The story of how ancient crocodiles conquered a continent, diversified in spectacular ways, and ultimately vanished, leaving only shadows of their former glory.
When we picture crocodiles today, we imagine formidable predators lurking in waterways, seemingly unchanged since the age of dinosaurs. This popular image, however, hides a far more dramatic evolutionary story. Recent paleontological discoveries have revealed that Australia and its surrounding regions were once home to an astonishing array of crocodyliforms that defied our modern expectations.
Their 113-million-year history in Australasia—filled with migrations, diversifications, and eventual extinctions—is only now coming to light, challenging everything we thought we knew about these ancient archosaurs.
For much of the Cenozoic Era (the past 66 million years), Australasia was dominated by an extraordinary group of crocodiles known as mekosuchines. These were not mere variations of today's crocodiles but a distinct lineage that evolved into forms never seen in the modern world.
The mekosuchine fossil record in Australasia spans from the early Eocene (around 56 million years ago) until the Holocene (as recent as a few thousand years ago).
Unlike the semi-aquatic generalists that survive today, mekosuchines explored diverse ecological niches across the ancient Australian landscape.
For over two decades, the origins of these peculiar crocodiles have puzzled paleontologists. Recent phylogenetic analyses present two compelling hypotheses about their ancestry:
The mekosuchine story became more complex in 2021 with a groundbreaking discovery that challenged the notion of mekosuchine dominance throughout the Australasian Cenozoic. Scientists described Australia's first tomistomine crocodylian, Gunggamarandu maunala, from the Pliocene or Pleistocene of south-eastern Queensland 6 .
The holotype specimen represents not only a new species but an entirely new crocodylian lineage for Australia.
Gunggamarandu marks the southern-most global record for Tomistominae.
The cranial proportions indicate it was the largest crocodyliform yet discovered from Australia.
The evolutionary history of crocodyliforms in Australasia is part of a broader pattern of experimentation and adaptation that characterized the group globally. For much of their evolutionary history, crocodyliforms explored ecological niches far beyond those occupied by their modern descendants 8 .
| Principal Component | Percentage of Variance Explained | Key Features Described |
|---|---|---|
| PC1 | 55.9% | Length to width ratio of the snout, skull height, position of quadrate condyles relative to occipital condyle |
| PC2 | 12.5% | Size of pterygoid flange, dorsoventral flexion of snout dorsal surface, position of the eye |
| PC3 | 5.5% | Dorsoventral tapering of the snout |
| PC4 | 4.2% | Lateral tapering of the snout |
A 2021 study revealed that the pace of crocodyliform cranial evolution was initially high, particularly in the extinct Notosuchia (which included herbivorous forms), but slowed near the base of Neosuchia, with a late burst of rapid evolution in crown-group crocodiles 8 .
Surprisingly, the study found that modern crocodiles, especially Australian, southeast Asian, and Indo-Pacific species, have high rates of evolution, despite exhibiting low variation 8 .
The pattern of crocodyliform evolution continues to be revealed through new discoveries. In September 2025, paleontologists announced a remarkable find from southwest Montana with implications for understanding global crocodyliform evolution—a tiny, heterodont neosuchian named Thikarisuchus xenodentes 4 .
First crocodyliforms appear in Australasia (Two known genera)
Mekosuchines arrive from Asia (Early mekosuchines)
Mekosuchine radiation peaks (Diverse mekosuchines)
Tomistomine arrival (Gunggamarandu) (Mekosuchines, Crocodylus, tomistomines)
Mekosuchine extinction (Only Crocodylus species remain)
Unraveling the evolutionary history of Australasian crocodyliforms requires specialized techniques and technologies. Paleontologists employ a diverse toolkit to extract maximum information from often fragmentary fossil remains.
When Harrison Allen discovered the tiny Thikarisuchus specimen, he and his colleagues spent over 100 hours coloring digital, 2D segment slices from CT scans to visually distinguish the bones from the surrounding rock. This digital reconstruction process allowed them to study the exceptionally fragile fragments without physically reassembling them 4 .
Scientists performed several phylogenetic analyses to test the relationships of Mekosuchinae and other extinct crocodylians. These complex statistical methods use anatomical characteristics to reconstruct evolutionary relationships between different species, both living and extinct 1 .
The study of crocodyliform skull evolution using high-density three-dimensional geometric morphometrics (incorporating 1291 landmarks and semilandmarks) represents a technological leap forward. This approach quantifies shape data in extremely high detail by distributing landmarks across the entire surface of the skull, capturing nuanced information about phenotypic variation that simpler techniques miss 8 .
Fossil preparators use specialized consolidants (such as Butvar and Acryloid B-72) and adhesives to preserve fragile specimens. The fundamental principle in fossil conservation is: "Minimal intervention is best"—do as little as possible to a specimen that will change its nature 2 .
| Research Method | Application | Significance |
|---|---|---|
| CT Scanning | Digital reconstruction of fragile specimens | Allows study without physical manipulation |
| Phylogenetic Analysis | Testing evolutionary relationships | Reconstructs family trees of extinct species |
| Geometric Morphometrics | Quantifying skull shape variation | Reveals patterns of evolutionary adaptation |
| Fossil Preparation | Conserving delicate fossil material | Preserves specimens for future study |
The evolutionary history of crocodyliforms in Australasia is far richer than the modern fauna suggests. What appears today as a modest diversity of crocodiles represents the last survivors of a much grander evolutionary drama that played out over millions of years. The mekosuchines that dominated for most of the Cenozoic, the unexpected tomistomine that arrived later, and the modern Crocodylus species that eventually replaced them all tell a story of migration, adaptation, and replacement.
As paleontologists continue to unearth and analyze fossils across Australasia, using increasingly sophisticated technologies, our understanding of this remarkable evolutionary saga will undoubtedly grow richer, revealing new insights into the processes that shape biological diversity across deep time.