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Description
Australia's Mesozoic fossil record is notoriously poor except for the Mid Cretaceous, where most of the known non-avian dinosaurs and associated fossils (pterosaurs, marine reptiles, early monotremes, etc.) come from. Even more specifically, most of the Mesozoic taxa that are known from reasonably complete material (not fragments like the majority of the Dinosaur Cove finds) hail from the Rolling Downs Group that covers Queensland and New South Wales, its strata dating from the upper Aptian to the earliest Turonian (115-93 mya), a relatively narrow timespan and unsuprisingly, we find the same kinds of dinosaurs, pterosaurs, and marine reptiles in various layers throughout this group (mainly from the mid Albian to the end of the Cenomanian, 110-94 mya), giving us a pretty good idea of what the common groups of vertebrates (both large and small) were in the land down under during the Mid Cretaceous (further corroborated by similar finds at the older Dinosaur Cove, late Barremian-early Albian). At that time, most of Australia was flooded by the shallow Eromanga Sea, hence the abundance of marine reptiles, fish and sea pterosaurs, but it gradually subsided and was gone by the dawn of the Turonian.

One of the more common but often overlooked groups found in Mid Cretaceous Australia are various small ornithopods that were traditionally classed as "hypsilophodonts" but nowadays, most of the named taxa are classed within the newly erected elasmarians (named in 2007), a very diverse group of small-bodied (occasionally mid-sized) ornithopods whose fossils have been found in Australia, Argentina and the James Ross Island region of Antarctica, showing that they ranged all across southern Gondwana and were well suited for high-latitude environments, one of many groups of animals that serve as biogeographic links which showcase how Australia, Antarctica, and South America used to be connected. Many of Australia's elasmarians have been found at Dinosaur Cove, and are mainly represented by fragments, other than two headless and long-tailed skeletons from Eumeralla tentatively assigned to Leaellynasaura. Further north, fossils of them are rare, with the first record being a femur fragment from Griman Creek that was named as Fulgurotherium (1932), and more recently, we described Weewarrasaurus from the same site based on a dentary fragment (2018). More notably, we have recently found a skull, mandible, and three partial skeletons from the Winton Formation, belonging to a small, yet-unnamed elasmarian, as well as the digested remains of a juvenile in the gut content of the crocodile Confractosuchus, and they might be the same species that left numerous footprints at the Lark Quarry (Wintonopus). Here, I dubbed the new Winton elasmarian "Austrodromeus".

1. Kunbarrasaurus ieversi is a basal parankylosaur and the most complete non-avian dinosaur known from Australia, with its remarkably complete holotype stemming from the upper Albian Allaru Formation, and a partial skull and other fragments also being known from the Toolebuc Formation. The former was originally included as a specimen of the earlier-named Minmi in 1996, first described from a partial postcranial skeleton from the older Bungil Formation, but the Allaru specimen was named as a distinct genus in 2015. Like many Australian dinosaurs, parankylosaurs are also known from Argentina and Antarctica, with Stegouros, Patagopelta, and the larger Antarctopelta. They were quite distinct from the northern euankylosaurs, with Stegouros being known for its unique, macuahuitl-esque tail, and unsuprisingly, Kunbarrasaurus sports a much more rudimentary version of that tail weapon . Fragmentary fossils of animals similar to Kunbarrasaurus (namely teeth and osteoderms) are also known from the younger Griman Creek and Winton Formation (the latter having a dentary and right maxilla too).

2. Fossils of pterosaurs aren't uncommon in the Albian Toolebuc Formation, but all of them are isolated pieces of skull or postcrania , many specimens being known since at least the 1980s. The first taxon to be properly named (in 2008) is the anhanguerid Mythunga camara, known from a partial skull . It would later be joined by the larger and sympatric Thapunngaka, and the smaller and geologically younger Ferrodraco from Winton, all three being tropeognathines, related to the giant Brazilian Tropeognathus , and Thapunngaka and Ferrodraco (the latter also being Australia's most complete pterosaur ) both preserve similar keel crests on their rostrums. A third pterosaur was named from Toolebuc, Aussidraco molnari (known from a symphysis), but it's classified as a targaryendraconid and is the smallest of the four named taxa, and yet another link to South America, as the much more complete Barbosania gracilirostris is known from Romualdo (alongside Tropeognathus). Anhanguerians were the most common pterosaurs during the Mid Cretaceous and the shallow Eromanga Sea was an ideal feeding ground for these soaring piscivores.

3. For the longest time, the theropods of Australia were an enigma, known only from isolated scraps like a finger bone (Rapator) or an ankle bone ("Allosaurus robustus"), but the description of Australovenator wintonensis (2009) from the upper Cenomanian-early Turonian Winton Formation, known from two incomplete skeletons , revealed the existence of a wholly new group of tetanuran theropods, with countless close relatives like Megaraptor and Aerosteon being known from Argentina. Sporting large arm claws akin to spinosaurids, long snouts, and gracile builds designed for speed, megaraptorans are thought to be a basal stock of coelurosaurs (related to tyrannosaurs) and they were seemingly quite successful in southern Gondwana. Other megaraptoran specimens in Australia include the "Lightning Claw" from the latest Albian-early Cenomanian Griman Creek (possibly belonging to Rapator) and the "Otway Claw" from the early Albian Eumeralla Formation, as well as numerous other fragments from Dinosaur Cove. So far, they are the only large theropods confidently identified from the continent, other than the Cenozoic dromornithids and extant ratites.

4. Muttaburrasaurus langdoni was the first well-preserved dinosaur to be described in Australia, named based on a partial skeleton from the upper Albian-earliest Cenomanian Mackunda Formation in 1981, with a skull and postcranial fragment of a more basal species being later found at Allaru, as well as isolated teeth and other fragments from Toolebuc and Griman Creek, the latter also having more complete material of the related Fostoria. Muttaburrasaurus was a large ornithopod (8 meters, 3 tons) and sported a distinct bulbous snout (less exaggerated in Muttaburrasaurus sp. from Allaru), though its classification has been controversial, originally being described as an iguanodont but current thinking is that it and the smaller Fostoria are rhabdodontomorphs, as they lacked thumb spikes and based on their more basal position among ornithopods, may have been obligate bipeds, not facultative ones like Iguanodon and its kin. Footprints of a similar large ornithopod (Tyrannosauropus/Amblydactylus) are also known from the Lark Quarry at Winton.

5. Several types of marine reptiles are known from the Albian Toolebuc Formation, the most obscure one being a giant turtle, Cratochelone berneyi, an early protostegid known from incomplete material, and yet it has been estimated to have reached comparable sizes to the much later, Campanian-aged Archelon from the Western Interior Seaway, though size estimates for Cratochelone have been inconsistent, with some suggesting a smaller size of just 2 meters, but the most recent studies do favor it being close to 4 meters long! Cratochelone and other sea turtles, like its smaller cousin Notochelone and the dermochelyid Bouliachelys, would have been the only marine reptiles in the Eromanga Sea to crawl back onto land to lay their eggs, the rest were viviparous and fully marine, and in the case of plesiosaurs, possibly even engaging in parental care!

6. Four different types of plesiosaurs have been found in the Eromanga Sea, with the leptocleidid Umoonasaurus demoscyllus being the smallest, bite-sized for the giant pliosaurids that swam Eromanga. Known from three very complete, opalized skeletons from the late Aptian Bulldog Shale, Umoonasaurus sports unusual ridges on its snout and over its eyes, that are thought to have sported small keratinous structures in life , quite unusual for marine reptiles. At the younger Toolebuc, we found a very complete polycotylid skeleton (QM F18041 ), along with other fragments across Queensland and New South Wales (such as Bulldog Shale), with polycotylids being the sister group to the leptocleidids (together forming Leptocleidia), who convergently evolved a similar body plan to the pliosaurids, despite being part of the otherwise long-necked plesiosauroids.

7. Speaking of which...long-necked plesiosauroids, specifically early elasmosaurids, are known from Australia, the first prolific specimen being the fragmentary Woolungasaurus from the Aptian Wallumbilla Formation, with a 40-cm skull and associated jaw from Toolebuc also being historically assigned to it, but in 2005, the latter was reclassified as its own genus; Eromangasaurus australis. If proportioned similar to later elasmosaurids such as Styxosaurus and Thalassomedon, Eromangasaurus was likely a very long animal, just shy of 10 meters in length, and the holotype skull sports bite marks attributed to the top predator of the Eromanga Sea. A partial skeleton from Bulldog Shale offers evidence of another elasmosaurid, as well as the well-preserved skeleton of Opallionectes, a plesiosauroid of uncertain classification (possibly a late-surviving cryptoclidid).

8. Of course, the top predator and undisputed king of the Eromanga Sea, as well as one of the most iconic Cretaceous animals from Australia, is the giant brachauchenine pliosaurid Kronosaurus queenslandicus. Known from several skulls and partial postcranial skeletons from Toolebuc, this animal was initially estimated at 13 meters in length (some textbooks went as far as 15 meters!), based on the famous Harvard mount , but improved understanding of brachauchenine anatomy, based on the more complete Sachicasaurus and Monquirasaurus from Columbia (the latter being historically lumped into Kronosaurus) offered a slightly smaller but still imposing size of around 10-11 meters and 10-12 tons, nearly 3 times the length of the average great white that patrols Australia's coastal waters today, with its skull alone being around 2.5 meters long (one-quarter of its body length). Remains of other plesiosaurs and turtles have been found in its gut content, as well as the aforementioned tooth marks on the Eromangasaurus skull, testifying to Kronosaurus's prowess as an apex predator. Teeth and other fragments that might belong to the genus have also been found in the older Bulldog Shale and Wallumbilla.

9. Pliosaurids weren't the only macropredators in Eromanga though. Though their heyday was the Triassic and Jurassic, ichthyosaurs (the oldest of marine reptiles) still thrived in the Early to Mid Cretaceous in the form of platypterygiines, many of which sported robust dentition indicating that they hunted more than just small fish, unike the more basal ophthalmosaurids from the Jurassic. Many species across the globe were historically placed in the genus Platypterygius (first erected with P. platydactylus from Aptian Germany) but most of them have since been split into their own genera. Toolebuc's Platypterygius australis, or perhaps more accurately Longirostria australis, is known from several well-preserved skeletons and it's one of the largest known platypterygiines (barring some fragments ), with an estimated length of 7 meters (comparable to an orca), and next to Kronosaurus, it was perhaps the biggest predator in Eromanga. Fragmentary fossils of "Platypterygius" and occasionally more complete specimens are known from other sites in the Rolling Downs Group as well.

10. And lastly, there are the biggest animals from the Rolling Downs Group, the giant titanosaurs (or macronarians of some sort). In the past, the best (and only) known Cretaceous sauropod from Australia was Austrosaurus from Allaru (named in 1933), known from incomplete material, but in the 2000s, it was eclipsed by several new species described from the Cenomanian-Turonian Winton Formation. All are known from partial remains and three separate species were initially described, all macronarians and all midsized by sauropod standards (falling in the 15-meter range); Savannasaurus elliottorum (2016), Wintonotitan wattsi (2009), and perhaps the most famous one, Diamantinasaurus matildae (2009). Initially, these animals were only known from postcranial remains, but in 2023, we found additional specimens, includinged skull material , attributed to D. matildae, showing that it had a more diplodocid-esque head in contrast to the boxy ones often shown in older restorations, and that its skull was rather similar to the similar-aged but Argentinian Sarmientosaurus, suggesting that diamantinasaurians are yet another biogeographic link between Australia and South America. Another diamantinasaurians from Winton, only known from its scapula, sacrum, and front limbs, is the aptly named Australotitan cooperensis (2021), which grew an estimated 25 meters or more in length , far larger than any other land animal known from Australia.

Anatomical References:
www.deviantart.com/plastosplee…
twitter.com/AshleyPalaeo/statu…
twitter.com/AshleyPalaeo/statu…
www.deviantart.com/plastosplee…
twitter.com/fishboy86164577/st…
twitter.com/fishboy86164577/st…
www.deviantart.com/drscotthart…
upload.wikimedia.org/wikipedia…
www.deviantart.com/horsthippol…

Description
Australia's Mesozoic fossil record is notoriously poor except for the Mid Cretaceous, where most of the known non-avian dinosaurs and associated fossils (pterosaurs, marine reptiles, early monotremes, etc.) come from. Even more specifically, most of the Mesozoic taxa that are known from reasonably complete material (not fragments like the majority of the Dinosaur Cove finds) hail from the Rolling Downs Group that covers Queensland and New South Wales, its strata dating from the upper Aptian to the earliest Turonian (115-93 mya), a relatively narrow timespan and unsuprisingly, we find the same kinds of dinosaurs, pterosaurs, and marine reptiles in various layers throughout this group (mainly from the mid Albian to the end of the Cenomanian, 110-94 mya), giving us a pretty good idea of what the common groups of vertebrates (both large and small) were in the land down under during the Mid Cretaceous (further corroborated by similar finds at the older Dinosaur Cove, late Barremian-early Albian). At that time, most of Australia was flooded by the shallow Eromanga Sea, hence the abundance of marine reptiles, fish and sea pterosaurs, but it gradually subsided and was gone by the dawn of the Turonian.

One of the more common but often overlooked groups found in Mid Cretaceous Australia are various small ornithopods that were traditionally classed as "hypsilophodonts" but nowadays, most of the named taxa are classed within the newly erected elasmarians (named in 2007), a very diverse group of small-bodied (occasionally mid-sized) ornithopods whose fossils have been found in Australia, Argentina and the James Ross Island region of Antarctica, showing that they ranged all across southern Gondwana and were well suited for high-latitude environments, one of many groups of animals that serve as biogeographic links which showcase how Australia, Antarctica, and South America used to be connected. Many of Australia's elasmarians have been found at Dinosaur Cove, and are mainly represented by fragments, other than two headless and long-tailed skeletons from Eumeralla tentatively assigned to Leaellynasaura. Further north, fossils of them are rare, with the first record being a femur fragment from Griman Creek that was named as Fulgurotherium (1932), and more recently, we described Weewarrasaurus from the same site based on a dentary fragment (2018). More notably, we have recently found a skull, mandible, and three partial skeletons from the Winton Formation, belonging to a small, yet-unnamed elasmarian, as well as the digested remains of a juvenile in the gut content of the crocodile Confractosuchus, and they might be the same species that left numerous footprints at the Lark Quarry (Wintonopus). Here, I dubbed the new Winton elasmarian "Austrodromeus".

1. Kunbarrasaurus ieversi is a basal parankylosaur and the most complete non-avian dinosaur known from Australia, with its remarkably complete holotype stemming from the upper Albian Allaru Formation, and a partial skull and other fragments also being known from the Toolebuc Formation. The former was originally included as a specimen of the earlier-named Minmi in 1996, first described from a partial postcranial skeleton from the older Bungil Formation, but the Allaru specimen was named as a distinct genus in 2015. Like many Australian dinosaurs, parankylosaurs are also known from Argentina and Antarctica, with Stegouros, Patagopelta, and the larger Antarctopelta. They were quite distinct from the northern euankylosaurs, with Stegouros being known for its unique, macuahuitl-esque tail, and unsuprisingly, Kunbarrasaurus sports a much more rudimentary version of that tail weapon . Fragmentary fossils of animals similar to Kunbarrasaurus (namely teeth and osteoderms) are also known from the younger Griman Creek and Winton Formation (the latter having a dentary and right maxilla too).

2. Fossils of pterosaurs aren't uncommon in the Albian Toolebuc Formation, but all of them are isolated pieces of skull or postcrania , many specimens being known since at least the 1980s. The first taxon to be properly named (in 2008) is the anhanguerid Mythunga camara, known from a partial skull . It would later be joined by the larger and sympatric Thapunngaka, and the smaller and geologically younger Ferrodraco from Winton, all three being tropeognathines, related to the giant Brazilian Tropeognathus , and Thapunngaka and Ferrodraco (the latter also being Australia's most complete pterosaur ) both preserve similar keel crests on their rostrums. A third pterosaur was named from Toolebuc, Aussidraco molnari (known from a symphysis), but it's classified as a targaryendraconid and is the smallest of the four named taxa, and yet another link to South America, as the much more complete Barbosania gracilirostris is known from Romualdo (alongside Tropeognathus). Anhanguerians were the most common pterosaurs during the Mid Cretaceous and the shallow Eromanga Sea was an ideal feeding ground for these soaring piscivores.

3. For the longest time, the theropods of Australia were an enigma, known only from isolated scraps like a finger bone (Rapator) or an ankle bone ("Allosaurus robustus"), but the description of Australovenator wintonensis (2009) from the upper Cenomanian-early Turonian Winton Formation, known from two incomplete skeletons , revealed the existence of a wholly new group of tetanuran theropods, with countless close relatives like Megaraptor and Aerosteon being known from Argentina. Sporting large arm claws akin to spinosaurids, long snouts, and gracile builds designed for speed, megaraptorans are thought to be a basal stock of coelurosaurs (related to tyrannosaurs) and they were seemingly quite successful in southern Gondwana. Other megaraptoran specimens in Australia include the "Lightning Claw" from the latest Albian-early Cenomanian Griman Creek (possibly belonging to Rapator) and the "Otway Claw" from the early Albian Eumeralla Formation, as well as numerous other fragments from Dinosaur Cove. So far, they are the only large theropods confidently identified from the continent, other than the Cenozoic dromornithids and extant ratites.

4. Muttaburrasaurus langdoni was the first well-preserved dinosaur to be described in Australia, named based on a partial skeleton from the upper Albian-earliest Cenomanian Mackunda Formation in 1981, with a skull and postcranial fragment of a more basal species being later found at Allaru, as well as isolated teeth and other fragments from Toolebuc and Griman Creek, the latter also having more complete material of the related Fostoria. Muttaburrasaurus was a large ornithopod (8 meters, 3 tons) and sported a distinct bulbous snout (less exaggerated in Muttaburrasaurus sp. from Allaru), though its classification has been controversial, originally being described as an iguanodont but current thinking is that it and the smaller Fostoria are rhabdodontomorphs, as they lacked thumb spikes and based on their more basal position among ornithopods, may have been obligate bipeds, not facultative ones like Iguanodon and its kin. Footprints of a similar large ornithopod (Tyrannosauropus/Amblydactylus) are also known from the Lark Quarry at Winton.

5. Several types of marine reptiles are known from the Albian Toolebuc Formation, the most obscure one being a giant turtle, Cratochelone berneyi, an early protostegid known from incomplete material, and yet it has been estimated to have reached comparable sizes to the much later, Campanian-aged Archelon from the Western Interior Seaway, though size estimates for Cratochelone have been inconsistent, with some suggesting a smaller size of just 2 meters, but the most recent studies do favor it being close to 4 meters long! Cratochelone and other sea turtles, like its smaller cousin Notochelone and the dermochelyid Bouliachelys, would have been the only marine reptiles in the Eromanga Sea to crawl back onto land to lay their eggs, the rest were viviparous and fully marine, and in the case of plesiosaurs, possibly even engaging in parental care!

6. Four different types of plesiosaurs have been found in the Eromanga Sea, with the leptocleidid Umoonasaurus demoscyllus being the smallest, bite-sized for the giant pliosaurids that swam Eromanga. Known from three very complete, opalized skeletons from the late Aptian Bulldog Shale, Umoonasaurus sports unusual ridges on its snout and over its eyes, that are thought to have sported small keratinous structures in life , quite unusual for marine reptiles. At the younger Toolebuc, we found a very complete polycotylid skeleton (QM F18041 ), along with other fragments across Queensland and New South Wales (such as Bulldog Shale), with polycotylids being the sister group to the leptocleidids (together forming Leptocleidia), who convergently evolved a similar body plan to the pliosaurids, despite being part of the otherwise long-necked plesiosauroids.

7. Speaking of which...long-necked plesiosauroids, specifically early elasmosaurids, are known from Australia, the first prolific specimen being the fragmentary Woolungasaurus from the Aptian Wallumbilla Formation, with a 40-cm skull and associated jaw from Toolebuc also being historically assigned to it, but in 2005, the latter was reclassified as its own genus; Eromangasaurus australis. If proportioned similar to later elasmosaurids such as Styxosaurus and Thalassomedon, Eromangasaurus was likely a very long animal, just shy of 10 meters in length, and the holotype skull sports bite marks attributed to the top predator of the Eromanga Sea. A partial skeleton from Bulldog Shale offers evidence of another elasmosaurid, as well as the well-preserved skeleton of Opallionectes, a plesiosauroid of uncertain classification (possibly a late-surviving cryptoclidid).

8. Of course, the top predator and undisputed king of the Eromanga Sea, as well as one of the most iconic Cretaceous animals from Australia, is the giant brachauchenine pliosaurid Kronosaurus queenslandicus. Known from several skulls and partial postcranial skeletons from Toolebuc, this animal was initially estimated at 13 meters in length (some textbooks went as far as 15 meters!), based on the famous Harvard mount , but improved understanding of brachauchenine anatomy, based on the more complete Sachicasaurus and Monquirasaurus from Columbia (the latter being historically lumped into Kronosaurus) offered a slightly smaller but still imposing size of around 10-11 meters and 10-12 tons, nearly 3 times the length of the average great white that patrols Australia's coastal waters today, with its skull alone being around 2.5 meters long (one-quarter of its body length). Remains of other plesiosaurs and turtles have been found in its gut content, as well as the aforementioned tooth marks on the Eromangasaurus skull, testifying to Kronosaurus's prowess as an apex predator. Teeth and other fragments that might belong to the genus have also been found in the older Bulldog Shale and Wallumbilla.

9. Pliosaurids weren't the only macropredators in Eromanga though. Though their heyday was the Triassic and Jurassic, ichthyosaurs (the oldest of marine reptiles) still thrived in the Early to Mid Cretaceous in the form of platypterygiines, many of which sported robust dentition indicating that they hunted more than just small fish, unike the more basal ophthalmosaurids from the Jurassic. Many species across the globe were historically placed in the genus Platypterygius (first erected with P. platydactylus from Aptian Germany) but most of them have since been split into their own genera. Toolebuc's Platypterygius australis, or perhaps more accurately Longirostria australis, is known from several well-preserved skeletons and it's one of the largest known platypterygiines (barring some fragments ), with an estimated length of 7 meters (comparable to an orca), and next to Kronosaurus, it was perhaps the biggest predator in Eromanga. Fragmentary fossils of "Platypterygius" and occasionally more complete specimens are known from other sites in the Rolling Downs Group as well.

10. And lastly, there are the biggest animals from the Rolling Downs Group, the giant titanosaurs (or macronarians of some sort). In the past, the best (and only) known Cretaceous sauropod from Australia was Austrosaurus from Allaru (named in 1933), known from incomplete material, but in the 2000s, it was eclipsed by several new species described from the Cenomanian-Turonian Winton Formation. All are known from partial remains and three separate species were initially described, all macronarians and all midsized by sauropod standards (falling in the 15-meter range); Savannasaurus elliottorum (2016), Wintonotitan wattsi (2009), and perhaps the most famous one, Diamantinasaurus matildae (2009). Initially, these animals were only known from postcranial remains, but in 2023, we found additional specimens, includinged skull material , attributed to D. matildae, showing that it had a more diplodocid-esque head in contrast to the boxy ones often shown in older restorations, and that its skull was rather similar to the similar-aged but Argentinian Sarmientosaurus, suggesting that diamantinasaurians are yet another biogeographic link between Australia and South America. Another diamantinasaurians from Winton, only known from its scapula, sacrum, and front limbs, is the aptly named Australotitan cooperensis (2021), which grew an estimated 25 meters or more in length , far larger than any other land animal known from Australia.

Anatomical References:
www.deviantart.com/plastosplee…
twitter.com/AshleyPalaeo/statu…
twitter.com/AshleyPalaeo/statu…
www.deviantart.com/plastosplee…
twitter.com/fishboy86164577/st…
twitter.com/fishboy86164577/st…
www.deviantart.com/drscotthart…
upload.wikimedia.org/wikipedia…
www.deviantart.com/horsthippol…