Getting inside the head of a fuxianhuiid

If you were to look at the diversity of life on Earth today, you could be forgiven for thinking that animals have always been around and have dominated the planet since time memorial. However, you would in fact be completely wrong! Animals have only been around for roughly 600 million years whilst life first evolved over 3.5 billion years ago and remained in single-celled form for the majority of the Earth’s history.

The period when animals rapidly diversified into the majority of extant phyla is known as the ‘Cambrian explosion’, which began approximately 545 million years ago during the Cambrian period. One particularly enigmatic example of this is the Burgess Shale, where beautifully preserved animals, some of which are unlike anything alive today, have been found.

This odd looking creature is Opabinia, one of the enigmatic products of the Cambrian explosion. It possessed five eyes which were on stalks, and a long proboscis which it used to grab its food! Image from paleobiology.si.edu
This odd-looking creature is Opabinia, one of the enigmatic animals of the Burgess Shale. It possessed five eyes which were on stalks, and a long proboscis which it used to grab its food! Image from paleobiology.si.edu

Until the past few decades, the Burgess Shale has stood out as our best glimpse into this stage of the evolution of life on Earth. However in China, several localities (e.g. Chengjiang) have been found, producing fossils of equally exquisite detail which scientists have been excitedly studying. The advantages of localities like these is that we can decipher how living groups first evolved and what would most likely have been the ancestral state for our modern animal groups.

Two new fossil species, described this past week in the journal Nature give us just such an insight for arthropods, the group containing animals such as insects, crustaceans, centipedes, spiders and the extinct trilobites. The fossils, named as Chengjiangocaris kunmingensis and Fuxianhuia xiaoshibaensis are from a group known as the fuxianhuiids, which are regarded as representatives of early arthropods.

Chengjiangocaris kunmingensis, the image on the left shows a reconstruction of the animal with the feeding tube and nerve cord shown running the length of the animal. The image on the right shows a specimen with the head 'taphonomically dissected', allowing the researchers to see the limbs and nerve cord properly for the first time. Images from Yang et al. 2013.
Chengjiangocaris kunmingensis, the image on the left shows a reconstruction of the animal with the limbs shown on the head of the animal and the feeding tube and nerve cord shown running the length of the animal. The image on the right shows a specimen with the head shield ‘taphonomically dissected’, allowing the researchers to see the limbs and nerve cord properly for the first time. Images from Yang et al. 2013.

The fossils, which were found in a Lagerstätte (a locality of exceptional preservation) near the city of Kunming in the Yunnan province of China, have been dated to approximately 520 million years old, meaning they are from a relatively early stage of the ‘Cambrian explosion’. Previous specimens of fuxianhuiids have had their heads covered by their head shield, part of the tough exoskeleton that is synonymous with arthropods. This has meant that debate over what exactly the paired post-antennal structures in other fuxianhuiids actually represented has never had a clear resolution. Until now that it is. In a stroke of geological good fortune, numerous specimens of the two new fuxianhuiid species have experienced ‘taphonomic dissections’, where the conncective tissues of the head shield have softened before final burial allowing the head shields to rotate forwards, exposing the structures underneath and making them visible to scientists for the first time.

The holotype specimen of Fuxianhuia xiaoshibaensis. At the top of the image you can see where the head shield has rotated forward, revealing the structures underneath. Image from Yang et al. 2013.
The holotype specimen of Fuxianhuia xiaoshibaensis. At the top of the image you can see where the head shield has rotated forward, revealing the structures underneath. Image from Yang et al. 2013.

The fossils are so well-preserved that the functional articulation of these post-antennal structures can be explained. The limited range of movement in the limbs means that they would most likely have been used to sweep detritus into the mouth, where the food particles would then have been filtered out of it.  The nerve cord is also the first documented
case of a preserved post-cephalic central nervous system in a stem group arthropod. It is simple in structure, especially compared to animals alive today (perhaps as expected).

The locality these fossils were found has just begun to be explored. With the potential for more insights into this pivotal period in the evolution of life and finds with this quality of preservation, I could very well be writing more articles on invertebrates sooner than I think!

References

Jie Yang, Javier Ortega-Hernández, Nicholas J. Butterfield, Xi-guang Zhang. Specialized appendages in fuxianhuiids and the head organization of early euarthropods. Nature, 2013; 494 (7438).