TSoSC #3: The floating lily fields of Guanling

Triassic Seas of South China #3: Among the floating lily fields of Guanling

Acrylics, digital and photography, 2012-2013

3rd in a series for the following review paper =
Michael J. Benton, Qiyue Zhang, Shixue Hu, Zhong-Qiang Chen, Wen Wen, Jun Liu, Jinyuang Huang, Changyong Zhou, Tao Xie, Jinnan Tong & Brian Choo (accepted manuscript), Exceptional vertebrate biotas from the Triassic of China, and the expansion of marine ecosystems after the Permo-Triassic mass extinction, Earth Science Review. www.sciencedirect.com/science/…

ca. 233,000,000 bp, Late Triassic (early Carnian), near the city of Guanling, Guizhou, China (Xiaowa Formation).
Note = the Guanling Formation produces the Middle Triassic Luoping biota, the Guanling biota, shown here, is from the Late Triassic Xiaowa Formation.

In the midst of a major marine transgression, the shorelines have retreated and the seafloor, dozens if not hundreds of metres beneath the surface, is an anoxic wasteland populated only by a few specialised bivalves. A different story plays out in the sunlit surface waters in the form of a truly unique early Mesozoic habitat = the Traumatocrinus floating lily field. In the absence of marine wood-borers, the trunks and branches of Triassic trees remain afloat at sea for many years, providing anchorage for great colonies of huge pseudopelagic crinoids. Small fish and cephalopods flock to the lily fields, seeking food or shelter. They in turn support one of the heaviest concentrations of predatory marine reptiles found anywhere in the Triassic world. Thalattosaurs and ichthyosaurs dominate this oddly skewed fauna at the expense of sauropterygians, who are represented only by a few small placodonts. Also foraging among the crinoids are some of the world's first turtles.

INVERTEBRATES
Traumatocrinus hsui - giant sea lilies
Perfect colonies of this huge encrinid crinoid are the signature fossil of the Guanling biota. Each individual consists of a long ropy stalk ending in a cluster of frond-like arms that trapped plankton and edible detritus from the surrounding water. Mature colonies reached gargantuan proportions, with stalks up to 11 metres in length. 

As free-swimming larvae, swarms of infant crinoids affixed themselves to pieces of floating wood. The sediments of Guanling are filled with the remains of smaller colonies whose rafts were too small or badly decomposed to support their growing bulk, sinking to a premature doom. Elsewhere the world, Traumatocrinus fossils are found as broken segments, likely the remains of colonies that drifted ashore and perished. Only on large, fresh logs that remained far out at sea could the crinoids reach their full majesty.

The colonies must have had a profound impact on their surroundings. Today, the shelter offered by large pieces of flotsam, natural or man-made, attract swarms of small fish that in turn draw in big ocean predators. The sheer volume of marine reptiles at Guanling (ichthyosaur and thalattosaur fossils outnumber the fish!) is likely a result, at least in part, of the floating sea lilies and the unique habitat they provided.

Traumatocrinus did not survive the end of the Carnian, but other crinoids would continue this pseudopelagic lifestyle until well into the Jurassic. The appearance of shipworm (wood-boring bivalves) in the Middle Jurassic probably doomed this niche by drastically reducing the time afloat of seaborne logs.

Trachyceras multituberculatus - ammonoid
Up to 7cm shell diameter. Shells of these small ammonoids are found in dense concentrations at Guanling

FISHES
Fish fossils at Guanling are much rarer than the marine reptiles. Some authors have taken this at face value and assume that Guanling represents some sort of unusually stressed, fish-free environment. To me, the abundance of big predatory reptiles (along with crinoids and cephalopods) makes this unlikely and suggests an alternative scenario = Guanling represents such a highly productive, predator-rich community that fish were rarely preserved simply because their bodies were gobbled up by all the reptiles. A similar situation presents itself in the Late Cretaceous Niobrara Formation of Kansas where you find plenty of fossils of giant marine predators, but few of the numerous small fish that must have supported all these huge appetites.

Pholidopleurus xiaowaensis (30cm SL) - slender blue fishes shoaling in the background.

Peltopleurus brachycephalus (8cm SL) - small black-headed fishes congregating around the crinoid crowns.

Birgeria guizhouensis (1.5 m SL) - big grey fishes. The various Birgeria spp. were sleek predatory actinopterygians with reduced scale cover, perhaps the Triassic equivalent to modern spanish mackeral and amberjack.

Guizhoucoelacanthus guanlingensis (c.1 m SL) - blue coelacanth in foreground.

Generic hybodont shark - based on isolated ichthyoliths, there was a diverse array of small elasmobranchs at Guanling. Alas no articulated body fossils have been found.

MARINE REPTILES
Odontochelys semitestacea - turtles in right foreground
About 40 cm long. The smallest and most famous of the Guanling reptiles, this is the world's oldest and most primitive turtle. This really was a hero in a half-shell = the ventral part of the shell (the plastron) was fully developed but the back armour consisted only of small neural plates. Unlike modern turtles, Odontochelys also had a full set of teeth.

Odontochelys has been described as an inhabitant of marginal seas or river deltas. I am not convinced of this. The fossils of the Guanling biota are set in black shale with no bioturbation, evidently a deep, anoxic seabed. The only remains of terrestrial life are bits of wood and other vegetation which may have drifted in from a great distance. Fossils of terrestrial arthropods found in older sites (Luoping, Panxian, Xingyi) are nowhere to be seen, nor are there any of the predatory sauropterygians and semi-aquatic archosaurs which also frequented these presumably more coastal settings. By my reckoning, Guanling represents a community that was far out to sea. However, the holotype of Odontochelys is 100% complete so the body probably hasn't travelled far from where it died.

Speculation = Odontochelys was a fully marine turtle, spending at least part of its life far from shore as a resident of the floating lily fields. It and the other smaller reptiles (placodonts and xinpusaurs) would seek prey among the giant crinoids, hauling themselves out onto the log rafts to rest and bask.

Qianichthyosaurus zhoui - small ichthyosaurs in the foreground.
Up to 2m long with females sexually mature at 1.3m. These sleek toretocnemids were abundant and, based on the presence of preserved pregnant females, breeding in the area. Qianichthyosaurus and the thalattosaur Xinpusaurus account for the vast bulk of Guanling reptile fossils.

Guanlingsaurus (= Shastasaurus) liangae - huge ichthyosaurs in the background.
At over 10m in length, the shastasaurid Guanlingsaurus dwarfed the other vertebrates in the region. With long, slender bodies and short, toothless snouts, it has been proposed that Guanlingsaurus was a suction-feeder of soft-bodied prey akin to modern beaked whales, although this hypothesis has been challenged.

Xinpusaurus bamaolinensis - small 1.5m long thalattosaur in left foreground.
In no other part of the world did the thalattosaurs attain such success as they did at Guanling, with at least five species. X. bamaolinensis was the oddest, with sharp elongated snout and a heterodont dentition of sharp fangs up front and crushing teeth at the back. Most sources recognise two Guanling xinpusaurs (separate long and short-snouted forms) although it has been proposed that there is only a single ontogenetically variable species to which the name X. suni would have priority.

Miodentosaurus brevis - big slender reptile in middle distance.
A sleek 5m long giant among thalattosaurs, Miodentosaurus had an unusually short snout with only a few small teeth restricted to the tips of the upper and lower jaws while the unguals were flat and blunt unlike the sharp claws of others of its family. Clearly this creature had a specialised diet, though precisely what it ate is not clear.

Psephochelys polyosteoderma - armoured reptile in left foreground
About 60cm long. The only Guanling sauropterygians were small cyamodontoid placodonts, bizarre forms that resembled reptilian banjos with flattened, armoured-plated torsos and slender tails. Their crushing dentition suggests a diet of hard-shelled prey.