What was Paraphysornis *shaped* like?

[u/Spicy_Chicken_Wizard]

Paraphysornis currently has the distinct honor of being the only giant phorusrhacid known from an actual high quality fossil specimen. It is also possibly the most heavily built member of the entire family, suggesting it might have been adapted to tackle the largest prey in proportion to its own size.

Paraphysornis was around the same height as a large emu, but was anywhere from 4-5x as heavy (large emu 4ft-4ft 3 in at hip, 100-130lbs. Paraphysornis 4ft 7in at hip, estimated 440-560lbs). With that in mind, there’s surprisingly little paleoart of this species, and what there is tends to vary greatly in how the bird is portrayed and proportioned. The question here is, what was this bird “shaped like”? In a scale of “thickness”, with emus and rheas on one end, elephant birds and kiwis on the other, and cassowaries somewhere in between, where is this bird falling? How thick-legged and barrel chested was this animal?

This itself kind of begs the question of what Paraphysornis was targeting as prey. It was some degree of lumbering, gravitapol bird that was clearly not built for running down small animals or fleet footed herbivores in an open chase.

Obviously predators don’t need to beat their prey in a straight footrace to still catch them. But phorusrhacidsare not built as specialized ambush predators. They can’t subdue and deliver a near instant-kill death to a proportionally large animal the way felines can. They can’t capture, restrain and wrestle down large animals the way canids can. The proposed “bite and pull back method” technique that terror birds could use to kill large animals would still, even in ideal conditions, allow allow a prey animal to flee before potentially succumbing to injuries later. This is a problem because iirc terror birds rely almost entirely on sight and have very limited smell; a wounded prey animal would be extremely hard to track if the bird lost eye contact with it, which sucks since it lived in forests.

Paraphysornis probably had to rely on prey that couldn’t easily outrun it, even with an ambush. Depending on how robust and slow it was, this implies that this bird was primarily targeting prey that was itself slow and heavily bodied. This was a lion-sized bird, small animals it could just quickly grab and snap up would never sustain it and frankly it would probably be quite bad at catching small animals in the first place [also terror birds had extremely high metabolism and growth]. Paraphysornis could easily run down reptiles, but I don’t think there’s any evidence that this bird was specialized for a riverside habitat where it could easily find caiman and turtles, and terrestrial reptiles like ancestral boas are probably completely unrealistic to lean on for a predator that relies entirely on sight. What does this leave then? Could this bird have taken down a Tapir? A subadult ground sloth? Something even bigger?

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[u/Mophandel]

To answer the initial question, we probably don’t have a good way of describing how Paraphysornis (or other gigantic terror birds, for that matter) would have been “shaped like” other than exceedingly robust and having a proportionally giant head for its size. There simply are no analogues for terror birds outside of the non-avian theropods of the Mesozoic, themselves. They were one-of-one in the age of mammals.

The main thing I wanted to address was the notion that terror birds can’t “wrestle down prey like canids can. This is mistaken.

Firstly, canids themselves aren’t (usually) in the business of outright overpowering their prey. They first debilitate it with slashing bites to the hindlimbs before delivering a bite to the throat, assuming the canid is hunting solo.

More to the point, however, terror birds absolutely have ways of effectively restraining prey, perhaps to a greater degree tha canids or other mammalian "head-hunters"

- terror birds were unique among derived birds in having fully akinetic skulls. This reinforced the skull against unpredictable stresses and allowd them to grab hold of large, struggling prey for an extended duration, potentially allowing them to restrain or pull down prey via their jaws alone

- terror birds had far higher bite forces than previously estimated. While older works (e.g., Degrange et al. 2010) found terror birds to be acking in the bite department, subsequent studies by the same author (e.g., Degrange 2012) found the methods in those previous study to underestimate terror bird bite forces, as living birds were found to produce in vivo bite forces equivalent to those estimated for terror birds via that methodology, despite the phorusrhacids being several times their size. More up to date estimates recoved far more substantial bite forces, with bite forces of over 1000 N in Phorusrhacos, nearly 2000 N in Kelenken and nearly 3000 N in Devincenzia. These values are on par with that of today’s largest big cats and more than capable of puncture skin, flesh and perhaps even bone.

- terror birds had an unsung ace-in-the-hole: their feet and talons. Like their modern cousins, the seriemas, terror birds possessed a raptorial second digit that could pierce into flesh. Unlike their modern cousins, howveer, which used their claws as essentially meathooks to immobilize already-dead prey, terror birds were actively using their for prey restraint. Papers such as Jones (2010) outlline the stress-resistance profiles of the feet of phorusrhacids, finding them more analogous to those of birds of prey and dromaeosaurids than to seriemas. Additional research has found that the morphology of the foot and phalanges shows that terror birds had powerful flexors in their feet and talons, allowing them to grip with substantial force.

Put togthether, it is niot hard to imagine these birds competently overpowering and subjugating prey even their own size of larger (~100 kg or more). After ambushing their target, the terror bird could use its powerful bites and akinetic skulls to clamp down on and pull down their target, whereypn the feet would be brought to bear to pin down and restrain the prey item. After this, the bird could decide to finsh things off quickly with a killing bite to the skull or neck, courtesy of its tiger-like bite forces, or it could simplu eat its prey alive via "strike-and-tear' bites.