What traits distinguish one dinosaur from another? For many, that
question probably conjures characteristics like the massive jaws of
tyrannosaurs, the distinctive horns and plates of ceratopsids and
stegosaurs, and the exceedingly large bodies of sauropods and their kin.
But for paleontologists looking to classify different dinosaurs, it’s
all about the shapes, or morphologies, of the animals’ bones.
With a knowledge of the morphological traits of dinosaur bones and
enough of a fossil skeleton, paleontologists can broadly identify where
on the dinosaur tree a specimen fits. Then they can look to other traits
— specifically those shared by subsets of dinosaurs — to figure out
where the animal belongs in more detail.
The dinosaur family tree we use today was first published in 1887.
After studying the orientation of the bones that make up dinosaurs’
hips, paleontologist Harry Seeley
proposed
that all dinosaurs fit into one of two groups, based on the structure
of their pelvises. As in all tetrapods, the dinosaur pelvis, where the
hips attach to the spine, is made up of three bones: the illium, the
ischium and the pubis. He found that, in some species, like the
sauropods, the pubic bones point toward the front of the body, while in
other species, such as the duck-billed hadrosaurs, they point toward the
back, running parallel to the ischia. Seeley called the former group
Saurischia, meaning “lizard-hipped,” because of the resemblance of the
orientation of these animals’ hip bones to those in lizards; animals in
the latter group became Ornithischia, meaning “bird-hipped,” because of
the similarity of their hip structure to that of modern birds. From
those two broad branches in Seeley’s classification sprung more
specialized groups of dinosaurs, including the theropods — saurischians
that include tyrannosaurs, dromaeosaurs and birds, among others.
The skeleton of the Tyrannosaurus rex named “Sue” at Chicago’s Field
Museum of Natural History displays the forward-facing pubic bone in its
hip. Theropods like T. rex have traditionally been considered part of
the dinosaur group Saurischia, meaning “lizard-hipped,” because of the
resemblance of the orientation of these animals’ hip bones to those in
lizards. Credit: Connie Ma.
This view of a Stegosaurus skeleton shows the backward-facing pubic
bone in its hip. Stegosaurs are part of Ornithischia, which means
“bird-hipped,” because of the similarity of their hip structure to that
of modern birds. Credit: Susannah Maidment, Natural History Museum,
London.
This split between bird-hipped and
lizard-hipped dinosaurs has been entrenched among dinosaur researchers
since Seeley’s days. But in a 2017 study in Nature, a team led by
paleontologist
Matthew Baron of the University of Cambridge in England
proposed
a radical reconfiguration of that arrangement. Baron and his team
suggested that the ornithischian-saurischian classification should be
set aside in favor of a revised tree in which the dichotomy in hip
structure is not all-encompassing. This new classification combines
theropods with ornithischians in a new group called Ornithoscelida,
while the sauropods and herrerasaurids — an early group of carnivores
that died out in the Triassic — remain as the saurischians (technically
sauropodomorphs).
Such a reorganization, involving branches near the base of the
dinosaur family tree, represents a major overhaul — one that challenges
an orthodoxy built on roughly a century and a half of research. Among
the many implications, the new classification suggests that carnivory in
dinosaurs evolved independently at least twice, in herrerasaurids and
theropods. With both of those groups previously considered saurischians,
it appeared carnivory had evolved just once. Also, feathers, once
thought to occur across multiple dinosaur lineages, now seem to have
evolved in only the theropods — the line that led to the birds.
Additionally, it was unclear from Seeley’s long-standing family tree
whether the last common ancestor of all dinosaurs was lizard- or
bird-hipped, or how many times the evolutionary transition from one to
the other conformation might have happened. The new tree implies that
the first dinosaur was lizard-hipped and that bird-hipped morphologies
evolved later in multiple ornithoscelidan lineages, specifically among
the ornithischians and several theropod groups.
But many paleontologists are not
ready to accept the revised tree; such a big shift in our understanding
of dinosaur evolutionary history is a steep proposition, and some
researchers are calling for caution and more study.
All About the Bones
Left: The traditional dinosaur family tree, with Ornithischia and
Saurischia separated based on differences in hip-bone morphology. Right:
The dinosaur family tree proposed by Matthew Baron and his colleagues,
in which Ornithischia forms a new group with Theropoda called
Ornithoscelida. Credit: K. Cantner, AGI.
The key to differentiating dinosaur
groups is anatomical data, specifically related to bones. However, those
data, because they can be somewhat subjective, are not always easy to
define. A paleontologist looking to distinguish one dinosaur species or
group from another logs differences in bone morphologies. If enough
differences are logged, a picture emerges of how dinosaurs are distinct
animals — from the clade level all the way down to individual species.
Broadly, dinosaurs as a whole are distinguished from other reptilian
groups like pterosaurs by the presence of a hole in their hip sockets.
Although hip bones have been the major focus, there are numerous other
bones to consider as well: femurs and teeth, for example. While
sauropods like Apatosaurus had relatively straight femurs resembling the columns that support ancient Greek temples, theropods like Tyrannosaurus rex
had femurs with more of a bowed shape. Within sauropods, meanwhile,
there are distinctions in tooth morphology: Some have spatula-shaped
teeth while others have more peg-like teeth.
This is essentially what Baron and his team did in their 2017 study:
They assembled a dataset of morphological features that they thought
best distinguished dinosaurs, including everything from unique openings
in the skull to the varying shapes of different limb bones. In total,
they looked at 457 traits from 74 different dinosaur taxa, with most of
their data coming from previously published scientific literature.
Although the researchers didn’t set out to fundamentally reshape the
dinosaur family tree, Baron explains, their findings led them to do just
that.
A Gnarly Tree Trunk
Eoraptor lunensis is an early dinosaur that may be key to settling the
current debate over the dinosaur family tree. Credit: Fossil Lab,
University of Chicago.
The exercise of parsing features to
distinguish dinosaur species gets trickier the closer you get to the
base of the dinosaur evolutionary tree, where the major groups diverge.
The early species that are known all tend to look somewhat similar, and
paleontologists often have a hard time telling who’s who. That is the
root of the debate, because those early dinosaur species — called
dinosauromorphs — are key to deciphering the base of the dinosaur family
tree, says
Steve Brusatte, a vertebrate paleontologist at the University of Edinburgh in Scotland, who co-authored a
response in Nature rebutting the Baron team’s revised tree.
One of those early species is the Late Triassic Eoraptor lunensis, known from a nearly complete skeleton found in the Valley of the Moon in northwestern Argentina. Eoraptor
was a bipedal dinosaur that lived about 230 million years ago on the
supercontinent Gondwana. About the size of a dog, it lived before
dinosaurs were the top predators on the landscape, and before they
diversified into the many famous forms known today.
Eoraptor lunensis had forelimbs that appear adapted for
grasping, and it looks like it is the “perfect, standard-issue basal
dinosaur,” Baron says, because it appears it could be almost any
dinosaur’s ancestor — which is a problem when it comes to figuring out
which dinosaurs might be its direct descendants. “Eoraptor is a nightmare … there are as many opinions on what Eoraptor is as there are workers in the field,” Baron says.
The Late Triassic Eoraptor is known from skeletons found in Argentina, such as this one. Credit: public domain.
Baron and his colleagues think the grasping forelimbs resemble those of later theropods. Other researchers, including
Paul Sereno, a paleontologist at the University of Chicago who helped discover
E. lunensis
in the 1990s, think it is a member of the sauropodomorphs — ancestors
of the sauropods — based on things like its thumb. The bones of its
thumb twist such that the side of the thumb contacts the ground, which
is a hallmark trait of later sauropodomorphs, Sereno says.
Another early dinosaur,
Heterodontosaurus tucki,
categorized as an ornithischian based on features like its bird-hipped
pelvis, was found in Early Jurassic rocks in South Africa in 1961. But
it “has a large number of features that were only otherwise seen in the
meat-eating dinosaurs, the theropods,” says
Paul Barrett,
a dinosaur specialist at the Natural History Museum in London and a
co-author of the 2017 study that proposed the new tree. It was
H. tucki’s
similarities to theropods — including long, slender shoulder blades, an
outgrowth at the front of the hip for muscle attachment, and forelimbs
adapted for grasping, as in
E. lunensis — that first led Baron,
Barrett and their colleagues to think that “something more complicated
was going on” regarding the dinosaur tree.
H. tucki “is an ornithischian,” Barrett says, but based on
the proposed new tree, it appears it’s more closely related to the
theropods than researchers have thought.
Where these early dinosaurs reside at the base of the family tree
has significant implications for the structure of the overlying
branches. But the uncertainty surrounding the identity of early
dinosaurs — what they were, and which later dinosaurs they were most
closely related to — means that their positions on the evolutionary tree
are not settled. “They move all around the family tree” depending on
whom you ask, Brusatte says. “Some of these weird species have
combinations of [characteristics from] different groups. They can cause
trouble, but those species can also provide clarity. They can unite
groups together,” he says.
Deciding What’s Most Important
But before they can define anything, scientists have to come to an
agreement on how important or relevant various morphological
characteristics, such as thumb shape, are compared to others when it
comes to deciphering relationships. And how should these characteristics
even be assessed? It’s all too subjective, Sereno says.
Sereno says that the current way dinosaur paleontologists assemble
and use datasets, and thus how they build evolutionary trees, needs to
change. “You can make a hypothesis and literally ignore a huge part of
the data that somebody else used to look at the same group,” he says.
Like Eoraptor, Heterodontosaurus tucki (above and below) is one of the
early dinosaurs discovered in the last few decades that prompted some
paleontologists to rethink the dinosaur family tree. Credit: top: Tyler
Keillor; below: Funk Monk, CC BY-SA 3.0.
When researchers assess bone
morphology, for example, they might describe a particular part of a bone
as being round or square-shaped, but that assessment is up to each
researcher. Furthermore, if someone can’t decide whether the morphology
is round or square, they might simply note the characteristic as
“absent.” But, Sereno explains, absence or presence has nothing to do
with shape — the quality of interest — so describing a shape as absent
obscures the meaning or relevance of the characteristic with respect to
deciphering evolutionary relationships.
Standardizing the assessment of dinosaur characteristics and the use
of datasets is one of the only ways to get around the issue of
researcher subjectivity, Sereno says. “We need more rules.”
How those new rules might take shape is unclear, but until
standardization occurs, different results and different evolutionary
trees will continue to emerge whenever different researchers run
analyses. Following the proposal to revise the dinosaur tree in 2017,
for instance, Brusatte and his co-authors studied the same set of
characteristics that Baron’s team had used in its study. They went
through each morphological trait considered in the study and
reinterpreted them based on their own expertise. What they came up with
was a tree that supported the traditional ornithischian-saurischian
arrangement. Brusatte says subjectivity is “just the nature of the
game.”
Much in paleontology is left to subjective descriptions and
assumptions, such as whether Eoraptor’s “grasping forelimbs” more
closely resemble those of later theropods or later sauropods. Credit:
James Kuether.
What everyone involved in the debate
seems to agree on is that “we really need more fossils: especially new
fossils of Triassic dinosaurs from near the base of the dinosaur family
tree that can help untangle the relationships of these lineages,”
Brusatte says. “[The debate] has revealed that something we thought was a
certainty is actually a mystery. And so that mystery has to be solved.”
Baron agrees: “It could be that two or three discoveries make all the difference.”
For now, which version of the dinosaur family tree — the traditional
or revised — comes closer to the truth remains unclear. Meanwhile, to
surmount differences in opinion and create a standard set of metrics
with which to parse fossils, “we need a lot more collaboration” among
researchers, Baron says.
Joel (
www.lvjwriting.com)
is a freelance science journalist. He has a master's in paleontology
from the University of California, Riverside, and is currently based in
Boulder, Colo.
Monday, February 4, 2019 - 06:00
Nenhum comentário:
Postar um comentário
Observação: somente um membro deste blog pode postar um comentário.