SVP abstract 20: Squamate variability within a single species

Petermann and Gauthier 2020 bring us their views on the 
“potential consequences of our inability to assess intraspecific variability in growth rates.”

From the Petermann and Gauthier abstract:
“An investigation of life-history parameters in the extant iguanian lizard Sauromalus ater (the Common Chuckwalla), a sexually dimorphic species from the SW U.S.A., revealed remarkable intraspecific variability.”

“We found expected differences in growth strategies between males and females, but also within each sex, relating to body size and the timing of sexual maturity. Males and females can grow rapidly to size-at-sexual-maturity, producing above-average adult body sizes. Or, they can grow slowly to size-at-sexual-maturity, yielding adults at or below average body sizes. Neither growth strategy influences longevity. As a result, we found that body size of similar-aged individuals varied by 53% for males and 38% for females, and maximum differences in ‘adults’ of 64% for males and 38% for females.”

Further ranging results were found here earlier in the large pterosaur tree (LPT, 251 taxa) for the lepidosaur pterosaurs, Pteranodon (Fig. 1) and Rhamphorhynchus (Fig. 2). These both became fully resolved in phylogenetic analysis.

Figure 2. The DMNH specimen is in color, nesting between the short crest KS specimen and the long crest AMNH specimen.

Figure 2. The DMNH specimen is in color, nesting between the short crest KS specimen and the long crest AMNH specimen.

Figure 2. Rhamphorhynchus specimens to scale. The Lauer Collection specimen would precede the Limhoff specimen on the second row.

Figure 2. Rhamphorhynchus specimens to scale. The Lauer Collection specimen would precede the Limhoff specimen on the second row.

Continuing from the Petermann and Gauthier abstract:
“Our results add to previous reports of intraspecific variability in extant and extinct vertebrates. High levels of intraspecific size-variability have multiple implications for vertebrate paleontology.

  1. Morphologically similar specimens from the same locality could belong to the same species even if the size difference among adult individuals exceeds 50%, which is a higher level than previously thought.
  2. Specimens that have been analyzed skeletochronologically and have been found to be similar or identical in chronological age, may not exhibit similar sizes.
  3. Variability in growth strategies may lead to mistaking males and females (especially among sexual dimorphs), or individuals using different growth strategies, as belonging to separate species.”

This is the way evolution works in all vertebrate communities, including humans, where some are taller, some are robust, some are more colorful or sexier, some are brilliant, distinct from the others. In both Rhamphorhynchus and Pteranodon, no two specimens are alike.

“We previously presented evidence that a sequence of sub-terminal skeletal suture fusions relates to maximum body size in squamates, and not to chronological age. This indicates that late-ontogenetic, suture-fusion events could be used to evaluate whether two or more specimens of similar morphology and chronological age are differently-sized conspecifics. Likewise, skeletal suture fusions may aid discerning different growth strategies within a single species, as opposed to the presence of two morphologically similar, but nonetheless separate, species in a single taphonomic assemblage.”

This follows the work of Maisano 2002, who found fusion patterns were phylogenetic in lepidosaurs. a pattern continued in pterosaurs, where fusion patterns are also phylogenetic, distinct form archosaur growth patterns.


References
Maisano JA 2002. 
Terminal fusions of skeletal elements as indicators of maturity in squamates. Journal of Vertebrae Paleontology 22: 268–275.
Petermann H and Gauthier JA 2020. Intrespecific variability in an extant squamate and its implications for use in skeletochronology in extinct vertebrates. SVP abstracts 2020.

 

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