centra as Incan wall stones. There are no spaces between either. This compaction between vertebrae greatly restricts movement between individual cervicals and restricts cervical movement overall. Even so, even half a degree per centrum magnified by 76 cervicals can add up (Fig. 1) permitting some movement. Short, L-shaped cervical ribs are fused to each centrum.Their distal processes do not articulate with one another, but hypothetical ligaments extending from anteroposteriorly-oriented distal tips may have done so.

Figure 5. Muraenosaurus cervical sections from Noe et al. 2017 alongside a ghosted diagram of a complete Muraenosaurus neck.

Figure 4. Muraenosaurus cervical sections from Noe et al. 2017 alongside a ghosted diagram of a complete Muraenosaurus neck. The space between centra can be compared to the space between Incan wall stones. In other words: none. That is not shown in the ghosted reconstruction.

Noe, Taylor and Gomez-Perez conclude,
The consistent presence of numerous cervical segments that lack bony stiffening adaptations, however, is also strong evidence that flexibility was an important functional element in plesiosaur necks (Evans 1993), and gives the potential for a considerable range of movement in the living animal (cf. Zarnik 1925–1926).” The authors compare plesiosaurs to stiff-necked tanystropheids (with only 12 cervicals) to emphasize their point. They overlooked the tight articulations of each centrum with its neighbors. 

From a historical perspective, Noe, Taylor and Gomez-Perez report, 
“Previous workers have considered the degree of neck flexibility in plesiosaurs to range from: extreme mobility (Hawkins 1840; Zarnik 1925–1926; Welles 1943; Welles and Bump 1949), including the ability to arch the neck like a swan (Conybeare 1824; Andrews 1910; Brown 1981b); through relative inflexibility (Hutchinson 1897; Williston 1914; North 1933; Shuler 1950; Storrs 1997); to almost complete rigidity (Buckland 1836; Watson 1924, 1951; Cruickshank and Fordyce 2002; Figs. 3, 9); although some of this variation in interpretation may be due to differences between the species studied (Watson 1924, 1951).”

Clearly some of these workers were right and others were wrong.
But which ones? Zoe, Taylor and Gomez-Perez conclude, to their credit, “Overall, the range of movement available to the plesiosaur neck was strictly limited.”

Figure 7. Illustration from Noe, Taylor and Perez-Gomez showing their view of plesiosaur feeding and escape configurations.

Figure 5. Illustration from Noe, Taylor and Perez-Gomez showing their view of plesiosaur feeding and escape configurations. Usually paleo illustrations are more anatomically accurate than this.

Elasmosaurs were morphologically different than anything else in the sea. 
And they became more and more different as time went by (Fig. 2). So, something was working better and better as evolution selected for more extreme neck lengths.

Once again, let’s broaden our scope and look at the environs,
including coeval predators. All of these were robust, fast, streamlined, short-neck predators that swam horizontally preceding an attack from outside in. All of this is the opposite of elasmosaurs who hypothetically loitered below schools of fish unobtrusively rising to slip only their head in from below with minimum turbulence in order to remove fish or squid at leisure from the inside out.

Plesiosaur respiration at the surface
had to take place horizontally due to air pressure constraints. Alternatively, elasmosaurs could have gulped air, then assumed a horizontal or diving orientation to let the air bubble travel back through their long neck back or up to their lungs. With such tiny nostrils, gulping air seems more reasonable than narial inhalation.

Exhalation could have been more leisurely
and might have involved producing a ‘bubble net’ from stale air stored in the long trachea and released through the tiny nares. Extant baleen whales sometimes produce a bubble net to herd fish and plankton as they rise to feed on them. Perhaps elasmosaurs did the same, again based on their vertical orientation.

Fins at all four corners
Noe, Taylor and Gomez-Perez report, “With limbs at the four corners of the body, plesiosaurs could potentially produce vectored thrust from different limbs, to provide fine control of movement in all directions, and around all axes. This is more useful in slow swimming or hovering animals than simple shark-like control fins, which require movement in order to generate a current over the control surfaces.” Exactly. Unfortunately, these authors did not consider plesiosaurs to have a vertical orientation. Instead they focused on the ability of the neck to flex ventrally from a horizontal orientation.

Stomach stones
Noe, Taylor and Gomez-Perez report, “Swimming efficiency was further impaired by the mass of the neck, and the stomach stones commonly preserved in plesiosaurs. This stone ballast was probably needed to establish trim control and longitudinal stability to enable the animal to swim slowly horizontally and to hover, especially when diving in shallow water when the animal was positively buoyant.” The other explanation is that stomach stones helped weight the body below the more buoyant neck (filled with stagnant air), again supporting a vertical orientation when not swimming to other locations.


References
Noe LF, Taylor MA and Gomez-Perez M 2017. An integrated approach to understanding the role of the long neck in plesiosaurs. Acta Palaeontologica Polonica 62 (1): 137–162.

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