are terror birds still alive

66 million years ago, a meteor the size of a mountain slammed into the sea, right off the coast of what is now Mexico. Soot, ash and burning smolders filled the skies, earthquakes struck across the planet, and tsunamis, a hundred meters tall, crashed over the coasts. This was the end of the Mesozoic, and with it, the Age of Dinosaurs—or so we say. Yet some dinosaurs did survive, members of a small, feathered lineage, graced with the ability of flight. It is an amusing twist of fate—or more rightly, of evolution—that some descendants of these furtive, flying animals would abandon these traits once more, turning away from those very adaptations that saved them where their larger cousins vanished, and returning to a shape and lifestyle much more “dinosaurian” than their flighted relatives. These were the ’terror birds’, more rightly the members of the family Phorusrhacidae, whose outsized role in the prehistory of South America is matched only by their ubiquity in the prehistoric pop-culture of our modern day. Few groups of the Cenozoic, mammal, bird or reptile, have captivated the modern imagination as much as the terror birds. Denizens of South- and, for a time, North America, they ranged from the Eocene all the way until the Pliocene and, as we will discuss in this article, probably the Pleistocene as well.

Fig 1. A red-legged seriema (Cariama cristata), one of the closest living relatives of the terror birds.

Terms of use: This is licensed under an Attribution-ShareAlike 3.0 Unported. The is attributed to José Reynaldo da Fonseca and is unedited.

The family Phorusrhacidae is fairly well-defined and united through several skeletal traits, the most iconic of which is a huge upper jaw ending in a single downfacing point (2), but a host of other features distinguish them from other birds. The closest relatives are the Seriemas (Cariamidae) (2, Worthy et al 2017) along with a number of other extinct families such as the Bathornithidae (2), they are united in the clade Cariamiformes.

Traditionally, the Phorusrhacidae is divided into 5 subfamilies: Brontornithinae, Mesembriornithinae, Patagornithinae, Phorusrhacinae and Psilopterinae (2). Substantial disagreements regarding this taxonomy exists, and relationships between subfamilies is equally controversial.

First up are The Psilopterines, a group of small, lightly built Phorusrhacids, which hardly inspire the name ‘terror bird’ as they may superficially have resembled the seriemas (Cariamidae) and secretary birds (Sagittaridae) (Though up to twice as big as the latter). The family contains its namesake Psilopterus, with the genera Procaraima and Paleopsilopterus also originally included (2), though the former has also been placed within Mesembriornithidae (10) and the latter is quite controversial and may not actually constitute a phorusrhacid at all (4). A recently discovered genus Eleutherornis has also been assigned to the psilopterines (4). This clade is accepted by most phylogenetic studies (1, 10), though 1 considers them to be a family separate from but closely allied to Phorusrhacidae (1). Psilopterines are undoubtedly the oldest known subfamily, depending on the status of Paleopsilopterus dating back to either the Palaeocene or Eocene epochs (2, 4), and potentially the very last survivors of the clade (16).

A size of up from the Psilopterines, we have the Mesembriornithinae. Initially erected based solely on the genus Mesembriornis which is a medium-small genus of terror bird, with a somewhat gracile build (Alveranga & Hofling 2003). An additional two similarly sized genera are suggested to belong to this group now, the aforementioned Procariama and the newly discovered Llallawarvis (10). All genera of this subfamily are known from the Late Miocene and Early Pliocene (2, 10). Much like the psilopterines, the group seems well supported by phylogenetic analyses (10, 1), though 1 once again does not consider it to belong to Phorusrhacidae and instead places it as the immediate sister family.

Most messy are the remaining three subfamilies, which are much closer to the mental evoked by the term ‘terror birds’ all possessing especially large beaks and considerable sizes. Starting with the Patagornithinae, named after the genus Patagornis, but also comprising Andrewsornis and the particularly well-studied Andalgornis (2), these medium sized phorusrhacids were relatively slim in build though with skulls much greater in size than either the Mesembriornithines or Psilopterines and are known from the middle Oligocene to the Early Pliocene (Alveranga & Hofling 2003). Brontornithinae on the other hand were amongst the largest of the terror birds and the bulkiest, the group consists of the namesake Brontornis as well as Physiornis and Paraphysiornis and these genera were known from the middle Oligocene to the middle Miocene (2). Recent analyses have shown however that Brontornis itself is not a Phorusrhacid at all! In fact, it seems to belong either to a separate branch of the Cariamiformes (Worthy et al 2017) or the Waterfowl (Anseriformes) much like the giant Gastornithidae and Dromornithidae (Agnolin 2021), where exactly this leaves Physornis and Paraphysornis within Phorusrhacidae is unclear. Lastly are the best known of the terror birds, the Phorusrhacines which include famous genera such as Phorusrhacos, Titanis and Kelenken (5) as well as the more obscure Devinsenzia (2), Kelenken probably constitutes the as of yet largest terror bird (5). Phorusrhacines struck somewhat of a middle ground between the gracile Patagornithines and the bulky Brontornithines. As neat as these subdivisions are, phylogenetic analysis both by 1 and Degrage et al 2015 contest the validty of them. Simply put members of these three subfamilies all cluster together and should probably be treated a single subfamily (or in the opinion of 1, a single family), whether any of these three subfamilies form distinct clades is less clear, 1 found monophyly in the Brontornithinae, but paraphyly in the Phorusrhacines (1), whereas 10 found no monphlyly at all (10).

This overview has omitted a number of controversial remains, including the probable phorusrhacid Lavocatavis and additional phorusrhacids which have recently been reassigned to other groups (8). Clearly much more work needs to be done on terror bird taxonomy and phylogenetics before a clear picture can be generated.

Likewise, the origins of the terror birds are controversial. Phorusrhacids are best known from South America where they have a rich fossil record which includes the vast majority of the known genera and species. Conclusive remains dating back to the early Eocene (4) are known, with Paleopsilopterus possibly pushing the date back to the Palaeocene (Alvarez & Hofling 2003), and highly controversial material even suggested from the cretaceous period (2, 8). Until recently it was widely accepted that Terror birds originated from South America and were endemic there until they entered North America during an event known as the Great American Biotic Interchange (GABI) where North and South America formed the landbridge in Panama during the Late Pliocene (4). However, the recent discovery of Lavocatavis from Algeria, and Eleutherornis France and Switzerland suggests that during the Eocene Phorusrhacids were present in Europe and Africa as well (4). Findings from Antarctica have also been documented, though highly disputed (2, 8). This places uncertainty on the point of origin of the Phorusrhacids. Likely they did not originate in Europe as the earliest remains are younger than those of Africa and South America, and no other remains from Laurasia are known (Barring the late remains from North America). Instead, early Phorusrhacids likely crossed into Europe from Africa across the Tethys (4). Between Africa and South America, it is much harder to ascertain a point of origin, the two continents likely split apart in the Early Cretaceous but there seems to have been a corridor of dispersal between the two lasting into the Cenozoic (4) as other groups such as monkeys seem to have crossed during this time. So whilst we don’t know which landmass they evolved on, they likely made it across from one to the other. This hypothesis seems to be favoured over a gondwanan vicariance hypothesis, however, should strongly evidence of phorusrhacids from the cretaceous or Antarctica appear this would change.

The size of Phorusrhacids varied heavily as might be expected of such a diverse group. 2 estimates a mass of the smallest species, Psilopterus bachmani at 5kg with a height of about 80cm and the largest estimate for a species at 180kg for Paraphysornis brasiliensis (2) (not including the Brontornithines), though Kelenken guillermoi likely reached larger sizes (5, 10). The degree of size differences within each species is unclear: sex, age or even different spatial or temporal differences in populations are impossible to establish without an abundant fossil record.

Most Phorusrhacids were undoubtedly flightless, their wings were heavily reduced and absolutely could not carry their heavy mass (2). Some controversy exists regarding the smaller Psilopterines, if they were capable of flight it would have been in a very limited capacity as their wing to body ratios are inferior to those of the related sereimas (Cariama spp.) which are only capable of gliding flight (2). However, whether Psilopterines (and indeed early Phorusrhacids in general) could fly at all has large rammifications on their ability to disperse across continents, which would lend credence to the continent hopping to Europe and between South America and Africa.

More controversial is the cursorial abilities of the terror birds, Alverenga & Hofling 2003 suggests that most species were rather slow moving due to their relatively short tarsometatarsus (The lowest leg bone in birds), though noting some members of the family such as Patagornis exhibited a higher degree of cursoriality (2). A more recent mechanical analysis suggests otherwise, it estimated the running speed of Mesembriornis at a staggering 97km/h and Patagornis at an impressive 50km/h, demonstrating that the birds were capable of rapid movement (6). An unclassified large Phorusrhacine was estimated at 50km/h as well, though this number may a slight overestimate according to the authors, even so these were quite clearly not slow-moving organisms (6, 9). Interestingly in a morphometric analysis terror birds clustered with different bird groups in terms of hing leg and pelvic proportions, indicating a wide range of locomotion methods across the subfamilies. The small Psilopterines clustered closely with the bustards (Otididae), indicating they may have been walkers rather than runners and may have been aided by limited flight capabilities, though affinities to wading birds is also noted (22). The rapid Mesembriornithinae on the other hand appear adept runners clustering closely with the ratities (22) and the Patagornithines showing affinities to both these groups (22).

Terror birds also had a characteristically large skull with a downwards pointing tip on the upper beak, much like many birds of prey, these undoubtedly point towards a predatory diet, but their prey species and feeding methods are still debated. 14 found that the cranium of the terror bird Andalgornis best withstood dorso-ventral (top to bottom) stresses, and very weakly lateral (sideways) stress, and this probably was true for all non-psilopterine terror birds, though the skulls of some large terror birds were more compact (9). This indicates that the group weren’t capable of holding down prey like many mammalian predators (14) or that they did so with their feet (14). Instead, it seems they either fed on animals significantly smaller than themselves, which would not put up too much resistance or alternatively used their tip like a hatchet and produced well-targeted strikes to vulnerable parts of the prey species, such as the skull or neck (14). Both the structure of the ear canal and neck would support the use of rapid movements of the skull which would be expected in these strikes or pursuit of swift prey species (10, 19). Little is known of the sensory capabilities of the terror birds, except their auditory capabilities which appear comparable to sereimas and ratites (13). Overall Phorusrhacids appear to be widely different, but show tendencies towards a pursuit predatory lifestyle. 10 suggests a niche partitioning between contemporary phorusrhacids with the large Phorusrhacines preying on megafauna, medium sized patagornithines on medium sized prey in the 10-20kg range and the small psilopterines on small animals (10(2)), but further details need to be resolved this can be fully evaluated.

The decline and extinction of the Phorusrhacids is usually ascribed to the Great American Biotic Interchange, which took place during the early Pliocene period and was an event in which North and South America collided via the Isthmus of Panama. Particularly the influx of various groups of carnivorous mammals such as big cats (Felidae) and bears (Ursidae) (Cione et al 2015) are often thought to have outcompeted the terror birds. This narrative is not so clear-cut as terror birds are quite rare in the fossil record from the early Pliocene (2), whereas these new groups of hypercarnivores do not become well established until the Pliocene-Pleistocene boundary (Cione et al 2015). This gap could simply be a result of the poor taphonomy of Pliocene South America or be a result of an early immigration event such as the canids and procyonids (Which grew to large sizes in the Pliocene)(Cione et al 2015). A detailed investigation of the decline and extinction of the Phorusrhacids is outside the scope of this article, however regardless of whether they declined due to the GABI or some other event, the known diversity was limited past the Late Pliocene. And yet, terror birds did not go extinct immediately following the GABI, remains from three of four species of terror bird have been described from the time period following this event.

Blanco, R., & Jones, W. (2005). Terror birds on the run: a mechanical model to estimate its maximum running speed Proceedings of the Royal Society B: Biological Sciences, 272 (1574), 1769-1773 DOI: 10.1098/rspb.2005.3133

Unfortunately, the birds fail to live up to their title. The birds get an overhaul from Smith, including claws, a long tail, the ability to instantly blend in with their surroundings, and the ability to perfectly mimic human speech. Despite this, the birds do very little more than dither for roughly 300 pages, and in an awkward attempt to leave the story open for a sequel, their fate is ultimately left unresolved. I have a feeling that The Flock will wind up being one of those low-budget SyFy original films, though I have to give Smith credit for igniting my curiosity about Titanis. Just how bad-ass was Florida’s own terror bird, exactly?.

Zimmer proposed that Tyrannosaurus and other Cretaceous monsters vanished over 60 million years ago, but their avian cousins continued their voracious legacy. And it makes sense that Smith took some artistic license in giving The Flock’s terrifying birds long tails; with just a tiny bit more effort, he was able to bring dinosaurs back to life without the aid of intricate genetic labs or a lost world tucked away on a rainforest pleateau. It makes sense to ignore a twelve-thousand-year gap rather than a sixty-five-million-year or longer one, which is why Titanis is a useful monster to bring up to the present day.

Gould, G. C. , and Quitmyer, I. R. (2005). BONES OF CONTENTION: TITANIS WALLERI Bulletin of the Florida Museum of Natural History, 45 (4), 201-229

What brought the terror birds to North America was one of the greatest exchanges of animals to have ever occurred on the planet. Around three million years ago, the isthmus of Panama formed a solid connection between North and South America. What George Gaylord Simpson called the “splendid isolation” of South America was finally broken. Small animals had already been dispersing between the continents for millions of years by this point, but this final connection allowed many of the larger species – those incapable of island-hopping, flying, or rafting – to move between continents. Prehistoric elephants and saber-toothed cats moved south, giant sloths and armored glyptodonts moved north, and among the last of the terror birds also made the trek northwards.

The remaining three subfamilies are the messiest; they are much more akin to the idea conjured up by the term “terror birds,” all of which have particularly large beaks and substantial sizes. Beginning with the Patagornithinae family, which takes its name from the genus Patagornis and also includes Andrewsornis and the extensively researched Andalgornis (2), these medium-sized phorusrhacids were known to have existed from the middle Oligocene to the Early Pliocene. Despite having much larger skulls than either the Mesembriornithines or Psilopterines, they were relatively slender in build. The group, which includes the namesake Brontornis as well as Physiornis and Paraphysiornis, was known from the middle Oligocene to the middle Miocene (2). On the other hand, Brontornithinae were among the largest and bulkiest terror birds. However, recent analyses have revealed that Brontornis is not at all a Phorusrhacid! Rather, it appears to belong to either the Waterfowl (Anseriformes) or a different branch of the Cariamiformes (Worthy et al., 2017), much like the enormous Gastornithidae and Dromornithidae (Agnolin 2021). It is unclear, therefore, exactly where this leaves Physornis and Paraphysornis within the Phorusrhacidae. The last group of terror birds are the best-known, the Phorusrhacines, which include well-known genera like Phorusrhacos, Titanis, and Kelenken (5) in addition to the less well-known Devinsenzia (2). Kelenken is most likely the largest terror bird as of right now (5). A compromise between the graceful Patagornithines and the hefty Brontornithines was achieved by Phorusrhacines. Even though these divisions are well-organized, phylogenetic analysis conducted by 1 and Degrage et al. (2015) challenges their validity. It is unclear whether any of these three subfamilies form distinct clades; 1 found monophyly in the Brontornithinae but paraphyly in the Phorusrhacines (1), whereas 10 found no monophyly at all (10). To put it simply, members of these three subfamilies all cluster together and should probably be treated a single subfamily (or, in the opinion of 1) a single family.

That final fossil discovery might or might not offer that information. 16 reported on a phorusrhacid humerus from Curupi Creek, Uruguay; sediments in the area were dated to the Late Pleistocene, or roughly 96,040 years before the present (16). The specimen has been assigned to the binomial Psilopterus sp., which clearly indicates that this bone belongs to the Psilopterine terror birds, and especially to Psilopterus. , with the possibility of a species designation coming later when more information is discovered (16) The fossil’s fragmentary state means that future research based on new data may need to reclassify it. This is especially true since the current designation would extend the Psilopterus genus’ age by approximately 30 million years, from the middle Oliogocene to the Late Pleistocene (Alvarenga Although the relationship between this humerus and the previously mentioned tarsometatarsuses is unclear, it is possible that they are members of the same Late Pleistocene species because 16 mentions the tarsometatarus’ affinity for psilopterines. Although the humerus does seem to come from a slightly smaller specimen, intra-specific variation may be the cause of this. With independent datings of small phorusrhacids and large terror birds to both 17,620 and 96,040 years ago, it seems to be quite clear that these birds survived until the Late Pleistocene, and possibly even later, until the Middle Pleistocene.

Since its remains are fragmentary and are only known from a single tibiotarsus, the species of Phorusrhacine from Uruguay that Tambussi et al. described in 1999 is still unnamed. However, given the size of the bone, it was clearly an apex predator and a large species, resembling Titanis in size (20) Regretfully, dating this species is problematic, much like dating Titanis. The tibiotarsis was discovered in the Raigon formation, which is typically dated to the Pliocene–Early Pleistocene (20), though there are some age uncertainties associated with it (1). However, based on the faunal assemblages preserved there, the specific strata in which the phorusrhacine was discovered have been determined to be the Late Pliocene-Middle Pleistocene (7, 16). This would place the species in line with, if not slightly younger than, Titanis, and offer compelling proof of the survival of massive terror birds in South America following the GABI.

The first group are the Psilopterines, a group of small, light-built Phorusrhacids that are up to twice as big as secretary birds (Sagittaridae) and seriemas (Cariamidae), hardly deserving of the moniker “terror bird.” The genera Procaraima and Paleopsilopterus were originally included in the family, along with the namesake Psilopterus (2). However, the latter is highly debatable and may not even be a phorusrhacid (4), and the former has also been placed within Mesembriornithidae (10) The psilopterines also include the recently discovered genus Eleutherornis (4). The majority of phylogenetic studies (1, 10) accept this clade, however one believes that they are a distinct family that is closely related to Phorusrhacidae (1). Depending on the status of Paleopsilopterus, which dates to either the Palaeocene or Eocene epochs, Psilopterines are unquestionably the oldest known subfamily and may even be the last members of the clade (16).

The terror birds’, Alverenga, cursory abilities are more contentious. On the other hand, a more recent mechanical analysis indicates that Mesembriornis could run as fast as 97 km/h, and Patagornis as fast as 50 km/h, proving that the birds were able to move quickly (6). Even though the authors speculate that this estimate of 50 km/h for an unclassified large Phorusrhacine may be a little high, it is still evident that these were not slow-moving organisms (6, 9). Surprisingly, morphometric analysis revealed that terror birds’ hing leg and pelvic proportions clustered with various bird groups, suggesting a variety of locomotion strategies used by the subfamilies. Although affinities to wading birds are also mentioned, the small Psilopterines clustered closely with the bustards (Otididae), suggesting they may have been walkers rather than runners and may have been helped by limited flight capabilities (22). Conversely, the swift Mesembriornithinae seem to be skilled runners, forming close clusters with the ratities (22) and the Patagornithines exhibiting similarities to both of these groups (22).

FAQ

What killed off the terror birds?

This lead palaeontologists to initially say that it was competition with mammalian predators (e. g. Smilodon) that killed off the terror birds. Whilst this certainly didn’t help the terror birds, more recent theories suggest it was climate change that led to their extinction due to loss of habitat and prey species.

What happened to the terror bird?

But no, this animal was not outcompeted, due it being the last large South American terror bird, going extinct 1.5 million years before the GABI. The bird was driven extinct by climatic changes.

What is the closest thing to a terror bird?

The seriemas are the only known living relatives of the terror birds, an extinct group of very large, carnivorous, flightless birds.

What made terror birds so terrifying?

The terror bird – also known as Gastornis – was a flightless, around two-metre-tall bird sporting an enormously large, intimidating beak. Due to its size and terrifying appearance, most scientists have assumed that it was carnivorous, and it would have been the top predator of the Eocene epoch.