Rosa Rubicondior: Refuting Creationism - Jaw-Dropping Evolution in Reptiles

December: Jaw evolution in lizards and snakes | News and features | University of Bristol

A research team from Bristol University, UK with Professor Anthony Herrel of the Mécanismes Adaptatifs et Evolution, Muséum national d'Histoire naturelle, Paris, France has recently published a paper showing the close link between the lower jaw shape of lizards and snakes, known collectively as the lepidosaurs, and their ecological niche as well as factors such as their phylogeny and scaling for body size (allometry).

They found that evolution was rapid in specialised groups such as burrowing and aquatic species.

What information do you have on the evolution of the lower jaw in the lepidosaurs and in particular the genetic control of lower jaw morphology in this group? The evolution of the lower jaw in lepidosaurs and the genetic control of its morphology provide fascinating insights into the group's adaptation and diversity. Here's an overview of key aspects related to this topic:

Evolution of the Lower Jaw in Lepidosaurs

General Morphology:

Lepidosaurs, which include lizards, snakes, and tuataras, exhibit considerable diversity in lower jaw morphology. This diversity is tied to their varied feeding strategies, ecological niches, and evolutionary history.
The lower jaw of lepidosaurs is typically composed of multiple bones, including the dentary (the main tooth-bearing bone), surangular, angular, splenial, and coronoid, among others.
In snakes, the lower jaw has become highly modified for extreme kinesis (flexibility), including the loss of the mandibular symphysis (fusion point between left and right dentary bones), enabling them to consume large prey.

Ancestral Features:

Fossil evidence suggests that early lepidosaurs (dating back to the Triassic) had relatively robust and less kinetic jaws compared to modern species. These ancestral forms were likely generalist feeders.
Over time, selective pressures related to diet and predation led to adaptations such as increased jaw kinesis in some lineages (e.g., snakes) and more robust, crushing jaws in others (e.g., herbivorous lizards).

Functional Adaptations:

Carnivorous lepidosaurs typically show elongated dentaries and sharp teeth for slicing flesh.
Herbivorous and omnivorous species may exhibit flatter teeth and more robust jaws adapted for grinding plant material.

Genetic Control of Lower Jaw Morphology

Developmental Genes:

The development of the lower jaw in lepidosaurs, as in other vertebrates, is regulated by conserved genetic pathways, including:

Hox genes: These play a role in patterning the craniofacial structures, including the jaws.
Bmp (Bone morphogenetic proteins) and Shh (Sonic hedgehog): These signalling molecules are critical for jaw development, influencing the growth and differentiation of craniofacial mesenchyme and epithelial cells.
Dlx genes: These homeobox genes are essential for mandibular arch patterning. They show distinct expression patterns that influence the development of specific jaw components.

Evolutionary Modifications:

Changes in the regulation of these genes are hypothesized to underlie morphological variation in the lower jaws of lepidosaurs. For example:

Alterations in Shh expression can lead to variations in jaw size and shape.
Modifications in Dlx gene activity might explain differences in the development of the dentary and post-dentary bones.

Genetic Basis of Jaw Kinesis:

In snakes, the evolution of extreme jaw kinesis involves both morphological changes (e.g., loss of the mandibular symphysis) and underlying genetic changes that impact jaw ligament and bone development.
Studies of Runx2, a gene involved in bone formation, suggest its role in regulating the relative flexibility and strength of jaw bones.

Epigenetic and Environmental Influences:

In addition to genetic factors, environmental factors and mechanical loading during development can influence jaw morphology, highlighting the interplay between genetics and functional demands.

Research Gaps and Future Directions

While the genetic pathways controlling lower jaw development are broadly conserved across vertebrates, lepidosaur-specific studies remain limited. Comparative genomic and transcriptomic analyses in lizards, snakes, and tuataras could provide deeper insights.
Fossil evidence combined with molecular developmental data can help reconstruct the evolutionary trajectory of jaw morphology in this group.

New research unlocks jaw-dropping evolution of lizards and snakes
A ground-breaking University of Bristol study has shed light on how lizards and snakes -the most diverse group of land vertebrates with nearly 12,000 species - have evolved remarkably varied jaw shapes, driving their extraordinary ecological success.
This research, led by a team of evolutionary biologists and published in the Proceedings of the Royal Society B today, offers a new understanding of the intricate factors influencing the evolution of lower jaw morphology in these animals, known collectively as lepidosaurs.

The researchers discovered that jaw shape evolution in lepidosaurs is influenced by a complex interplay of factors beyond ecology, including phylogeny (evolutionary relatedness) and allometry (the scaling of shape with size).

In terms of jaw shape, the team found that snakes are morphological outliers, exhibiting unique jaw morphologies, likely due to their highly flexible skulls and extreme mechanics that enable them to swallow prey many times larger than their heads.

Interestingly, we found that jaw shape evolves particularly fast in ecologically specialised groups, such as in burrowing and aquatic species, and in herbivorous lizards, suggesting that evolutionary innovation in the lower jaw was key to achieve these unique ecologies. Our study shows how lizards and snakes evolved their disparate jaw shapes which adapted to their wide range of ecologies, diets, and habitats, driving their extraordinary diversity.

Dr. Antonio Ballell Mayoral, lead author
Bristol Palaeobiology Group
School of Earth Sciences University of Bristol, Bristol, UK.
This work underscores the critical role of morphological innovation in promoting the diversification of highly biodiverse groups like lepidosaurs. The lower jaw - a vital component of the vertebrate feeding apparatus - has been a key element in their ecological experimentation and adaptation.

Looking ahead, the team plans to delve deeper into the evolution of the lepidosaur head.

Lower jaws are important, but they work together with the jaw closing muscles to support essential functions like feeding and defence. We are exploring the relationship between skull shape and the arrangement of the jaw closing musculature through evolution, and how it has impacted the diversification of feeding mechanics and habits.

Dr. Antonio Ballell Mayoral.
Publication:

Antonio Ballell, et al
Ecological drivers of jaw morphological evolution in lepidosaurs
Proceedings of the Royal Society B 291(2036). DOI: 10.1098/rspb.2024.2052

Abstract
Ecology is a key driver of morphological evolution during adaptive radiations, but alternative factors like phylogeny and allometry can have a strong influence on morphology. Lepidosaurs, the most diverse clade of tetrapods, including lizards and snakes, have evolved a remarkable variety of forms and adapted to disparate ecological niches, representing an ideal case study to understand drivers of morphological evolution. Here, we quantify morphological variation in the lower jaw using three-dimensional geometric morphometrics on a broad sample of 153 lepidosaur species. Our results suggest that phylogeny has significantly influenced mandibular shape evolution, and snakes have diverged from a lizard-like jaw morphology during their evolution. Allometry and ecological factors like diet, foraging mode and substrate also appear to drive the diversification of mandibular forms. Ecological groups differ in patterns of disparity, convergence and rates of evolution, indicating that divergent evolutionary mechanisms are responsible for the acquisition of different diets and habitats. Our analyses support that lepidosaurs ancestrally use their jaws to capture prey, contrary to the traditional view favouring lingual prehension as ancestral. Specialized or ecologically diverse lineages show high rates of jaw shape evolution, suggesting that morphological innovation in the mandible has contributed to the spectacular ecomorphological diversification of lepidosaurs.

1. Introduction
Phenotypic diversity in unequally distributed throughout the tree of life. During evolutionary radiations, clades explore different areas and extent of morphological space during the conquest of new ecological niches. Thus, ecological opportunity drives morphological diversification through the process of adaptation [1]. However, phylogenetic history and developmental processes may impose additional controls on the evolution of form [2–4]. The relative importance of each of these factors in the evolution of morphology, and the degree of interaction between them, is a major question in evolutionary biology, and appears to vary depending on the biological system and taxonomical group of study (e.g. [5–7]). Thus, identifying the evolutionary patterns and drivers of morphological diversification within and across lineages is fundamental to understand how clades radiate and biodiversity is generated.

Lepidosaurs are the clade of diapsids comprising lizards, snakes and the tuatara, and with over 11 000 species, represent the most speciose group of tetrapods today [8]. Since their origin at more than 240 Ma [9], lepidosaurs have diversified into a myriad of sizes and body plans, expressed in a remarkable disparity of cranial and postcranial morphologies. Among living species, the range in body size spans three orders of magnitude, as exemplified by the approximately 17 mm long Sphaerodactylus geckos and the approximately 10 m long green anaconda [10]. Extremes in large body size become even more dramatic when extinct mosasaurs are considered (up to 17 m in length; [11]). Disparity in body form is reflected in the different degrees of body elongation, and reduction or modification of limb elements seen in multiple lineages, with snake-like body plans evolving at least 25 independent times [12]. Similarly, lepidosaurs show a rich variety in skull configurations [13,14] shaped by the loss and gain of skull bones during their evolutionary history [15,16], and the acquisition of different kinds and degrees of cranial kinesis [17]. As a result of this diversification of forms, lepidosaurs have conquered diverse ecological niches across most of the globe [18,19]. In the terrestrial realm, lepidosaurs inhabit diverse microhabitats on the ground, in trees, on rocks and in crevices [20,21], while several lineages of lizards and snakes have independently evolved fossorial lifestyles, usually associated with limb reduction and body elongation [22]. Moreover, various groups have independently acquired semiaquatic habits [23], while a few snake lineages and the extinct mosasaurs adapted to aquatic environments [11,24]. Diet is another aspect of ecology that has played a major role in the diversification of lepidosaurs [18,25,26]. Living species show an outstanding breadth of feeding habits, comprising predators feeding on vertebrate (carnivores) and invertebrate prey (insectivores), and species that feed partly (omnivores) or strictly (herbivores) on plant matter [27,28]. This striking diversity of forms and ecologies makes lepidosaurs an exceptional model system to understand the drivers of morphological evolution and biodiversity.

The skull is a particularly interesting system to decipher the ecological drivers of morphological evolution, since it is involved in multiple functions like feeding, locomotion and defence [29]. In lepidosaurs, the relationship between skull morphology and ecology has been investigated, revealing that both ecological aspects such as diet or habitat, and other factors like phylogeny, allometry and heterochrony, have shaped the evolution of this structure [7,30–34]. Surprisingly, the mandible has received much less attention than the cranium. Despite being historically assumed to be more tightly linked to feeding [35], a two-dimensional morphometric study of lower jaw shape in lizards suggested that its relationship with diet is weak [31]. This may be because the tongue has a central role in food acquisition and processing, as lepidosaurs are the clade of amniotes that shows the greatest diversity in tongue morphology and function [36,37]. Prey capture is achieved with either the tongue, the jaws or both, and the type of prehension used varies among clades. Iguanian lizards use lingual (i.e. tongue) prehension, while most other squamates, including snakes, capture prey with their jaws [18,36–38]. Sphenodon primarily uses lingual prehension, but it catches large prey with the jaws [37,39]. This led to the interpretation of lingual prehension as the ancestral condition of lepidosaurs and squamates [18]. However, this hypothesis has more recently been undermined by the rejection of an early diverging position for Iguania in all recent phylogenies of Squamata [40–42], and the discovery of species that use both types of prehension within clades that were considered exclusively jaw feeders [43–45], rendering the evolutionary pattern of prehension in lepidosaurs unclear. Moreover, the degree to which prehension mechanism and other aspects of ecology such as diet or habitat are related to the morphological evolution of the lower jaw remains elusive.

In this study, we present the first broadscale quantification of mandibular shape variation across all major clades of living lepidosaurs (153 extant species), and one of the very few studies of this kind in a major vertebrate clade [46,47]. For the first time to our knowledge, this study quantifies the significance of multiple potential drivers of morphological evolution (prehension, foraging mode, substrate, dietary ecology, phylogeny and allometry) using three-dimensional geometric morphometrics and phylogenetic comparative methods. We test the hypothesis that diet and ecology are the main drivers of jaw shape evolution, and explore patterns of disparity, convergence and evolutionary rates of jaw morphology among ecological groups. We also estimate the tempo of mandibular evolution in Lepidosauria, predicting that high evolutionary rates are related to ecological innovation. Finally, we reconstruct the evolution of prehension mechanisms to test the hypothesis that prey capture was ancestrally performed via lingual prehension in lepidosaurs.

Figure 1. Phylomorphospace of lower jaw shape in Lepidosauria. Top panel shows the PC1–PC2 phylomorphospace; bottom panel shows the PC1–PC3 phylomorphospace. Extreme jaw morphologies are represented at the positive and negative ends of the three principal coordinate (PC) axes. Convex hulls indicate different lepidosaur clades. Within Serpentes, dashed hulls represent Colubroidea, and dotted hulls represent Viperidae. Silhouettes from PhyloPic.org.

Figure 2. Phylogenetic regression of allometry of jaw shape on size. Regression lines represent allometric trends in lizards and snakes. Within Serpentes, dashed hulls represent Colubroidea, and dotted hulls represent Viperidae. Silhouettes from PhyloPic.org.

Figure 4. Rates of evolution of mandibular morphology in Lepidosauria. Branch-specific evolutionary rates (log-transformed) represented by a colour gradient, where red represents high rates and blue, low rates.

Figure 5. Ancestral state reconstruction of prehension mechanisms. Pie charts represent the probability of each character state at each node, as the consensus of a leave-one-out cross-validation on four different models of character state transition. Abbreviations: Ig, Iguania; Le, Lepidosauria; Sq, Squamata; To, Toxicofera. Prehension mode icons modified from [18].

The only way creationists can explain evidence such as this that an entire order evolved in response to environmental pressures and ecological opportunities is to assume their allegedly all-loving god created the evidence to mislead them - which is a strange definition of all-loving. Or maybe creationists would see nothing wrong with lying to their children as some sort of test so their children could prove they loved them - with the threat of eternal pain and suffering if they failed the test.

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