Fossil bone histology reveals ancient origins for rapid juvenile growth in tetrapods

The rate of animal growth is a crucial life history trait, influencing factors like reproductive timing, physiology, and ecological interactions. Traditionally, the ancestral tetrapod growth pattern was considered slow-to-moderate, similar to modern amphibians. Faster growth and high metabolic rates were believed to be a specialized amniote adaptation. This study challenges that view by examining the bone histology of *Whatcheeria deltae*, an Early Carboniferous stem tetrapod. Understanding *Whatcheeria*'s growth pattern is vital because it lived during the early diversification of tetrapods, a critical period in vertebrate evolution. By analyzing bone histology, a key proxy for somatic growth rates, the researchers aimed to determine whether *Whatcheeria* exhibited rapid growth and if this trait is unique to specific lineages or indicative of a broader pattern in early tetrapod evolution. This research directly addresses the question of the ancestral growth pattern in tetrapods and its potential influence on their successful terrestrial colonization.

Previous studies have documented varying growth rates among modern and extinct vertebrates. Birds and mammals exhibit the highest growth rates, facilitated by elevated metabolic activity. This rapid growth was once thought to be restricted to crown mammals and birds, considered a derived amniote adaptation. However, bone histological analyses of fossils have expanded the phylogenetic scope of elevated growth rates, revealing its presence in various taxa including dinosaurs, pterosaurs, and some synapsids. Lissamphibians and other stem tetrapods generally show slower growth patterns, raising questions about the evolutionary history and distribution of rapid growth within the tetrapod lineage. The lack of detailed ontogenetic information in fossil species poses a challenge to interpreting growth patterns, highlighting the need for histological analysis of comprehensive ontogenetic series. The study of *Whatcheeria* is significant because it potentially bridges the gap between the presumed slow-growing early tetrapods and the faster-growing amniotes, providing critical insights into the evolution of growth strategies within the tetrapod clade.

The researchers examined nine *Whatcheeria deltae* femora representing a range of size classes using paleohistological and micro-computed tomography (µCT) techniques. µCT scanning was used to create three-dimensional models of the bones and to guide the selection of regions for thin sectioning. Four specimens, representing each size class, were chosen for destructive thin sectioning. Standard paleohistological techniques were then used to analyze the bone tissues. Transverse thin sections were prepared at the mid-diaphysis of each femur, as this region provides the longest record of bone growth. ImageJ software was used to measure the cross-sectional and cortical bone areas to determine the proportion of cortical bone in each specimen. The researchers observed and categorized the various bone tissues (fibrolamellar, parallel-fibered, lamellar) present, noting the organization, vascularity, and presence of remodeling. This meticulous examination across multiple size classes provided critical ontogenetic information, allowing for analysis of growth patterns from juvenile to adult stages.

The histological analysis revealed the presence of fibro-lamellar bone in the smallest *Whatcheeria* specimens (size class I), a tissue type strongly associated with rapid growth in vertebrates. This fibro-lamellar bone was subsequently remodeled during growth, with later-stage specimens (size classes II-IV) exhibiting predominantly parallel-fibered and lamellar bone, indicative of slower growth rates. The shift from fibro-lamellar to lamellar bone across the size classes suggests a deceleration of growth as *Whatcheeria* matured. The proportion of cortical bone also decreased with increasing size, further supporting this observation. The researchers observed two chronic changes in *Whatcheeria* femoral histology: a reduction in cortical thickness and decreased bone deposition rates with increasing size. The presence of fibro-lamellar bone in the juvenile stages of *Whatcheeria* marks a remarkably early occurrence of this tissue type in the tetrapod lineage, both temporally and phylogenetically. The detailed description of bone histology in each size class (I-IV) provided a comprehensive picture of bone tissue changes throughout *Whatcheeria*'s ontogeny. Specifically, it was observed that the fibrolamellar bone in the smallest specimen represented the earliest stages of growth, which were later remodeled as the animal matured. This finding highlights the importance of examining a range of size classes to understand the complete growth trajectory of a fossil species.

The discovery of fibro-lamellar bone in *Whatcheeria* challenges the traditional view that fast juvenile growth is a derived characteristic of amniotes. It demonstrates that elevated growth rates were present in stem tetrapods much earlier than previously assumed, extending the temporal and phylogenetic range of this adaptation. The rapid growth in *Whatcheeria* likely conferred significant ecological advantages, allowing individuals to attain a larger body size and reach reproductive maturity more quickly. This would have enabled them to compete effectively for resources and occupy a large-bodied predator niche in their paleoenvironment. The contrast between *Whatcheeria*'s rapid growth and the slower growth documented in other stem tetrapods such as *Greererpeton* suggests divergent ecological roles and life history strategies. The observed remodeling of juvenile bone tissue underscores the dynamic nature of bone and the importance of examining ontogenetic series to avoid misinterpretations of growth patterns. The study raises uncertainties about using exclusively the slow-growing Devonian stem tetrapods as models for early terrestrial adaptations.

This study provides compelling evidence for rapid juvenile growth in the Early Carboniferous stem tetrapod *Whatcheeria deltae*, significantly expanding the known temporal and phylogenetic range of this life history strategy. The discovery of fibro-lamellar bone in *Whatcheeria* suggests that elevated juvenile growth is a much older trait than previously appreciated and was likely an important factor in early tetrapod evolution and terrestrialization. Future research could focus on expanding the histological analysis to other stem tetrapods to test the generality of these findings and to further investigate the evolutionary drivers of this important life history trait.

The study focuses solely on femoral bone histology. Examining other skeletal elements could provide a more complete picture of *Whatcheeria*'s growth pattern. The absence of younger juvenile specimens might bias the interpretation of early ontogenetic stages. Taphonomic processes could have selectively preserved larger individuals, potentially leading to an incomplete representation of the full size range. Future research should strive to obtain a more complete ontogenetic series to clarify early growth patterns.