Sahul - Part I: Timeline

Introduction

At the end of the Pleistocene (2.58mya – 11.7kya) the world was swept by a wave of extinctions that devastated much of its megafauna. Near the beginning of this wave lies Sahul, the continent comprising Australia, New Guinea, and Tasmania, the first continent to experience a major collapse. Despite also being ravaged by extinctions, the other continents — save perhaps for South America — maintained a handful of large species (44). Sahul was not so fortunate, its largest terrestrial survivors are roughly man-sized, and save for a population of thylacines on Tasmania, there were no sizable native predators upon European arrival. To make matters worse the landscape itself deteriorated markedly with much of Sahul becoming desertified and scarred by wildfires. The extreme extent of this extinction begs the question, what could possibly cause this calamity? To explore this question, we will in part 1 establish what — and more controversially — when the extinction was. In part 2 we will examine how the landscape changed over the course of the Pleistocene and consider the merits of climate change as the driving force behind the extinction. Finally, part 3 will focus on the timing of human arrival and the evidence for a human hunting and/or burning hypothesis.

To fully appreciate the scale of the extinction it is important to comprehend the diversity of Sahul prior to the extinction, with its unique cast of species across all the megafaunal niches. First off, we have the grazers, this is the most intact guild of large fauna. Amongst the extant species we have the Red Kangaroo (Macropus rufus), and the Western and Eastern Grey Kangaroos (Macropus fuliginosus & L. giganteus) (26, 48). The Common Wombat (Vombatus ursinus) and the Southern and Northern Hairy Nosed Wombats (Lasiorhinus latifrons & L. krefftii) also constitute noteworthy grazers (20). None of these species exceed an average mass of 40kg (13), which is the usual cutoff point for megafauna. These species were once joined by various extinct species and subspecies of Macropus, including Macropus giganteus titan which was conspecific with the Eastern Grey Kangaroo but averaged about 5 times the size (13). The extinct wombats attained much larger sizes, with the species Ramsayia magna and Phascolonus gigas being the largest grazers weighing around half a tonne (19, 32). Additionally, there was the giant landfowl Genyornis Newtoni (20) around 200-300kg (4).

Fig 1. An Eastern Grey Kangaroo and its offspring. This species is the largest extant mammal native to Sahul alongside the Red Kangaroo. During the Pleistocene it grew to over 200kg (1).

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All sizable browers in Sahul went extinct in the Late Pleistocene extinction, yet its cast of species was vast. Most plentiful were the Short-faced Kangaroos represented primarily by the genera, Procoptodon, Simosthenurus, Sthenurus and a number of smaller (and often dubious) genera; they range from wallaby-sized to weights comparable to large deer (13), the short-faced kangaroos were specialised to browse upon tough vegetation (23). Constituting the largest animals in Sahul were the diprotodonts, relatives of the wombats (2). The largest and most common was Diprotodon optatum, weighing in just under 3 tonnes (55), however the group is diverse and contained many other taxa including the ‘small’ Maokopia ronaldi and the common Zygomaturus trilobus. Though traditionally considered browsers (19) the group may have been opportunistic herbivores (40) and some species may have been grazers (17). The enigmatic and highly specialised Palorchestes azael, sometimes called the marsupial tapir, appears to be convergently evolved with ground sloths or Chalicotheres to browse on high branches (32). There are even species of large arboreal browsing fauna, Bohra paulae, a species of giant tree-kangaroo and Phascolarctos stirtoni, the giant koala bear.

Rounding out the herbivores are the grazer-browsers, which are opportunistic herbivores that will feed on most available plant matter. The surviving members of this niche are the ratites: the emu (Dromaius novaehollandiae) and the Southern and Northern Cassowaries (Cassuarius cassuarius & unappendiculatus). However, it once also contained the extinct Protemnodon which was once a diverse genus of wallaby-like kangaroos (26).

The sizable predators native to Sahul are completely extinct, the last surviving member was the thylacine (Thylacinus cynocephalus) which survived until the 20th century in Tasmania, this was a medium-sized carnivore with a disputed diet (35, 57). Similarly, the rat-kangaroo Propleopus oscillans has a largely unknown lifestyle, being suggested as either an omnivore or a predator (1, 33). Thylacoleo carnifex, the marsupial lion, was the largest mammalian predator at around 100-150kg (53) and constituted an apex predator, with giant incisors and arboreal tendencies (49) - perhaps akin to leopards.

Reptiles comprise much of the predatory megafauna as well. The largest ever lizard was Megalania (Varanus priscus) and though its size is heavily disputed it was undoubtedly the largest terrestrial predator on the continent (25, 54). The Komodo dragon (Varanus komodoensis) is also known from Middle Pleistocene deposits of Australia (59), and Komodo-dragon-sized varanid fossils linger into the Late Pleistocene (15). In addition to this a roster of crocodylians inhabited the region; still extant are the Saltwater Crocodile (Crocodylus porosus), Freshwater Crocodile (Crocodylus johnstoni), and the New Guinea Crocodile (Crocodylus novaeguineae). Two extinct genera belong to an ancient Sahulian clade called Mekusuchinae, these are the putative land crocodile Quinkana fortirostrum (24) and the large broad-snouted Paludirex vincenti (36) - formerly called Pallimnarchus pollens. An anaconda-sized snake called Wonambi naracoortensis is also known from this time and was the last survivor of the ancient lineage of Madtsoiid snakes.

Finally, there are megafauna which do not neatly fit into any of these categories. Zaglossus hacketii was a species of echidna, similar in size to the giant anteater and was presumably myrmecophagous (ant-eating). Two species of giant horned turtle are also known from Pleistocene Sahul: Ninjemys oweni and Meiolania sp.

Fig 2. An artist’s interpretation of Diprotodon optatum, the largest known marsupial.

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Dating the Extinction

Understanding the timing of extinctions is key to evaluating any causation behind them. By knowing which events, whether climatic or anthropological, coincide with these extinctions we can better scrutinise hypotheses. Unfortunately, the dating of extinctions is particularly difficult in Sahul with estimates varying heavily depending on the dating technique employed. Until recently such estimates have used carbon dating. Radioactive Carbon-14 (C-14) is maintained in living organisms at a uniform proportion, but decays into Carbon-12 over time when the organism is dead. C-14 has a half life of 5,730 years, which means in that period half the C-14 decays. Based on the proportion of C-14 in a fossil you can infer its age. At older ages the amount of C-14 left is so miniscule that the dating is incredibly sensitive to contamination, and may be affected by organisms in the soil, therefore Carbon dating anything near or older than 50,000 years old is generally considered unreliable (5). The Sahul extinction occurred somewhere in this ballpark so Instead we must rely on more recent dating methods such as Uranium-Thorium dating and Optic Luminescence as well as ingenious proxies. Using these we can now weigh more recent evidence and make more confident inferences about the timeline.

It has been suggested that the extinction of the Australian megafauna was not a single event, but rather a series of extinctions dating back to the Middle Pleistocene (Ca. 400kya) and lasting until around 45-50kya (56, 58). This hypothesis is based on taking the youngest fossil age of each Pleistocene species and using these to estimate an extinction timeline. Using this method extinctions are staggered throughout the Pleistocene with only 8-14 Pleistocene species that have been dated immediately prior to the last extincton, whereas 74-80 species either disappear before or remain undated (58).

This may appear damning evidence against a single large extinction event, but this is not the case. Sudden extinctions may appear gradual due to preservation biases, this is known as the Signor-Lipps rule (41). Firstly, over 60% of the taxa with early terminal dates are known from one or two dated specimens (3). This is very problematic, consider a species living in Sahul between a million years ago and 40kya, if you randomly sample a single specimen or two on this timeline it is unlikely to be anywhere near the actual extinction date. Even if you have a good range of specimens, there can still be spatial biases at play, the Pleistocene Sahul fossil record is much more abundant in the south east of Australia than anywhere else (34) with almost no megafaunal fossils known from Northern Australia, range contractions could easily be mistaken for extinctions. This is especially true in habitat types that rarely yield fossils such as rainforest. A good example of these limitations is that just recently Hocknull et al 2020 demonstrated the survival of Quinkana and Paludirex until about 40kya in rainforest environments, two species which were previously only dated to the Middle Pleistocene (58).

We can’t fully dismiss a gradual extinction, but the evidence for it is tenuous. There is a wider array of evidence to suggest a sudden extinction occuring around 40-50kya (21, 34, 38). This date is suggested because the fossil record of extinct megafauna in Sahul ends abruptly at this time (Barring a few which we will touch on later)(34). This is corroborated by a model which integrates only species which have been widely and accurately dated and takes account of the Signor Lipps rule, it estimated the extinction of all these species between 35kya and 63kya, with most species clustering between 56kya and 40kya (38).

Even so terminal dates aren’t a great indicator of the extinction age, instead we can look at case study sites to check for sudden extinctions. Tight Entrance Cave is a great such example, it is the most plentiful fossil deposit from the Late Pleistocene of Western Australia and has a large collection dating continuously between 143kya and 31kya (31). The most abundant species of megafauna all disappear suddenly in the same stratum between 53 and 43kya, despite showing no signs of local decline in previous strata. A few megafaunal species did occur prior to the last strata, however these were rare, so the signor-lipps rule may be applied here (31), though it could indicate a minor extinction (6). Another such sudden crash is recorded at South Water Creek around 40kya (15).

Fig 3. Map showing the borders of the Pleistocene continent of Sahul (Dark grey) and modern day landmasses (Light grey). The location of each fossil site mentioned is indicated with Red dots.

In addition to fossil bones, there is an intriguing abundance of large fossil eggshells found across the arid areas of Eastern and Central Australia. These fragments have a consistent record going back to about 100kya but exhibit a very abrupt disappearance between 53.9kya and 43.4kya, depending on the dating method implemented (20, 21). The sudden lack of eggshells is unlikely to be due to changes in fossilisation conditions as the eggshells of the Emu  persist past this point (21), it probably marks a genuine extinction event. The parentage of these eggs is traditionally assigned to Genyornis newtoni (20, 21, 22), but recent studies has implicated the much smaller megapode Progura gallinaeceae (45, 46).

An ingenious method of estimating the extinction dates of megafauna has been using dung spores from the fungal genus Sporormiella, which can be found as microfossils in sediment deposits. The fungus feeds primarily on the excrement of large herbivores and the spores pass through megafaunal digestive tracts to germinate (28), thus the population is tied to their abundance. The legitimacy of this proxy has been established both in modern, archaeological and palaeontological studies and usually corroborates established extinction dates (28). Two studies using Sporormiella have been conducted on Pleistocene deposits in Australia. At Devil’s lair the spore concentrations appear to crash between 47-42kya, (47), this is highly significant as it aligns with the disappearance of megafauna at nearby Tight Entrance Cave. Similarly, at the second site — Lynch’s Crater in North East Australia —the spore concentrations plummet around 41kya (37). This site is further North East than any well documented contemporary fossil site and represents a rainforest habitat. It agrees on the timing of the local extinction event established by Hocknull et al 2020 at nearby South Water Creek. Sporomiella has its controversies however, spore concentrations have been demonstrated to be affected by wood availability, temperature, and humidity, so crashes could be caused by climatic shifts (28). though it should be noted no previous crashes in line with climatic shifts are recorded at either site (37, 47). Sporomiella also likely acts as a proxy of some extant species and so can’t solely be linked to lost Megafauna.

There are a few reports of megafauna surviving past the 40,000-year terminus. An especially significant faunal assemblage is reported from Cuddie Springs in South-Eastern Australia with fossil remains of Diprotodon optatum, Genyornis Newtoni and other species of large megafauna (9) dating to as late as 30-36 kya (8), extending the extinction date in the region by as much as 10ky (34). Cuddie springs is however a questionable site. The presence of younger soil being found in the layer suggests some degree of sediment mixing (34) indicating that the remains of megafauna and stratum may not be contemporaneous (8). Direct dating of faunal remains found specimens to be around 45-50kya (11), which would place the assemblage within the expected margins.

Nombe rockshelter in New Guinea is another site which adds a question mark to the extinction window. The diprotodontid Maokopia ronaldi and two species of Protemnodon were found alongside charcoal dated to between 25kya and 14kya (27). The flowstone stratum itself couldn’t be reliably dated (8) and both landsnail shells and fossils from the site has yielded various ages and remain tenuous (8). A young age for the fossils is not out of the question, New Guinea may constitute a last refuge for megafauna, unfortunately the fossil record of Late Pleistocene New Guinea is vastly understudied so it is currently impossible to say, but the evidence for extending the extinction event to 25kya is weak.

A few species may have survived the initial onslaught, though these were notably small, roughly the size of modern day Macropus species. Sthenurus orientalis is dated to 23kya at Cloggs cave (8), though this has since been revised to older than 44.5kya (5) and Procoptodon hopei to 16kya at Seton Rock shelter (16), though this relies on carbon dating. Furthermore, Thyalcinus cynocephalus (The Tasmanian tiger)and Sarcophilus harisii (The Tasmanian devil) disappear from the Australian fossil record 3500ya coinciding with the arrival of the dingo (Canis familiaris dingo) (50).

Fig 4. Photograph of ‘Benjamin’, the last known Thylacine. This is a late surviving species on the Australian mainland, lasting until about 3-4kya with a population existing on Tasmania until the early 20th Century.

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Limitations and Conclusion

Ultimately Sahul remains an understudied region mired with technical challenges associated with its sparse and uneven fossil record at the threshold of Carbon dating. In order to solve these problems, it will require more digs, especially in areas where little has been conducted before. New Guinea appears to be a promising location as it has yielded fossils on past expeditions, but little has been done with modern dating techniques. Surveys of areas away from population centres may also lead to additional discoveries. Even if direct bone material is lacking this can be compensated Sporormiella. There are certainly ways to fill in the gaps provided adequate funding.

Based on current evidence we can say with some degree of confidence that an extinction event did probably take place around 40-50kya that removed most of the megafauna. This is corroborated by terminal fossil dates, case study sites, eggshells and proxies. What remains an open question is how much of the megafauna was present prior to this time and how many species survived past the event.

It would be insightful to establish the order of extinctions across Sahul. Unfortunately, the available evidence is imprecise, this is the caviat of using dating methods with much higher half lives. The uncertainties associated with most sites overlap and it is difficult to say what order they occurred in. Even so the evidence suggests that the Centre and South of modern-day Australia had extinctions predating the North-East. Until more evidence is retreived from Northern Australia and New Guinea a full overview is impossible. Perhaps a large-scale study at multiple sites across Sahul using Sporormiella could yield some clarity on this.

In any case establishing that an extinction took place and roughly when it took place is all well and good, but it leaves the more important question unaddressed, why did the extinction occur at all? In the next part we will attempt to answer this by examining the change in the landscape of Sahul and whether climate change can offer a suitable explanation for the rapid decline of species.

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Taxa List

Table showing animal species and genera mentioned in text. ‘†’ Denotes a globally extinct taxon. It should be noted most species are known only from limited fossil material, so classifications, sizes and ecology might be unclear or wrong. Do note there are some additional genera and species not included in this essay, particularly some dubious genera of Kangaroos and Diprotodonts.

Mammalia
Diprodontidae†
Diprotodon optatum †	Diprotodont was the largest marsupial known, reaching an estimated 2-3 metric tons (32, 55). Diprotodon was probably a browser, but was probably capable of a generalised diet in tough times (40). It was probably sexually dimorphic, with males being much larger than females. Based on this it is inferred that they were also polygynous and may have lived in gender segregated herds (29).
Maokopia ronaldi †	The species appears endemic to New Guinea. It is estimated at just under 100kg, making it possibly the smallest Diprotodont (17). It may have been a grazing taxon since it inhabited the open plains of Pleistocene New Guinea (17) or a browsing taxa like other diprotodonts.
Zygomaturus trilobus †	A large species of diprotodont, potentially reaching over a tonne (32) found throughout Pleistocene Sahul.
Palorchestidae†
Palorchestes azael †	Often called the marsupial tapir, this species was probably less tapir-like and more similar to the giant ground sloths or the Chalicotheres. It had large long forelimbs with big claws, probably as an aid to browsing on high branches, and it may have been able to stand on its hindlimbs. It may have reached sizes as large as 2000kg (32).
Phascolarctidae
Phascolarctos stirtoni †	A species of giant koala, very similar in build and niche to the modern species, but about 30% larger based on molar size. It has been suggested that it is conspecific with the modern-day Koala and simply undergone a dwarfing ( Phascolarctos cinereus) (30)
Thylacoleonidae†
Thylacoleo carnifex †	Thylacoleo was a unique Marsupial with no comparable animals known. It weighted in at about 100-150kg (53) and probably constituted a hyper carnivore using powerful forearms and massive incisors to hunt prey (49).
Vombatidae (Wombats)
Lasiorhinus krefftii	Northern hairy-nosed wombat
Lasiorhinus latifrons	Southern Hairy-nosed wombat
Phascolonus gigas †	A large species of wombat potentially reaching 460-730kg in weight (32), though other estimates places it as significantly smaller (51). Unlike modern wombats Phascolonus probably did not burrow, but probably it also had a grazing diet like modern wombats (51).
Ramsayia magna †	A large species of extinct wombat, somewhat smaller than Phascolonus gigas. Thought to be a grazer.
Vombatus ursinus	Common Wombat.
Hypsiprymnodontidae
Propleopus oscillans †	Proleopus was a genus most closely related to the modern-day rat-kangaroos, P. oscillans being slightly smaller than the extant grey kangaroos (33). It is thought to have been either an omnivore (1) or a carnivore (33).
Macropodidae (Kangaroos and relatives)
Bohra paulae †	A giant species of Tree kangaroo, very similar in build to the extant tree kangaroos (Dendrolagus) but approximately the size of a human (10).
Macropus fuliginosus	Western grey kangaroo.
Macropus giganteus	Eastern grey kangaroo. Averaging around 40kg, however during the pleistocene appear to have reached sizes up to 180kg (13).
Macropus rufus	Red Kangaroo.
Procoptodon	A genus of shortfaced kangaroo, which is very variable in size from about 50kg in P. gilli to 230kg in P. goliath which is the largest known kangaroo (13). Much like other shortfaced kangaroos it has a short and robush face, well adapted for powerful cheek muscles used in browsing (Mitchel 2019).
Protemnodon	A genus large wallaby-like kangaroos with about a dozen species (13) ranging from 45kg to about 165kg (13). It was a versatile browser-grazer (26).
Simosthenurus	A species rich genus of shortfaced kangaroo, reaching about 120kg but with smaller species as well (13). It was probably a browser like other shortfaced Kangaroos (23)
Sthenurus	Another genus of shortfaced kangaroo with a wide range of species between 70kg and 170kg (13), the skull of Sthenurus has been found to be capable of very high bite forces, probably for browsing exceptionally tough vegetation (23).
Dasyuridae
Sarcophilus harrisii	Tasmanian devil.
Thylacinidae
Thylacinus cynocephalus †	The Tasmanian tiger was a species most closely related to the modern-day numbats, quolls, and Tasmanian devils (7). It was quite a small predator perhaps averaging as little as 17kg (35) and Its diet is the subject of much controversy perhaps only feeding on small animals (35) or being capable of taking down prey around its own size or larger (57). It survived on Tasmania until the 20 th century.
Monotremata
Zaglossus hacketti †	A species of large echidna, in the genus of extant long-beaked echidna, it was however significantly larger at about 20kg (14).

Aves
Dromornithidae†
Genyornis Newtoni †	Genyornis was probably around 200-300 kg and over 2m in Length (4), being primarily a grazer (20). Likely it was a relative of the galliformes (landfowl) (52). It has been suggested having layed the large fossilised eggshells, though the validity of this proxy is disputed (20, 21, 22, 45, 46).
Galliformes (Landfowl)
Progura gallinaeceae †	A species of megapode (Mount building fowl) most closely related to the contemporary giant megapode ( Progura naracoortensis) and suggested to have laid the large fossilised eggshells studied. The species would have been small, only somewhat larger than megapodes, which are a few kilograms (45).
Casuariiformes
Dromaius Novahollandiae	Emu.
Casuarius casuarius	Southern Cassowary.
Casuarius unappendiculatus	Northern Cassowary.

Reptilia
Squamata (Lizards and Reptiles)
Varanus priscus †	Commonly known as Megalania, a species of giant goanna. Estimated to be as large as 2000kg (25), though the size is highly contentious (54). Megalania is most closely related to the Komodo Dragon (Varanus komodoensis) (12) and probably had a similar lifestyle.
Varanus sp. †	Komodo dragon sized species of monitor lizard from North Eastern Australia (15)
Wonambi naracoortensis †	This was a large anaconda-sized snake (54) belonging to the ancient gondwana clade of Madtsoiidae (39). It had a relatively small head and probably did not take on large prey (54).
Crocodylia (Crocodiles)
Crocodylus johnstoni	Freshwater crocodile.
Crocodylus novaguineae	New Guinean crocodile.
Crocodylus porosus	Saltwater crocodile.
Paludirex vincenti †	A genus of mekusuchine crocodile, formerly known as Pallimnarchus (36). It was a broad snouted crocodile larger than C. porosus (36) associated with freshwater fossil locations (15).
Quinkana fortirostrum †	A genus of mekusuchine crocodile. The size of quinkana is unclear, but has been proposed to be about 3m, with weight estimates as high 200kg It bears similarities to other suggested terrestrial crocodiles and may have been land adapted (24), though it has mostly been found in sites associated with water and the morphological evidence has been challenged (54).
Testudina (Turtles)
Meiolania sp. †	An unnamed species of the horned turtle Meiolania, belonging to the ancient gondwana clade of Meiolanidae (43). It was probably herbivorous (42). Meiolania had a bony club, with unknown function, but could be for defence or have a social function. It was probably terrestrial (42), but another species found on Lord Howe Island has been found to be semi-aquatic (18).
Ninjemys oweni †	The humerously named Ninjemys (Meaning Ninja turtle) was the largest genus of Meiolanid turtle known (42), though no size estimates exist because of the unclear build of this extinct group, it was probably comparable to giant extant tortoises. Not much is known about its ecology, besides that it was probably a terrestrial herbivore like Meiolania.

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