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Figure 1. A superb crinoid fossil specimen at the National Geological Museum of China in Beijing. The figured specimen is part of a large slab, exhibited under spotlights on the third floor of the museum (10 May 2016). The museum is adjacent to exit D (southwest) of the Xisi subway station on Line 4.
"Rock of the Month #181, posted for July 2016" ---
The crinoid Traumatocrinus
from Guanling county in Guizhou province, southern China.
The casual fossil collector is familiar with crinoids ("sea lilies")
mostly from the frequent occurrence of their segmented, cyclindrical stems,
composed of individual ring-shaped ossicles.
These are readily preserved, whereas the delicate
platy head (calyx) and frond-like arms are generally disarticulated
and decomposed. The host rock is a black shale,
implying a deep, reducing marine environment
favourable to the preservation of organic matter.
But why is this specimen in such great shape?
To paraphrase a recent study by Hagdorn and Wang (2015)
"Traumatocrinus lived attached to driftwood and was widespread across the Palaeo-Tethys. In contrast to its benthic encrinid ancestors, Traumatocrinus
fixed its distal column ...to its substrate, preferentially to the ends of the driftwood logs. ... Crowns are preserved in star position with radially splayed arms or in lateral bell position with the cup disrupted in interbrachial position. As usual in black shales, the exposed upper sides of the crinoids are in a more or less progressive state of decomposition whereas the embedded lower sides are intact".
This genus of crinoid occurs in the Carnian-age Xiaowa Formation,
and became extinct at the end of the Carnian stage.
Hagdorn et al. (2007) found that this genus of crinoid
occurred as colonies attached to driftwood, and was
a specialised variant of the otherwise-benthic family Encrinidae.
They noted that
"The stem of Traumatocrinus individuals may exceed 11 m in length. Colonies attached by root cirri to logs of driftwood may consist of more than 150 individuals. After the end-Carnian extinction of Traumatocrinus, its ecological niche was taken over in Norian times by Seirocrinus and
Pentacrinites with convergent morphological modifications".
The burial of colonies of this crinoid around and under waterlogged driftwood
is an unusual example of a fossil death assemblage (thanatocoenosis), but
the association with the driftwood provides the evidence of the mode of life
of this organism.
More generally, crinoids or "sea lilies" are the only class of sessile
echinoderms to have survived from their mid-Cambrian origins
to the present day (Turek et al., 1988, pp.372-389).
The best-known Triassic crinoid, Encrinus, occurs in the
Muschelkalk of Germany, and was described by Agricola
in the 16th century (ibid, pp.376-377).
A more recent crinoid is Uintacrinus, of upper Cretaceous age.
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Figure 2. Two more illustrations of selected fossils in the large, circa 140x85 cm flat slab, which displays about 12 fine crinoid calices and a number of other partial impressions.Although the spotlights made it hard to get a good photograph of the whole slab, the contrasty light is excellent for close-ups such as these.
References
Hagdorn,H and Wang,X (2015) The pseudoplanktonic crinoid Traumatocrinus from the late Triassic of southwest China - morphology, ontogeny and taphonomy. Palaeoworld 24, 479-496.
Hagdorn,H, Wang,X and Wang,C (2007) Palaeoecology of the pseudoplanktonic Triassic crinoid Traumatocrinus from southwest China. Palaeogeography, Palaeoclimatology, Palaeoecology 247, 181-196.
Turek,V, Marek,J and Benes,J (1988) Fossils of the World: a Comprehensive Practical Guide to Collecting and Studying Fossils. Arch Cape Press, New York, 1990 edition, 495pp.