Apart from ichthyosaurs, plesiosaurs likely represent the best-known group of Mesozoic marine reptiles. This may partly be related to the fact that they were discussed as potential identification for the Loch Ness monster by cryptozoologists, which of course is total nonsense. The distinctive body plan of plesiosaurs has been compared to a hybrid of a sea snake with a sea turtle. Studies suggest that they had a unique mode of swimming, with two pairs of flippers (Muscutt et al. 2017), but it has been a matter of considerable debate and speculation “whether both flipper pairs were moved up and down in synchrony or in some sort of alternating or asymmetrical gait” (Naish 2017). The most striking trait of plesiosaurs is their extremely elongated neck, which may have been very useful for catching fish. This elongated neck is already present in the older pachypleurosaurs, nothosaurs, and pistosaurs, which represent the assumed stem group from which plesiosaurs are thought to have evolved. They all belong to the 15 groups of marine reptiles that abruptly appeared in the Early Triassic after the Great Dying at the end of the Permian period (Bechly 2023).
Just Allometric Growth?
A new study by Liu et al. (2023), recently published in the journal BMC Ecology and Evolution, found that the neck length of plesiosaur-related pachypleurosaurs increased dramatically and doubled in length within only 5 million years in the Early Triassic (Freedman 2023). At first glance, it may seem that such an elongation of the neck is just allometric growth and therefore not out of the reach of unguided Darwinian mechanisms, but not so fast: indeed the neck of these marine reptiles was not just growing in length, as in giraffes which retain the normal number of vertebrae, but by an extreme multiplication of neck (cervical) vertebrae to as many as 72. This is a very unusual phenomenon and not easy to achieve for mutations without fatal consequences for the organism.
A study on abnormal numbers of cervical vertebrae in humans by Varela-Lasheras et al. (2011) showed that such deviations are generally deleterious. It might be objected that this is only true in mammals, where the number of cervical vertebrae is strictly constrained to seven (the only exceptions are manatees, sloths, lorises, and pottos, where homeotic mutations were responsible; Galis 1999, Böhmer et al. 2018, Galis et al. 2021), but maybe less so in reptiles, where the number of cervical vertebrae is less conserved. However, even in lizards deviations from the usual number of eight cervical vertebrae are extremely rare compared to the considerable individual variability for many other osteological traits (Barbadillo & Barahona 1994). Likewise, the usual number of nine cervical vertebrae in crocodilians was not broken even in the extinct, highly aberrant giant caiman Purussaurus (Scheyer et al. 2019). Obviously, the number of cervical vertebrae is highly conserved in reptiles as well and usually numbers around seven to nine.
Hard to Reconcile with Darwinian Evolution
Consequently, the breaking of the conserved number of cervical vertebrae is hard to reconcile with an unguided evolutionary mechanism, and better explained by intelligent design, which could coordinate changes and avoid deleterious consequences. Also, the very fast growth rate of the neck length exceeds the limitations of population genetics for the accumulation of mutations in a population of marine reptiles within the available window of time of less than 5 million years, which is about the average longevity of a single vertebrate species. This rather suggests coordinated non-random adaptive macromutations as a better explanation, which is my preferred model of the ID mechanism.
What is the evolutionary explanation suggested by the authors of the new study? They simply postulate an extremely rapid rate of change in the time of crisis after the end-Permian mass extinction (Freedman 2023), as if the opening of new niches by itself could create the genetic information required for biological novelty to emerge.
- Barbadillo LJ & Barahona F 1994. The number of cervical vertebrae in lacertid lizards: some unusual cases. Herpetological Journal 4, 1666. https://www.thebhs.org/publications/the-herpetological-journal/volume-4-number-4-october-1994/1382-08-the-number-of-cervical-vertebrae-in-lacertid-lizards-some-unusual-cases
- Bechly G 2023. Fossil Friday: The Triassic Explosion of Marine Reptiles. Evolution News March 31, 2023. https://evolutionnews.org/2023/03/fossil-friday-the-triassic-explosion-of-marine-reptiles/
- Böhmer C; Amson E; Arnold P; van Heteren AH; Nyakatura JA 2018. Homeotic transformations reflect departure from the mammalian ‘rule of seven’ cervical vertebrae in sloths: inferences on the Hox code and morphological modularity of the mammalian neck. BMC Evolutionary Biology 18(1): 84, 1–11. DOI: https://doi.org/10.1186/s12862-018-1202-5
- Freedman E 2023. Ancient sea monsters grew their long necks super fast after Great Dying by adding more vertebrae. LiveScience September 11, 2023. https://www.livescience.com/animals/reptiles/ancient-sea-monsters-grew-their-long-necks-super-fast-after-great-dying-by-adding-more-vertebrae
- Galis F 1999. Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. Journal of Experimental Zoology 285(1), 19–26. DOI: https://doi.org/10.1002/(sici)1097-010x(19990415)285:1<19::aid-jez3>3.0.co;2-z
- Galis F, Van Dooren TJM & van der Geer AAE 2022. Breaking the constraint on the number of cervical vertebrae in mammals: On homeotic transformations in lorises and pottos. Evolution & Development 24(6), 196–210. DOI: https://doi.org/10.1111/ede.12424
- Liu Q-L, Cheng L, Stubbs TL, Moon BC, Benton MJ, Yan C-B & Tian L 2023. Rapid neck elongation in Sauropterygia (Reptilia: Diapsida) revealed by a new basal pachypleurosaur from the Lower Triassic of China. BMC Ecology and Evolution 23: 44, 1–10. DOI: https://doi.org/10.1186/s12862-023-02150-w
- Muscutt LE, Dyke G, Weymouth GD, Naish D, Palmer C & Ganapathisubramani B 2017. The four-flipper swimming method of plesiosaurs enabled efficient and effective locomotion. Proceedings of the Royal Society B: Biological Sciences 284(1861): 20170951, 1–8. DOI: https://doi.org/10.1098/rspb.2017.0951
- Naish D 2017. The Unique and Efficient 4-Flipper Locomotion of Plesiosaurs. Scientific American August 30, 2017. https://blogs.scientificamerican.com/tetrapod-zoology/the-unique-and-efficient-4-flipper-locomotion-of-plesiosaurs/
- Scheyer TM, Hutchinson JR, Strauss O, Delfino M, Carrillo-Briceño JD, Sánchez R & Sánchez-Villagra MR 2019. Giant extinct caiman breaks constraint on the axial skeleton of extant crocodylians. eLife 8: e49972, 1–19. DOI: https://doi.org/10.7554/eLife.49972
- University of Bristol 2023. Plesiosaurs doubled their neck-length by gaining new vertebrae. Press release by the University of Bristol September 4, 2023. https://www.bristol.ac.uk/news/2023/september/plesiosaur-necks.html
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