History of Discovery
Known
in popular culture the world over as ‘terror birds’ the scientific
term for these birds is actually phorusrhacids, after the
Phorusrhacidae group to which they all belong. Naming taxonomic
families is actually a simple process of naming it after the first
member of the group to be discovered by science, in this case
Phorusrhacos.
Upon its naming by Florentino Ameghino in 1887 no
precedent for large carnivorous birds existed in South America
(Although other large birds were known in other parts of the
world), and Ameghino thought that the lower toothless jawbone
(the actual skull was not yet known) actually belonged to some large
mammal. However Ameghino soon realised that his original description
was a mistake, something that was confirmed by new and more complete
fossil material.
Large
flightless birds are actually well-known across the world, but their
diets have always been a matter of strong debate as their beaks are
well adapted for either a carnivorous or herbivorous lifestyle.
Phorusrhacids however have one key difference in their beaks, and
that is the way they strongly curve downwards to form a ‘pick’ that
protrudes well below the lower jaw. This feature is seen on every
single living bird that feeds by ripping flesh from the body of other
animals and is seen as a clear indication that phorusrhacids were meat
eaters. This dietary preference combined with the immense size of
some of their associated genera is why they are known the world over as
terror birds.
Phorusrhacid Morphology
& Biology
Phorusrhacids
were similar in their general appearance to other large flightless
birds, and for size the ostrich is sometimes used for a rough
comparison. Phorusrhacids typically had very long legs which saw them
carrying themselves high off the ground. Not only would this provide
efficient locomotion over distances, but many of the larger
phorusrhacids are thought to have been capable of incredible speeds.
Actual speed estimates can vary greatly but larger members are thought
to have been capable of at least attaining forty-five kilometres an
hour, with more gracile members of the group being even faster.
The
head was held up high on a long neck something that not only would have
given phorusrhacids significant reach, but a high vantage point to
spot prey. Study of large phorusrhacid skulls has also revealed areas
for large jaw opening muscles. Not only would these have counteracted
the jaw closing muscles, they may have also allowed for more rapid
opening and closing of the mouth.
CAT
scans of phorusrhacid skulls confirm that they were strongest when
withstanding downwards stresses from below, but not so much against
side wards motions. The beak was also confirmed as being completely
hollow and while this could be taken as a weight saving feature, it
has also been interpreted as a resonating chamber. If this
interpretation is correct then phorusrhacids may have called out to one
another by closing their bill hard against the skull with the sound of
the impact resonating inside this chamber.
CAT
scans have also allowed for reconstruction of phorusrhacid brains and
inner ears. The inner ears themselves are well developed to aid a
sense of balance like you might expect, but reconstructions also show
that phorusrhacids could probably hear a wide range of sound
frequencies (something that lends some support to the resonating
chamber theory. The actual brain reconstruction shows that
phorusrhacids had excellent vision and were able to make full use of
their height advantage. Phorusrhacids also had a well-developed
cerebral cortex that meant the phorusrhacids were not restricted to
pre-defined thought patterns and could adapt their behaviour to deal
with new situations. One particularly interesting part of the brain
was the severe underdevelopment of the brains olfactory ability. This
means that a phorusrhacids sense of smell was so poor it was barely
useful. It also means that phorusrhacids most probably had a greater
reliance upon actively hunting prey rather than scavenging.
Phorusrhacids
are what are termed secondarily flightless in that they evolved from
the early flying birds that first appeared back in the Mesozoic. The
wings themselves are greatly reduced and probably useless for flying,
but they may have still served a display purpose for attracting
mates. Claws on the wings may have also been used in interspecific
combat between individuals, perhaps as an alternative weapon to the
devastating beak.
Phorusrhacid
eggs are so far largely unknown and would have varied in size depending
upon the genera in question. Larger phorusrhacids may have had eggs
comparable in size to ostriches (ovoid and fifteen by thirteen
centimetres), although larger eggs may have been laid. In the
absence of eggs the only possible way of estimating egg sizes would
come from looking at the post cranial remains (where possible) and
making an educated guess.
Seriema birds
It
is hard to talk about terror birds without hearing seriema birds
mentioned somewhere. This is because the two extant (not extinct
and still alive) genera, Cariama cristata and Chunga
burmeisteri,
are thought to be the closest living things to the extinct terror
birds. Their overall body shape is very similar to the
Phorusrhacids, but upon a scale similar to the smaller members of the
group. Seriema birds are known to be capable of flying short
distances and roosting in trees up to three meters high off the
ground, but it is still uncertain if the smaller Phorusrhacids were
capable of this. While some palaeontologists have claimed it is
possible, others have refuted this idea.
Hunting, Killing and Eating
Although
fast runners, phorusrhacids may have still used ambush tactics to
approach prey. This is because their large and more importantly tall
forms would be easy to identify from a distance, allowing herbivores
the chance to extend the amount of ground between them and their
hunter. A better tactic for a phorusrhacids would be to stay on the
fringe of an outcropping of trees and blend in amongst the trunks. As
long as they stayed amongst the sparser areas of growth, or even
better scared prey into the open then they could use their superior
speed to close the gap and attack.
How
phorusrhacids killed prey depends upon the size of the prey in
question. As mentioned above the skulls were not adapted to withstand
large amounts of side wards stress which means that phorusrhacids
probably would not have tied to hold onto larger prey for risk of
injury. This also makes it unlikely that smaller prey was shaken from
side to side as some mammalian carnivores are seen to do. The actual
shape of the skull and the fact that it can withstand strong downwards
forces is what gives clues to the two most widely accepted methods of
killing.
The
pronounced hook tip of the beak is seen in most predatory birds as a
feeding aid, but many phorusrhacids have such pronounced tips that
they seem unnecessarily large. It’s thought that the larger
phorusrhacids could combine this tip with their strong height advantage
to initiate a series of downward strikes on top of their prey,
specifically key critical areas such as the neck. spine and top of
the skull that would have been most exposed from above. When brought
down with enough force this tip could have shattered and separated
vertebrae as well as cave in the top of the skull, either injury
capable of bringing paralysis or even instant death to their prey.
Killing of smaller prey is based upon observation of seriema birds
which pick up small vertebrate prey like lizards in their beaks,
before throwing them back down against the ground and hard objects
with as much force as they can. Not only does this stun and kill the
prey but it also breaks the preys bones so that it can be more easily
swallowed whole.
Often
phorusrhacids would have killed prey that was simply too large to be
swallowed and would have needed to have been eaten in bites. Here the
hook tip of the beak would dig into the flesh and pull back. The
lower jaw would then close, shearing off the chunk of flesh so that
the bird had a bite sized morsel. Phorusrhacids may have also used
the claws on their feet to tear up carcasses as well as pushing down on
them so that they could use their body weight to help remove pieces of
flesh.
World Distribution
The
vast majority of phorusrhacids currently known have been found in South
America, especially in Argentina, Uruguay and Brazil. The precise
evolutionary beginning of phorusrhacids is not certain, but the
oldest known member, Paleopsilopterus,
was discovered in Brazil and
is dated back to sixty million years ago. So far the only exception
to the South American exclusiveness to the phorusrhacids is Titanis,
the first Terror bird to be discovered n North America.
Titanis
fossils discovered in Texas however have been dated to the early
Pliocene five million years ago, long before the establishment of a
permanent land bridge to North America. There does seem however to
have been the development of islands between North and South America
during this earlier time. Tectonic movement was still pushing up land
masses and fluctuating sea levels may have caused a large variance in
land coverage. Since Titanis could not fly it
would have had to
swim, raft or exploit changing coastlines all the way to North
America.
However
the same series of events that allowed Titanis to
colonise North
America may have also brought about the downfall of the Phorusrhacidae.
The Great American Interchange
and the Extinction of the Terror Birds.
Phorusrhacids
disappear from the fossil record around two million years ago, during
the Gelasian stage of the Pleistocene. Some sources however still
continue to incorrectly quote their final disappearance as being as
recent as ten thousand years ago. This figure is based upon the
discovery of Titanis remains along with other
creatures known to live
from this time period, but these remains were all at the bottom of
the Santa Fe River in Florida, and had been washed about by the water
currents which caused all of the fossils to be mixed up. Analysis of
the mineral content of the fossils confirmed that the Titanis
fossils
were actually formed two million years prior (more details for this
are on the Titanis
page on this website).
Around
half way into the Pliocene period (three and a half million years
ago) the Isthmus of Panama was formed by a combination of volcanic
and tectonic activity. Ocean cooling towards the end of the
Pliocene also resulted in falling sea levels which further increased
the new land masses. The formation of this land bridge had a profound
effect upon the world, disrupting ocean currents and migratory routes
of animals like whales. But the most significant development for the
phorusrhacids was that they were no longer isolated from the rest of
the world. This triggered the event now called the Great American
Interchange which saw new predators such as the sabre toothed cat
Smilodon
establish a presence in areas where phorusrhacids had
previously been the dominant predators. Phorusrhacids had already
lived alongside other predators such as Thylacosmilus,
but native
South American predators had always evolved and lived under the threat
of the phorusrhacids. The new predators now spreading into South
America however had evolved along their own lines, and were likely to
have had different hunting behaviour and strategies that could have put
them on a par with the phorusrhacids.
Another
factor that affected the phorusrhacids was South America’s steadily
changing climate. The rising Andes Mountains that run down the
eastern side of South America triggered most of the rain fall to fall
on them (as clouds rise over mountain ranges changing air currents
and pressures trigger rainfall). This resulted in other parts of
South America becoming much drier with forested scrublands becoming
arid steppes and open savannah. Not only would this change the kinds
of herbivores available, phorusrhacids would have found themselves
very exposed and not able to use cover to hide their tall forms. Even
fast running predators don’t have much chance of catching prey if
their prey can see them coming at a distance. It would at the very
least mean that phorusrhacids would have had to expend a greater amount
of energy in killing prey, reducing their efficiency and making it
harder to compete with new predatory rivals. The attrition of these
two factors combined is probably what saw the downfall of the
phorusrhacids that are now forever known as the terror birds.