Cranial anatomy of the Galápagos marine iguana Amblyrhynchus cristatus (Squamata: Iguanidae)

Abstract

Amblyrhynchus cristatus, the marine iguana, is unique among the ~7,000 species of living limbed lizards as it has successfully evolved adaptations that allow it to live in both terrestrial and marine environments. This species is endemic to the Galápagos Archipelago and has evolved a specialized feeding behavior, consuming primarily the algae that grow on the rocky seafloor. The intriguing questions arising around the evolution of the marine iguana concerns the use of exaptations of terrestrial features for aquatic and specifically marine adaptations. However, the lack of fundamental information about its anatomy currently prevents us from understanding how it became adapted to such a peculiar lifestyle in comparison to all other iguanids. The goal of this study is to provide the first ever description of the skull, mandible, and hyoid of Amblyrhynchus. We examined several specimens of marine iguana, including skeletal, wet, and ct‐scanned material, and individuals at different ontogenetic stages. We also analyzed specimens of all other modern iguanid genera (Conolophus, Iguana, Ctenosaura, Cyclura, Dipsosaurus, Brachylophus, Sauromalus) in order to make comparisons between Amblyrhynchus and its closest relatives. We were able to identify several autapomorphic features that distinguish the marine iguana from all other iguanids. These unique morphologies are mostly associated with the modified configuration of the snout (nasal chamber), increased muscle attachments in the temporal‐postorbital region of the skull, and dentition. Since Amblyrhynchus is the only nonophidian squamate currently able to exploit the ocean at least for some vital functions (i.e., feeding), we used comparisons to fossil marine lizards (e.g., mosasaurids) to discuss some of these unique traits. The new cranial features described for Amblyrhynchus may represent a source of novel morphological characters for use in future phylogenetic analyses of iguanian (or squamate) relationships, which will then serve as the foundation for the exploration of evolutionary patterns and processes that led to the development of such unique adaptations.

A Palaeogene stem crotaphytid ( Aciprion formosum ) and the phylogenetic affinities of early fossil pleurodontan iguanians

“…Regardless, the number of extinct taxa included in the present study is nearly or more than double that of most published studies that included extinct iguanians (e.g. [17,21,45]) and comparable to a recent study [22] that focused more on fossil acrodontans.…”

“…present study and in future studies. Second, the matrix will be used in the future for combined-evidence analyses (both calibrated and uncalibrated) using Bayesian inference-I did not attempt to create a new or heavily revised morphological matrix (for a recent character matrix, see [22]) or make inferences about iguanian relationships based on unconstrained analyses, nor did I perform parsimony analyses.…”

“…First, an updated matrix serves to place Aciprion formosum AMNH FR 11400 and other fossil iguanians (particularly pleurodontans) in the present study and in future studies. Second, the matrix will be used in the future for combined-evidence analyses (both calibrated and uncalibrated) using Bayesian inference—I did not attempt to create a new or heavily revised morphological matrix (for a recent character matrix, see [ 22 ]) or make inferences about iguanian relationships based on unconstrained analyses, nor did I perform parsimony analyses. Regardless, the number of extinct taxa included in the present study is nearly or more than double that of most published studies that included extinct iguanians (e.g.…”

“…The fossil described here (AMNH FR 11400) has not been formally described, although it is probably the most complete known specimen referred to Aciprion formosum [ 21 ]. Aciprion formosum was included in several phylogenetic analyses of squamate reptiles [ 10 , 16 , 21 , 36 , 44 ] and in analyses of iguanian or pleurodontan relationships [ 8 , 22 , 23 , 45 ]. Several of those studies (e.g.…”

“…Fittingly, there has been practically as much disagreement about the phylogenetic position of Aciprion formosum as there has been about the intrarelationships of Pleurodonta itself. Aciprion formosum has been placed in a polytomy at the crown-Pleurodonta node [ 21 , 22 , 36 , 45 ], as sister to pleurodontans excluding Crotaphytidae, Corytophanidae, Opluridae, Anolidae (see [ 46 ] for discussion of this name) and Polychrotidae [ 22 ], as sister to pleurodontans excluding Crotaphytidae, Corytophanidae, Iguanidae and Hoplocercidae [ 22 ], as sister to Phrynosoma platyrhinos [ 21 ], as a stem hoplocercid [ 8 , 10 , 23 , 44 ], as a stem member of the clade ((Polychrotidae, Corytophanidae), Hoplocercidae) [ 36 ], as a stem crotaphytid [ 22 ], or as a stem member of the clade ((Crotaphytidae, Leiocephalidae), Corytophanidae) [ 23 ]. Aciprion formosum was also previously considered to be a ‘messelosaurine', a hypothesized clade of extinct iguanians mostly composed of fossil pleurodontans from Europe that was reported to be closely related to Corytophanidae (basilisk lizards and relatives) (Rossman [ 47 , 48 ]).…”