The unexpected discovery of a nearly complete skull
from the Early Cretaceous epoch that has been preserved in three
dimensions provides profound insights into the evolution and
biogeography of early mammals.
The fossils that advance our understanding of
evolutionary history often come from parts of the world that have not
been well studied by palaeontologists. Occasionally, however,
game-changing fossils are discovered in heavily surveyed regions by
targeting poorly sampled rock layers in those areas. In a paper in Nature, Huttenlocker et al.1
present one such discovery from the Cedar Mountain Formation in North
America — an Early Cretaceous rock formation in Utah dated to between
139 million and 124 million years ago. The authors describe a complete,
3D fossil skull from a previously unknown genus and species, which they
name Cifelliodon wahkarmoosuch.
Palaeontological fieldwork in western North America has led
to the discovery of a greater number of Cretaceous mammal species and
their more primitive relatives (collectively called mammaliaforms) here
than in any other region in the world — more than 150 species have been
found2.
They are represented by tens of thousands of specimens, the vast
majority of which are isolated teeth from the Late Cretaceous epoch (100
million to 66 million years ago). A few of the specimens are lower
jaws, and many fewer are skulls or skeletons. The extreme rarity of 3D
skulls makes Huttenlocker and colleagues’ discovery momentous.
Aside from its 3D preservation (Fig. 1), the skull is remarkable in other ways. It is about 7 centimetres long, indicating that Cifelliodon
was about the size of a medium hare. This would have been unusual in an
Early Cretaceous world dominated by shrew- and mouse-sized mammals.
Among its North American contemporaries, only one known species is
larger — the carnivorous Gobiconodon ostromi2. In addition, the downturned face and relatively shallow snout make the skull unusual among early mammaliaforms.
Figure 1 | A fossil from the dawn of mammals. Huttenlocker et al.1
report the discovery of a nearly complete skull from the Cedar Mountain
Formation in North America, dated to between 139 million and 124
million years ago. They named the species Cifelliodon wahkarmoosuch,
and suggest that it belongs to a group of animals called haramiyidans.
The skull is shown in side view with the snout pointing left. Scale bar,
10 millimetres. (Image adapted from Fig. 1 of ref. 1.)
Huttenlocker et al. used an imaging technique called
micro-computed tomography (µCT) to reveal a wealth of anatomical detail
about the skull. For example, they found that Cifelliodon had a small brain with large olfactory bulbs. This combination is commonly seen in early mammaliaforms3, and is indicative of the keen sense of smell that is one of the hallmarks of mammals today.
Using
the same technique, the authors also demonstrated that the bones of the
occipital region at the back of the skull (in particular, the tabular
bones) remained unfused in Cifelliodon. Unfused tabulars are a rare trait in mammaliaforms — the tabulars are generally fused with the occipital bones in adult mammals, but remain separate in other vertebrates4.
But, as the authors point out, the supposition that these bones are
fused in early mammaliaforms could be biased by the rarity of
well-preserved skulls, or by the techniques used to analyse them. The
few mammaliaforms that have been found to have unfused tabular bones, as
in Cifelliodon, are known from 3D skulls or have been scanned using µCT1,5, which enables the detection of cranial structures that might be invisible on the skull’s surface.
The completeness of the Cifelliodon skull allowed Huttenlocker et al. to
assess the early branches of the mammaliaform family tree more robustly
than has previously been possible. This phylogenetic analysis places Cifelliodon
in the extinct Haramiyida — a group that has elicited controversy
because its position in the evolutionary tree has implications for the
timing of the origin of mammals.
Until five years ago, the fossil
record of haramiyidans consisted mainly of isolated teeth from a few
European sites from the Late Triassic (237 million to 201 million years
ago) and Early Jurassic (201 million to 174 million years ago), and a
lower jaw and a few postcranial bones from a Late Triassic site in
Greenland2.
In 2013, two almost-complete haramiyidan skeletons from the Middle
Jurassic (174 million to 164 million years ago) were discovered in
China, sparking controversy about mammalian relationships6,7. The two research groups involved came to very different conclusions: one placed Haramiyida in Mammalia6, the other outside it7. This difference is not trivial: it results in vastly different temporal estimates for the origin of mammals.
Placing
Haramiyida in Mammalia pushes the origin of the latter group back to
the Late Triassic (215 million years ago). Such a date would imply that
several mammalian clades, including the lineage that led to placental
and marsupial mammals, have earlier origins than was thought. This
earlier date also implies that there were long intervals of time in
which these early lineages were present but for which fossils have not
yet been found.
By contrast, placing Haramiyida outside Mammalia
suggests an origin in the Early Jurassic (about 185 million years ago).
This implies a relatively explosive diversification of early mammals. Several
further haramiyidans from the Jurassic period have since been
discovered in China, but the controversy has only intensified5,8–10.
Particularly problematic is the fact that, although the Chinese
haramiyidans are represented by complete skeletons, the specimens are
essentially 2D. Most of the skulls are little more than flattened
outlines, which limits their usefulness for informing mammalian
relationships. Cifelliodon is one of the first skulls
preserved in three dimensions from the haramiyidan lineage. As such, it
is a crucial piece of the evolutionary puzzle. Huttenlocker and
colleagues’ phylogeny puts Haramiyida (and so Cifelliodon) outside Mammalia (Fig. 2a). Thus, their work favours a model in which early mammals diversified rapidly during the Jurassic.
Figure 2 | Re-evaluating the evolution and biogeography of haramiyidans.a, Huttenlocker et al.1
analysed relationships between the early branches of the family tree
for mammals and their more primitive relatives. The resulting
evolutionary tree indicates that haramiyidans are not mammals, contrary
to some previous evidence5,6,8,9. The analysis also places the Cretaceous genus Vintana in Haramiyida for the first time. b,
Cretaceous haramiyidans (indicated by green circles) have previously
been found in northern Africa and possibly India. The authors’ analysis
expands the Cretaceous range of haramiyidans to Madagascar (Vintana) and North America (Cifelliodon).
Combined with the fact that other fossils of haramiyidans from the
Triassic (purple) have been found in Europe and Greenland, and that
haramiyidans from the Jurassic (blue) have been found in Europe, China
and Tanzania, this work implies a much broader temporal and geographical
distribution of haramiyidans than had previously been hypothesized.
Finally, Huttenlocker et al. provide evidence that Cifelliodon is closely related to Cretaceous species from northern Africa (Hahnodon taqueti) and Madagascar (Vintana sertichi),
the latter of which had not previously been assigned to Haramiyida.
This implies a much broader temporal and geographical distribution for
Haramiyida than has been assumed (Fig. 2b), indicating the need to
reassess the biogeographical history of the group. The authors conclude
that haramiyidans had a global distribution during the
Jurassic–Cretaceous transition, and that land bridges aiding vertebrate
dispersal existed long after the fragmentation of the supercontinent
Pangaea — much later than previously recognized. An alternative
hypothesis that is perhaps more consistent with current
palaeogeographical models11
is that haramiyidans, like many vertebrate groups, had a Pangaean
distribution in the Jurassic period and evolved in isolation thereafter,
as landmasses separated during the Cretaceous period. The best way to
test these competing hypotheses is with the discovery of more
well-preserved fossils, like this exquisite skull of Cifelliodon.
Nature558, 32-33 (2018)
doi: 10.1038/d41586-018-05134-9
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