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Anatomy and evolution of the Galápagos marine iguana, Amblyrhynchus cristatus (Squamata, Iguanidae), with a new phylogeny of Iguania and considerations about aquatic adaptations in extant and fossil lizards
- Author / Creator
- Paparella, Ilaria
Amblyrhynchus cristatus, the marine iguana, is unique amongst the ~7000 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 behaviour, 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. Here I present a comprehensive anatomical description and review of the skeletal anatomy of Amblyrhynchus in order to perform a revision of the morphological characters used to assess phylogenetic relationships across iguanians and to investigate its origins in the larger context of iguanian evolution.
Iguanian lizards are a highly diverse clade of squamates that, in addition to the common iguanas, include also anoles, dragon lizards, and chamaeleons. They are roughly divided into two main lineages, the Acrodonta and Pleurodonta, and represented by over 2000 living species and a fairly extensive fossil record, with the earliest undisputed iguanians known from the early Late Cretaceous of Gondwana. In this study, I perform a new phylogenetic analysis of Iguania based on a combined dataset of morphological and molecular data. I include representatives from all modern clades as well as the largest sampling of fossil iguanians ever tested in a phylogeny before. I analysed the data primarily using Bayesian inference methods and performing both calibrated and uncalibrated analyses of the combined and separate data matrices.
With the new phylogenetic hypothesis presented here, I was able to revise long-standing issues in the classification of Iguania, to propose a new taxonomic scheme that better encompasses the diversity of the iguanian fossil record, and to address questions about the evolutionary and biogeographic history of both crown and fossil lineages. For example, I suggest limiting the use of the high-level taxa Acrodonta and Pleurodonta to crown lineages, and to account for the sister-group relationship between Pleurodonta and its fossil relatives, I established the new clade Iguaniformes, in parallel to the existing Chamaeleontiformes that includes Acrodonta and the fossil clade Priscagamidae. Acrodonta and Pleurodonta were defined based on the macroscopic differences in tooth-jaw geometries found between most of the members of the two groups. Acrodont implantation, used to indicate that the teeth are positioned apically along the jaw bone, appears in most crown chamaeleontiforms; pleurodont implantation, which refers to the teeth being located on the lingual side of the jaw, characterises all iguaniforms and some chamaeleontiforms. However, recent studies on amniote dental anatomy show how superficial this dualistic interpretation can be. With my revised morphological characters, I shift the attention to the single features that contribute to determine an overall acrodont or pleurodont appearance, and provide a re-interpretation of these conditions in living and fossil iguanians based on the new knowledge.
Finally, I discuss the origins and radiation of the Galápagos iguanas and other similar cases of disjointed distribution within Iguania. I argue that iguanids may have colonized ancient Galápagos Islands as long as 20-25 Ma via a dispersal event from the Caribbean plate. The initial dispersal was followed by constant short-range hopping from older to newer islands that continued to emerge during the activity of the Galápagos Hotspot, until the archipelago reached its current position. This scenario is consistent with both the sister-relationships of the Galápagos iguanids and the unusually old divergence time estimates that are persistently inferred in phylogenetic studies. Persistence of long-lasting lineages rather than more recent dispersal events is presented as the most plausible explanation also for the presence of iguaniform oplurids in Madagascar and the iguanid Brachylophus in the Fiji and Tonga Archipelagos.
- Graduation date
- Fall 2021
- Type of Item
- Doctor of Philosophy
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