2012-2013. acrylics on card, digital & photography.
c. 419,000,000 bp, Late Silurian (Ludlow), Qujing, Yunnan, China.
A trio of Entelognathus
swim over a shallow Silurian seabed. Swarms of worm-like conodont animals (Ozarkodina snajdri
) wriggle about them while a jawless galeaspid (Dunyu longiforus
) rests on the sand. A marauding Guiyu oneiros
, one of the earliest bony fishes, cruises in the distance.
Cover art for =
Min Zhu, Xiaobo Yu, Per Erik Ahlberg, Brian Choo (that’s me!), Jing Lu, Tuo Qiao, Qingming Qu, Wenjin Zhao, Liantao Jia, Henning Blom & You’an Zhu (2013) A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. doi:10.1038/nature12617 www.nature.com/nature/journal/…
I was utterly blown away when I first laid eyes on this fossil.
The majority of fossil discoveries worth publishing about can either strengthen previous studies or dish out little parcels of new data. These allow us to slowly piece together the history of life on Earth, but do not significantly rock the boat. But every now and then you are confronted with a jaw-dropping specimen, a fossil that says, “forget the textbooks, THIS is how it happened…”
Momentous discoveries like Lucy the Australopithecus
and the first batch of Chinese feathered dinosaurs that unleashed a tsunami of new information, bringing sudden clarity to our view of the distant past, and forcing us to rethink what we thought we knew about evolution. Now joining their ranks is a little armoured fish called Entelognathus
, described in Nature by an international team of researchers led by Prof. Zhu Min at IVPP, Beijing.Entelognathus primordialis
(“primordial complete jaw”) lived in coastal seas during the Late Silurian, about 419 million years ago, over sediments that are now exposed as the Kuanti Formation, near the town of Qujing, Yunnan. About 20cm long, Entelognathus
had a heavily armoured head and trunk, toothless jaws and tiny eyes set within large bony goggles. Besides the weird eyes, at first glance it appears to be a fairly ordinary arthrodiran placoderm. Placoderms are an extinct group of heavily armoured fishes that are generally regarded as the most primitive of the gnathostomes (vertebrates with jaws). But Entelognathus
has proved to be something far more bizarre and significant.
Whereas all other known placoderms had simple outer surfaces on the jaws and cheeks, each containing a few large bones, Entelognathus
has a much more complex arrangement of smaller bones. They include a maxilla and premaxilla on the upper jaw, a dentary on the lower jaw, as well as multiple cheek elements. This is a configuration identical to that seen in the skull modern osteichthyans, the backboned animals with bony skeletons including the bony fishes and tetrapods (limbed vertebrates). Indeed, it is effectively the same configuration seen in our own human skulls.
This astounding discovery throws a spanner in the works of some long held ideas of vertebrate evolution. Besides osteichthyans, the other living gnathostomes are the chondrichthyans, the group including sharks and rays. These have almost no bone in their bodies and skeletons made of cartilage. Until very recently, it was widely accepted that this condition represented the primitive state among the living jawed vertebrates. In other words, the most recent common ancestor of all gnathostomes would have looked something like a shark, devoid of armour and with a largely cartilaginous skull. An extinct group of unarmoured fishes, the acanthodians (which kind of resemble small spiny sharks) were thought to be close to the base of the modern gnathostome radiation.
Both osteichthyans and placoderms have skulls made of large bony plates. Similarly, the paired appendages of both groups are supported by bony girdles (although a placoderm-like dermal pelvic girdle is only found in the earliest osteichthyans). However it has been widely assumed that the two groups were unrelated, thus implying that the immediate ancestors of the osteichthyans developed their bony skulls from scratch. It was even suggested that the jaws of placoderms evolved independently of other gnathostomes.
Based on the available fossil evidence from the Devonian Period (419.2-358.9 million years ago) the “shark-like” scenario was sound. But until very recently we had little idea of what the placoderms and osteichthyans from the earlier Silurian Period looked like (443.4–419.2 million years ago). Then, within the last decade, superbly preserved articulated fish skeletons began to appear from the Silurian Kuanti Formation in southwest China. The anatomy of these bizarre creatures confounded the scientists who examined them (including Zhu) and prompted them to start questioning the prevailing wisdom. For example, the fish Guiyu
, first described by Min and his colleagues in 2009, was clearly an osteichthyan, but one with placoderm-like pectoral and pelvic girdles.
What began as a trickle with Guiyu
has become a flood with Entelognathus
. Whereas Guiyu
is a bony fish with features previously thought to be restricted to placoderms, Entelognathus
is a placoderm with features previously thought to be restricted to bony fishes. Additionally, when we look at the inside of the skull of Entelognathus
, we find the anatomy intermediate between placoderms and one hand, and both osteichthyans and chrondrichthyians on the other. The implications are clear: The last common ancestor of all the living jawed vertebrates, including us, was an armoured placoderm. Osteichthyans did not independently acquire their bony skeletons, they simply inherited them from their placoderm ancestors. Acanthodians and chondrichthyans represent a single sister-lineage to the osteichthyans, one that progressively reduced the bone content of their skulls and skeletons.Entelognathus
was not directly ancestral to modern gnathostomes. Given that it lived alongside more advanced jawed fishes like Guiyu
, it was a little bit too late for that. Based on fragments from around the world, the two main living groups (osteichthyans and chondrichthyans) must have emerged at some point from the Late Ordovician to Early Silurian. However, Entelognathus
from the Late Silurian represents a hitherto unknown grade of animal whose anatomy has brought us much closer to the common ancestry of all the modern jawed-and-backboned critters than anything previously seen.
We have only just started to scratch the surface of Silurian gnathostome diversity. Expect more momentous discoveries like this in the near future.