A newly published scientific paper has described a new taxon from the famous Burgess Shale deposits of British Columbia.  The new taxon has been named Entothyreos synnaustrus.  Its discovery will help palaeontologists to better understand how arthropod-like features evolved in the ancestors of the Arthropoda.  Entothyreos helps to broaden our understanding about how major anatomical innovations within invertebrates came about.

Entothyreos synnaustrus fossil specimens. ROMIP 59505. A, full slab, with Entothyreos paratype specimens numbered 1–9. Insets as indicated. B, ROMIP 59505.2, anterior section with front limbs. C, ROMIP 59505. 3. dissociated anterior section. D, ROMIP 59505.5, dissociated spines and dorsolateral sheets. E, ROMIP 59505.6, anterior section F, ROMIP 59505.7, isolated spine and claw. G, H, ROMIP 59505.8, isolated spines and claws. G, full specimen. H, close-up of claws of posterior lobopod. I, ROMIP 59505.9. fragment of posterior section showing dissociated dorsolateral sclerotic trunk sheets and spines. Scale bars: A, 50 mm; B, D, E, G, I, 5 mm; C, F, 2.5 mm and H, 1 mm. Picture credit: Aria and Caron.

Entothyreos synnaustrus from the Tulip Beds

The fossil material comes from the Tulip Beds locality at the base of the Campsite Cliff Shale Member. This location was discovered in 1983 by a Royal Ontario Museum field team.  The site has yielded over ten thousand fossils, a rich array of soft-bodied organisms including the filter feeding Siphusauctum gregarium. The tulip-shaped body of S. gregarium inspired the location’s name.

To read an Everything Dinosaur blog post about Siphusauctum gregarium: Picking Tulips from the Burgess Shale.

The Burgess Shale biota was preserved by rapid mudflow deposits. The first specimens of the new species were collected in 1989.  Most of the fossil material having been collected in subsequent field expeditions in 1996, 2000, 2008 and 2016. A total of fifty-one specimens of E. synnaustrus have been identified to date.

Entothyreos synnaustrus taphonomy. A, Paratype ROMIP 53233. Full specimen (posterior region missing). Spine arrangement quasi in place but underlying plates not visible. B, paratype ROMIP 53241. Possible moult with spine arrangement little disturbed, missing endocuticular plates. C, paratype ROMIP 53244. Weathered specimen preserving annulated posterior lobopods (arrow). D–H, paratype ROMIP 53239. D, full specimen (composite image of the part and counterpart, specimen immersed in water). Insets as indicated. E, close-up of a pair of anterior lobopods. Inset is F. F, close-up of proximal-most portion of lobopods with dense array of setae or small spines. G, EDS imaging of carbon (red) showing minute elements along spines otherwise rich in aluminium (light blue). H, BSE imaging of anterior spines showing minute external ornamentation. Scale bars: A, B, 10 mm; C, D, 5 mm; E, H 2 mm; F and G 1 mm. Picture credit: Aria and Caron.

The Evolution of the Arthropoda

The evolution of an exoskeleton consisting of hardened body parts (sclerites) connected by flexible joints is a defining characteristic of the Arthropoda.  The development of this body structure, referred to as arthrodization is considered one of the most significant evolutionary advancements in the animal kingdom.  The arthrodization of their limbs, known as arthropodization, likely emerged before the arthrodization of their bodies. Both of these key features first appeared during the rapid diversification of life known as the Cambrian explosion.  Entothyreos synnaustrus has been classified as a lobopodian.  Lobopodians are animals such as the extinct, bizarre Hallucigenia from the Cambrian as well as extant tardigrades and the velvet worms (Onychophora).  These animals are close relatives of arthropods but do not possess the sclerites of true arthropods.

Entothyreos synnaustrus, demonstrates a distinct system of overlapping and hardened body plates along its trunk. It also had hardened rings surrounding the bases of its limbs. While the limb rings likely provided protection, the trunk plates may have helped this animal stand upright and feed.  It is thought to have been a filter feeder, filtering particles of food from the water column.

Technical drawings from Entothyreos synnaustrus in lateral view (A) and dorsal view (B). Picture credit: Danielle Dufault/Royal Ontario Museum.

Entothyreos synnaustrus Developing Specialised Parts of the Body

Other related lobopodian fossils also show evidence of having separate ring-like structures that connected the bases of their spiny appendages, with these structures covered by the outer layer of the lobopodian’s soft body.  Importantly, this newly described taxon has tubby clawed legs, appendages adapted for filter feeding and spines. This newly described taxon has different parts of its five-centimetre-long body performing different functions.  This specialisation is a trait found in arthropods and indeed in more derived invertebrates.

Entothyreos synnaustrus line tracings. A, paratype ROMIP 64650, full specimen and B, holotype ROMIP 53234, central trunk section. Paratype ROMIP 66325 (C) central trunk section. Colours: dark yellow, surface cuticular layer; light orange, sclerotic sheet; purple, intercalary element. Line legend as indicated in A. Picture credit: Aria and Caron.

The Late Neoproterozoic and the Early Palaeozoic saw dramatic changes in life on our planet. Organisms went from amorphous unstructured creatures to ones with a definitive shape and specialised body areas. During the Cambrian organisms became more derived than the Ediacaran biota and we begin to see the evolutionary changes and radiation of the major animal groups we see today.

The discovery of Entothyreos synnaustrus suggests that the evolution of arthropod-like features, such as segmented and hardened body parts, may have emerged in parallel among the ancestors of arthropods. This broadens our understanding of how major anatomical innovations can arise in the history of life.

A life reconstruction of the newly described Entothyreos synnaustrus, part of the Burgess Shale Cambrian biota. Picture credit: Danielle Dufault/Royal Ontario Museum.

The Challenge of Interpreting Cambrian Fossils

One of the huge challenges faced by palaeontologists studying Cambrian fossils is trying to work out where in the Animalia the organisms should be placed.  Numerous Cambrian organisms have been identified, some of which demonstrate some, but not all the features seen in the Arthropoda.  For example, the “Walking Cactus” – Diania cactiformis is one such animal.  D. cactiformis is known from the Lower Cambrian Maotianshan shale of China.  It is around twelve million years older than Entothyreos synnaustrus.  It has jointed legs, which is a trait associated with the Arthropoda.  However, it has no discernible head, eyes or gills.

To read an article about the amazing Diania cactiformis: A Transitional Fossil Between Worms and Arthropods.

Palaeontologists remain uncertain as to how and in which order the characteristics associated with the arthropods evolved.  The scientific description of E. synnaustrus helps to inform the debate.

Everything Dinosaur acknowledges the assistance of Associate Professor Jean-Bernard Caron at the University of Toronto in the compilation of this article.

The scientific paper: “Deep origin of articulation strategies in panarthropods: evidence from a new luolishaniid lobopodian (Panarthropoda) from the Tulip Beds, Burgess Shale” by Cédric Aria and Jean-Bernard Caron published in the Journal of Systematic Palaeontology.

The Everything Dinosaur website: Dinosaur and Prehistoric Animal Models.