Swinging the tail like a maestro: diplodocids and the spatial awareness

Updated: Jul 29, 2020


    Sauropod dinosaurs are, very grossly, no more than a long neck glued on an enormous torso, columnar limbs supporting that heavy body, and a very long tail attached behind. Tail more or less long by the way, according to the focal species in this large group. Diplodocids (grouping the well-known Diplodocus among others) had notably the most amazing tails within sauropods, that could reach more than 10 meters in length and include more than 80 bones (Fig. 1) [1]!


Figure 1. General scheme of a diplodocid sauropod, depicting the very elongated tail compared to the total length of the individual. Above: dorsal view. Below: lateral view. (cer): cervical vertebrae. (dor): dorsal vertebrae. (sac): sacral vertebrae. (cau): caudal vertebrae. From [2], adapted from [3].

    But to what purpose such a long tail was designed? Well, this what Dr. Matthew Baron asked himself not so long ago [2], and honestly, I was eager to read more about it!



A whip, a 'third leg'... I am not sure about the topic anymore...


    Even though sauropods are particularly well-known among dinosaurs, they remain quite enigmatic. As opposed to the tail of Spinosaurus aegyptiacus (whose case has been discussed here: Final Fantasy: when dinosaurs decided to swim), complete or a bit less preserved tails are more common in long-necked dinosaurs. However, this does not resolve questions one could wonder about them.   


    Indeed, as the subtitle suggests, the sauropod tail has been envisioned a long time ago as a 'third leg' making possible to stand up on the hindlimbs to reach the highest leaves. Subsequently, when palaeontologists were still thinking that sauropods were semi-aquatic animals, the elongated tail was thought to stand for a tool of aquatic propulsion. Finally, those hypotheses fell into oblivion, and according to Preuschoft and Klein [4]: "no realistic idea seems to exist as to what [sauropods] did with their tails." For these authors, a tail suspended horizontally at the rear of the body was used either as a general counterbalance of the whole body or as an insertion point for a muscle attached posteriorly to the femur.


    As you can see, many hypotheses were proposed but remained unclear. Mamenchisaurids, a sauropod clade, the tail end was a kind of club, just like other distant dinosaurs groups (ankylosaurids notably, the tank dinosaurs). A defense function is thus possible at least for those sauropod species. This idea has been generalised to sauropods as a whole, even the club-less ones. Indeed, unfolding such elongated tail laterally is able to generate enough forces to tame any threatening predator or scare it by cracking the tail like a whip in order to go beyond the sound barrier and simulate the cracking sound of the thunder (Fig. 2)... So many things to tell, but for another post (all of this is summarised in [2] with all associated references)!


Figure 2. Leinkupal laticauda, a diplodocid from the Early Cretaceous of Argentina. This picture represents the use of the tail as a whip, beating down theropod predators. © Jorge A. González.

For the herd ! La queue de la conscience spatiale


    Sauropod dinosaurs were gregarious animals. The study of their fossilised trackways (palaeoichnologic realm, the study of fossilised tracks) show that more than being gregarious, some species moved even in herds. Those gatherings were linked to survival habits against predators but also to continual research of food in order to reach their colossal mass and to sustain it [2]. For the safety of the offsprings walking together with adults [5] or to follow distinct diet strategies [6], an age-based segregation of the herd took place in sauropods [5]; given the significant size difference between juveniles and confirmed adults (Fig. 3), accidents between individuals were highly probable (the juveniles would have been todi spotchi as we say in Walloon).


Figure 3. Baby sauropods that just hatched and juvenile ones are way smaller than the adults. Here is depicted a growth series of Rapetosaurus krauseri, showing the tremendous difference between the juvenile and the adult of this sauropod species, based on the scaled outline of the femur. Modified, from [7].

    That being said, the displacement of such enormous animals would need lots of work, energy and mostly constraints. For instance, the leader had to open the pace, change directions when needed, make sure everybody was following the herd and that no one was getting lost, etc., just like today's herds. However, big issue: their gigantic size and their very singular anatomy. The tiniest movement is highly costly from an energetic point of view as well as very slow: sauropods would not have been able to walk faster then some meters per second (which is particularly slow for their size). As a consequence, any slowdown or stop of the herd before an obstacle or to make sure everybody was following by moving the whole body, was a pure waste of energy for each and every member of the herd. This is even enhanced by the lack of mobility of the neck, allowed to move only from its base. This means that one individual could not just turn its head to look behind, but had to completely turnover to do so (moreover, their vision was probably quite limited, from their cranial anatomy). Yet, that individual could not do that while walking because of anatomic reasons... You probably think they were budderballs... *heart broken*


    But how did they coordinate then? How could information went from top to bottom of the herd? This is where Dr. Baron and his interpretation quite original enter the game to explain the majestic tail of sauropods: the tail as an organ of 'spatial awareness'. Thus, the tail worked as an organ that allowed each individual of the herd to bring its position in the overall stream to its mind, but also to communicate between them without displacing the rest of their body (Fig. 4) (see for instance [8]). This sensorial usage of the tail would have been participated to a good performance of the herd, thanks to a more or less continual contact between every sauropod in movement, added to other possible methods (including sonorisations for example).



Figure 4. The tail of the spatial awareness. This hypothesis is depicted here, with a herd of the diplodocid Apatosaurus, including juveniles and adults. In order to know what is going on behind it, the leader ‘checks’ the other sauropods behind it to maintain a continual contact with them, to give directions, and to communicate. From [2].

    The very elongated tail of diplodocids would be, among others, a metronome-like tactile tool dedicated to check regularly the rear environment of the animal but also on its sides in order not to slowdown or turnover. This would limit energetic and time losses for the seek of food of these sauropods. The tail tip would butt against the following sauropds' flanks, replacing the eyes of the individual located just before, and from one sauropod to another, replacing those of the herd's head, leading all the individuals with its tail just like the conductor who leads his/her orchestra with his/her stick.


    This hypothesis of a 'spatial tail' could also ease the group cohesion as well as the smooth motion between the herd's individuals. Given the size difference between them, the possibility that an adult hurt a juvenile badly was a high risk to take into account, the adult failing to actually see the small sauropod in time. And even if adults did not walk per se with younger groups [5], behemoths of same bodymass reaching tens of tons could also hurt each other in case of impact of congeners. As a response, manoeuvring one's tail could thus offer a good feel of the group compaction, as well as the general motion of all the individuals, limiting injuries due to herd desynchronisation during long migrations! The leader had to be a good musician to give the rythm and the tempo to the other sauropods, affraid of playing a wrong note, and wasting someone...



And what about the sort tails?


    Some sauropod species show significantly shorter tails than diplodocids' (however, size does not matter). Actually, it has been observed that short-tailed sauropods had a neck posture quite vertical compared to other clades, notably the horizontally-necked diplodocids (Fig. 5) [9-10]. As a comparison, the tail of Giraffatitan (brachiosaurid) represents around 27% of its total length, whereas the one of Diplodocus represents around 55%! Nonetheless, it is difficult to quantify accurately the link between the neck and the tail because of the general absence of near complete specimens throughout the whole sauropod evolutionary tree.


Figure 5. Comparison of sauropods with different neck postures. A: Diplodocus (diplodocid); B: Camarasaurus (camarasaurid); C: Giraffatitan (brachiosaurid); D: Sauroposeidon (titanosauriform). From [2], adapted from [11].

    It is worth noting that 'vertical' taxa with longer forelimbs than hindlimbs develop more tilted torso in the antero-posterior direction, minimising a counterbalance need. Moreover, the posture difference modifies the anatomic needs such as muscles linking hindlimbs and the tail, but can also facilitate the apprehension of the surroundings because of a better field of view that 'horizontal' sauropods lack. A recent study of Vidal and colleagues [12] would tend to show that such shortened tail would be held closer to the ground, losing its spatial awareness role but developing other abilities needed for 'vertical' sauropods.



An original theory quite unexpected, but is it plausible?


    Should originality of a theory overcome the traditionality of another, more or less accepted by the scientific community? Such question may be asked seriously sooner or later in a scientific career.


Actually, sometimes, you have to dare to think further, to take a different view of a well-knowm issue in order to find out new, fresh hypotheses that can be unusual for sure, but at least sobering. The tactile tail hypothesis is a wonderful example of this. From a beam of information pointing towards a gregarious behaviour of sauropods and mass movements in herds, anatomic properties (neck biomechanics, cranial anatomy, musculoskeletal peculiarities between the forelimbs and the neck, etc.) and observations from today's herds, this hypothesis brings a new answer that is functional what so ever.


    However, one should not go from one extreme to the other. On the one hand, being innovative, a bit creative or even audacious may add value to a study and mostly bring a new vision in a framework that could be, perhaps, too much traditionalist somehow. On the other hand, proposing wacky thesis does not help much. The study of Dr. Baron presented here is remarkable for its modernity in the general framework of the sauropod elongated tail mystery and by leaving almost no room to break down his hypothesis. Anything that could be addressed was, and the whole thing is functional, i.e. this new hypothesis is consistent, or even plausible. Nonetheless, translating theory to practice by proving once and for all this new idea in the fossil record will be a hard task in the future.


My very own understanding of Baron (2020), memefied. © Francisco Gascó.


References


  1. Wedel M.J. & Taylor M.P.0 (2013). Caudal pneumaticity and pneumatic haituses in the sauropod dinosaurs Giraffatitan and Apatosaurus, PLoS One 8, e78213. doi: 10.1371/journal.pone.0078213

  2. Baron M.G. (2020). Tactile tails: a new hypothesis for the function of the elongate tails of diplodocid sauropods, Historical Biology, 1-10. doi: 10.1080/08912963.2020.1769092

  3. Paul G.S. (1998). Terramegathermy and Cope's rule in the land of titans, Modern Geology 23, 179-217.

  4. Preuschoft H. & Klein N. (2013). Torsion and bending in the neck and tail of sauropod dinosaurs and the function of cervical ribs: insights from functional morphology and biomechanics, PLoS One 8, e78574. doi: 10.1371/journal.pone.0078574

  5. Myers T.S. & Fiorillo A.R. (2009). Evidence for gregarious behavior and age segregation in sauropod dinosaurs, Palaeogeography, Palaeoclimatology, Palaeoecology 274, 96-104. doi: 10.1016/j.palaeo.2009.01.002

  6. Reisz R.R., Scott D., Sues H.-D., Evans D.C. & Raath M.A. (2005). Embryos of an Early Jurassic prosauropod dinosaur and their evolutionary significance, Science 309, 761-764. doi: 10.1126/science.1114942

  7. Curry Rogers 2016

  8. Majolo B. & Huang P. (2020). Group living. In: Vonk J., Shackelford T. (Eds). Encyclopedia of animal cognition and behavior. Springer, pp 1-12. doi: 10.1007/978-3-319-47829-6

  9. Gallina P.A. & Otero A. (2009). Anterior caudal transverse processes in sauropod dinosaurs: morphological, phylogenetic and functional aspects, Ameghiniana 46, 165-176.

  10. Ibiricu L.M., Lamanna M.C. & Lacovara K.J. (2013). The influence of caudofemoral musculature on the titanosaurian (Saurischia: Sauropoda) tail skeleton: morphological and phylogenetic implications, Historical Biology 26, 454-471. doi: 10.1080/08912963.2013.787069

  11. McPhee B.W., Mannion P.D. De Klerk W.J. & Choiniere J.N. (2016). High diversity in the sauropod dinosaur fauna of the Lower Cretaceous Kirkwood Formation of South Africa: implications for the Jurassic-Cretaceous transition, Cretaceous Research 59, 228-248. doi: 10.1016/j.cretres.2015.11.006

  12. Vidal L.S., Pereira P.V.L.G.C., Tavares S., Brusatte S.L., Bergqvist L.P. & Candeiro C.R.A. (2020). Investigating the enigmatic Aeolosaurini clade: the caudal biomechanics of Aeolosaurus maximus (Aeolosaurini/Sauropoda) using the neutral pose method and the first case of protonic tail condition in Sauropoda, Historical Biology, 1-21. doi: 10.1080/08912963.2020.1745791


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