New insights into the postcranial morphology of Lithornis vulturinus from the Eocene London Clay

This study utilizes high-resolution microCT scanning to redescribe the neotype of *Lithornis vulturinus* and characterize a new, larger specimen from the Eocene London Clay, providing detailed postcranial insights that refine the species' diagnosis, reveal potential sexual dimorphism, and contribute to understanding early palaeognath evolution and the origins of flightlessness.

The Gist

Imagine you are a detective trying to solve a mystery about a family of birds that lived millions of years ago. These birds, called Lithornithids, are the great-great-grandparents of today's ostriches, emus, and kiwis. But here's the twist: unlike their modern, flightless cousins, these ancient birds could fly! They were the "superheroes" of the early bird world, capable of long-distance travel across oceans.

For decades, our understanding of these birds was like trying to solve a puzzle with half the pieces missing. The main clue we had was a fossil called the "neotype" (a replacement for a lost original specimen), but it was stuck inside a hard rock nodule, and many of its bones were broken or hidden.

This paper is the story of how scientists used high-tech "X-ray vision" to finally see the whole picture.

The High-Tech Detective Work

The researchers didn't just chip away at the rock with a hammer (which would destroy the fragile bones). Instead, they used micro-CT scanning, which is like a super-powered medical CT scan for fossils.

• The Neotype (The Old Clue): They scanned the original, famous fossil from the Isle of Sheppey. The scan acted like a digital X-ray, peeling away the layers of rock to reveal bones that were previously invisible. They found hidden vertebrae, ribs, and parts of the shoulder blade that had been covered up for years.
• The New Clue (The Seasalter Specimen): They also scanned a brand-new fossil found just a few miles away in a place called Seasalter. This one was a treasure trove! It contained a nearly complete skeleton, including two perfect shoulder blades and a whole breastbone (sternum) that were missing or broken in the old fossil.

What Did They Learn? (The "Aha!" Moments)

1. The "Hooked" Shoulder
One of the most famous features of these birds is a hook-like bump on their shoulder blade. The old fossil had this broken off, so scientists couldn't be 100% sure. The new Seasalter fossil had this hook perfectly intact. It's like finding a missing puzzle piece that confirms the picture is a bird, not a dinosaur or a reptile.

2. The "Crossed" Breastbone
The most fascinating discovery is about the bird's breastbone (sternum). In most birds, the grooves where the wing muscles attach run straight. But in Lithornithids, these grooves cross over each other like an "X" or a pair of crossed swords.

• The Analogy: Imagine a zipper that crosses over itself in the middle. This is a very rare feature in birds. The paper suggests this might be an ancient trait that the very first modern birds had, which was later lost in most species but kept in these ancient flyers. It's a "fossilized handshake" between ancient and modern birds.

3. The Size Mystery (The "Tall" vs. "Short" Bird)
The new Seasalter bird was about 33% heavier than the old neotype bird.

• The Analogy: Think of it like finding a giant and a small person from the same family living in the same village at the same time.
• The Theory: The scientists suspect this might be sexual dimorphism. In many modern flightless birds (like emus) and their flying cousins (tinamous), the females are often much larger than the males. The researchers think the big bird might be a female and the small one a male. This would be a huge clue about how these birds raised their families and laid eggs.

Why Does This Matter?

1. Rewriting the Family Tree
For a long time, scientists argued about whether these flying birds were the ancestors of the flightless "ratites" (ostriches, etc.) or a separate branch. By looking at the bones in such detail, the study confirms they are indeed a single, unique family (monophyletic) that sits right at the base of the bird family tree.

2. The Flight Connection
These birds are the "missing link" in the story of flight. They show us that the ancestors of the flightless ostriches were actually excellent flyers. It helps us understand how and when birds decided to stop flying and start running. It's like seeing a photo of a whale's ancestor with legs, proving that whales once walked on land.

3. Hunting for Older Fossils
The paper gives scientists a new "cheat sheet" of what to look for. Because we now know exactly what these bones look like (even the hidden ones), paleontologists can go back and look at fossils from the Cretaceous period (before the dinosaurs died out) to see if they missed any ancient Lithornithids hiding in plain sight.

The Bottom Line

This paper is like upgrading from a blurry, black-and-white sketch of an ancient bird to a high-definition, 3D color movie. By using modern technology to look at old rocks, the scientists have filled in the missing pieces of the puzzle, revealing that our ancient bird ancestors were not just flightless runners, but agile, flying travelers with unique body quirks that connect them to the very first birds of all time.

Technical Summary

1. Problem Statement

• Phylogenetic Uncertainty: The evolutionary origins of Palaeognathae (the clade containing flightless ratites and volant tinamous) remain obscure due to a sparse fossil record, particularly regarding the earliest stages of divergence. While molecular data suggests a Cretaceous divergence between Palaeognathae and Neognathae, definitive Mesozoic palaeognath fossils are lacking.
• Taxonomic Ambiguity: Lithornis vulturinus, the type species of the extinct, volant lithornithids, was historically known only from a holotype destroyed in WWII and a neotype (NHMUK A5204) described by Houde (1988). The neotype is incomplete, with many elements obscured by matrix or missing, leading to an incomplete understanding of the species' morphology and its placement within Lithornithidae.
• Systematic Challenges: The monophyly of Lithornithidae and their specific position within Pan-Palaeognathae (stem vs. crown) remain debated. Detailed anatomical descriptions are required to identify potential Cretaceous palaeognaths and clarify the independent loss of flight in Cenozoic palaeognaths.

2. Methodology

• Specimens: The study focuses on two specimens from the early Eocene (Ypresian) London Clay Formation:
  1. The neotype of Lithornis vulturinus (NHMUK A5204) from the Isle of Sheppey.
  2. A newly discovered, nearly complete postcranial skeleton from a clay nodule in Seasalter, Kent.
• Imaging: High-resolution micro-CT scanning was employed to digitally extract bones obscured by the phosphate/clay matrix.
  • Scanning: Performed on Nikon Metrology XT H 225 ST scanners at the Natural History Museum (London) and Cambridge Biotomography Centre.
  • Processing: Digital segmentation and 3D rendering were conducted using VGSTUDIO MAX 3.4 to visualize internal structures and separate bones from the matrix.
• Comparative Anatomy: The authors provided a comprehensive redescription of the neotype and a full description of the Seasalter specimen, comparing them against other lithornithids (e.g., MGUH 26770, Calciavis grandei) and extant tinamous (Eudromia elegans).
• Phylogenetic Analysis:
  • The new specimens were scored into the morphological data matrix of Nesbitt and Clarke (2016).
  • Analyses: Both Maximum Parsimony (TNT) and Bayesian Inference (MrBayes) were performed.
  • Constraints: Analyses were run both unconstrained and with constraints based on recent molecular phylogenies (e.g., Phillips et al., 2009; Prum et al., 2015) and specific constraints placing Lithornithidae as the sister to all other palaeognaths.
• Body Mass Estimation: Body mass was estimated using equations from Field et al. (2013) based on various skeletal correlates (e.g., humeral shaft circumference, coracoid articular facet diameter).

3. Key Contributions

• Redescription of the Neotype: The study reveals previously hidden anatomical details of the L. vulturinus neotype, including partial vertebrae, scapulae, and the sternum, allowing for a revised differential diagnosis.
• Description of a New Specimen: The Seasalter specimen provides the first nearly complete, three-dimensionally preserved postcranial skeleton of L. vulturinus, including undamaged scapulae with the diagnostic hooked acromion, complete coracoids, and a nearly complete sternum.
• Revised Differential Diagnosis: The authors propose new diagnostic features for L. vulturinus, including a proximally directed lateral process of the coracoid, caudally divergent lateral margins of the sternum, and an arcuate deltopectoral crest.
• Identification of Asymmetry: The study highlights a unique, non-pathological bilateral skeletal asymmetry in the sternum where the coracoidal sulci cross at the midline (left sulcus dorsal to the right), a feature likely ancestral for crown birds but rare in extant taxa.

4. Results

• Taxonomic Referral: The Seasalter specimen is confidently referred to Lithornis vulturinus based on shared apomorphies (hooked acromion, crossed sternal sulci, specific humeral and tarsometatarsal features) and geographic proximity to the neotype.
• Morphological Insights:
  • Flight Adaptations: The humeri are markedly sigmoid and elongated, and the sternum lacks the caudal elongation and lateral trabeculae seen in tinamous. This suggests L. vulturinus was adapted for sustained, long-distance flapping flight, unlike the burst-flight capabilities of modern tinamous.
  • Ground Foraging: The asulcate (lacking canals) hypotarsus and bill morphology support the hypothesis that lithornithids were ground-dwelling probe feeders.
  • Body Size Variation: The Seasalter specimen is estimated to be ~33% heavier than the neotype (mean estimate ~1487g vs. ~956g) and significantly larger than the smaller specimen MGUH 26770.
• Phylogenetic Position:
  • Monophyly: Lithornithidae is recovered as a monophyletic group in all analyses.
  • Internal Resolution: Resolution within the clade is generally poor (polytomies), though Bayesian analyses occasionally recover L. celetius + L. plebius and Calciavis + Pseudocrypturus as sister clades.
  • Stem vs. Crown: When unconstrained, Lithornithidae often groups as sister to Tinamidae. When constrained by molecular topologies, they frequently appear as stem palaeognaths, though the exact position remains sensitive to the constraints applied.
• Sexual Dimorphism: The significant size difference between the Seasalter specimen and the neotype, combined with the known reverse sexual dimorphism in extant tinamous (females larger than males), suggests the size variation in L. vulturinus may reflect sexual dimorphism rather than distinct species.

5. Significance

• Paleobiology: The study confirms that early Cenozoic palaeognaths were volant and capable of long-distance dispersal, challenging vicariant speciation models and supporting a scenario of overseas dispersal followed by independent flight loss.
• Fossil Identification: The detailed anatomical descriptions and identification of key synapomorphies (e.g., crossed sternal sulci, specific coracoid features) provide a critical toolkit for identifying potential Cretaceous palaeognath fossils that may have been previously overlooked or misidentified.
• Evolutionary History: The findings contribute to understanding the transition from volant ancestors to flightless ratites and the evolution of the palaeognathous palate and postcranial skeleton. The identification of crossed sternal sulci as a potential ancestral trait for crown birds offers new insights into the developmental and functional evolution of the avian pectoral girdle.
• Future Directions: The paper underscores the need for histological analysis of smaller specimens (like MGUH 26770) to confirm ontogenetic stage and further investigation into sexual dimorphism within lithornithids.