Inside an odd Triassic ichthyosaur: an odd embryo, not a meal

Summary for those in a hurry:
A new 5m adult ichthyosaur displays reversals (limb-like fins, a deep pelvis and a long neck) that went unnoticed, until it came to the embryo, which was misidentified as an incomplete thalattosaur meal.

Jiang et al. 2020 brought us news
of a “4m Triassic thallatosaur” swallowed by a 5m ‘megapredator’ ichthyosaur (Fig. 1; (XNGM-WS-53-R4). “The prey is identified as the thalattosaur Xinpusaurus xingyiensis based on close similarities of appendicular skeletal elements in both shape and size. The similarity is most characteristically seen in humeral morphology—it is a robust bone with a limited shaft constriction, and with an expanded proximal extremity.”

“The skull, mandible, and tail of the prey are unlikely to be present in the bromalite (= fossil of digested or digestible remains, i.e. coprolite), given that no isolated elements from these body regions are mixed in with what is preserved.”

Figure 1. Guizhouichthyosaurus ate a Xinpusaurus

Figure 1. Images from Jiang et al. proposing their hypothesis of a thalattosaur, Xinpusaurus, as stomach contents within the much larger Guizhouichthyosaurus. This hypothesis is based on several errors.

From the Jiang et al. abstract:
“Here we report a fossil that likely represents the oldest evidence for predation on megafauna, i.e., animals equal to or larger than humans, by marine tetrapods—a thalattosaur (∼4 m in total length) in the stomach of a Middle Triassic ichthyosaur (∼5 m). The predator has grasping teeth yet swallowed the body trunk of the prey in one to several pieces.”

After tracing published photos:

  1. The larger specimen is distinct from the holotype Guizhouichthyosaurus tangae (Fig. 4; Cao & Luo, 2000; IVPP V 11853) and reconstructions (Figs. 2, 3) are distinct from the Jiang et al. reconstruction. The limb-like fins of the adult were not reported. Several bones were misidentified in the embryo.
  2. Phylogenetic analysis (Fig. 9) nests the XNGM-WS-53-R4 specimen with Shonisaurus popularis (Fig. 5), two nodes away from Guizhouichthyosaurus.
  3. The embryo is folded in thirds and surrounded by an oval membrane. The unfolded morphology of the embryo matches the adult (Fig. 3).
  4. The size of the 1m embryo is much smaller than the estimated 4m prey item.
  5. The location of the embryo is in the posterior half of the abdomen near the uterus, distinct from the location of the more anterior stomach.
Figure 8. The skull of the new specimen wrongly assigned to Guizhouichthyosaurus by Jiang et al. 2020.

Figure 2. The skull of the new specimen wrongly mistakenly assigned to Guizhouichthyosaurus by Jiang et al. 2020.

Figure 1. The XNGM-WS-53-R4 specimen does not nest with Guizhouichthys but with Shonisaurus and has a distinct morphology.

Figure 3. The XNGM-WS-53-R4 specimen does not nest with Guizhouichthys (Fig.4). but with Shonisaurus (Fig. 5) and has a distinct morphology. Note the long neck and limb-like flippers/

Figure 2. Two closely related ichthyosaurs, Guizhouichthyosaurus tangae and "Cymbospondylus" buchseri, one with large flippers, one with small.

Figure 4. Two closely related ichthyosaurs, Guizhouichthyosaurus tangae and “Cymbospondylus” buchseri, one with large flippers, one with small.

The original diagram of the far from complete ‘stomach contents’
(Fig. 6) overlooked the skull, mandible, tail and many other bones here (Figs. 3, 4) here reconstructed (Fig. 7) as a complete skeleton of an embryo folded into a soft and pliable egg-like shape. Even the kink of the ichthyosaur tail is preserved. Both ends of the embryo were overlooked by those with firsthand access to the specimen (Fig. 1).

Figure 5. Shonisaurus popularis is a larger relative of the XNGM WS 53 R4, but retains the long slender flippers of Guizhouichthyosaurus.

Figure 5. Shonisaurus popularis is a larger relative of the XNGM WS 53 R4, but retains the long slender flippers of Guizhouichthyosaurus.

According to Laura Geggel, writing for LiveScience.com
“About 240 million years ago, one giant sea monster ate another, and then died with chunks of the beast in its belly. Researchers in China have now discovered and analyzed the fossilized corpses of these beasts, which they are calling the oldest evidence of megapredation — when one large animal eats another — on record.”

“The ichthyosaur may have attacked and killed the thalattosaur before eating it, but it’s also feasible that the ichthyosaur was simply scavenging the thalattosaur’s remains, the researchers said.”

Figure 8. Photo from Jiang et al. 2020. The XNGM-WS-53-R4 embryo in situ. Colors added.

Figure 6. Photo from Jiang et al. 2020. The XNGM-WS-53-R4 embryo in situ. Colors added. Note the posterior mandible was misidentified as a humerus. The distal humerus was tentatively misidentified as an interclavicle. One ilium is another jaw element .The other ilium is an ulna.

Figure 7. The XNGM embryo traced, unfolded and reconstructed from the tracing using DGS methods, as in the adult.

Figure 7. The XNGM embryo traced, unfolded and reconstructed from the tracing using DGS methods, as in the adult.

The IVPP holotype of Guizhouichthyosaurus
has much longer fins with more phalanges than the Jiang et al. adult and embryo specimens.

In the large reptile tree
(LRT, 1737+ taxa) thalattosaurs and mesosaurs are sister clades to ichthyosaurs. Why is this important? This XNGM specimens have long proximal limb element proportions and short digits. They also have more cervical vertebrae creating a longer neck. This odd morphology is more similar to those of thalattosaurs, mesosaurs and basal ichthyopteryigians like Wumengosaurus and Thaisaurus (Fig. 7) than to the XNGM specimen’s closer ichthyosaur relatives (Fig. 9), like Shonisaurus.

Phylogenetic reversals like this are rare.
Now we have one more example to add to that list.

Figure 2. Basal Ichthyosauria, including Wumengosaurus, Eohupehsuchus, Hupehsuchus and Thaisaurus

Figure 7. Basal Ichthyosauropterygia. The limb-like flipper and additional cervicals in the XNGM-WS-53-specimen are reversals to these more primitive taxa.

Figure 2. Guizhouichthyosaurus tangae skull preserved in three dimensions.

Figure 8. Guizhouichthyosaurus tangae skull preserved in three dimensions.

Figure 9. Subset of the LRT focusing on ichthyosaurs.

Figure 9. Subset of the LRT focusing on ichthyosaurs.

Displaying an unexpected limb/fin reversal,
a deep pelvis and a long neck, the XNGM adult and embryo were not typical of closely related ichthyosaurs. This odd morphology was originally overlooked in the adult and only partly observed in the embryo. This resulted in an incorrect assessment of the embryo as a thalattosaur meal. Tracing, reconstruction and phylogenetic analysis of both adult and embryo corrected the relationship and revealed the overlooked reversals in this unusual ichthyosaur. The XNGM specimen needs a new generic name because it is not congeneric with the holotype of Guizhouichthyosaurus.


References
Cao and Luo 2000. Published in: in Yin, Zhou, Cao, Yu & Luo, 2000. Geol Geochem 28 (3), Aug 8, 2000.
Jiang D-Y et al. (7 co-authors) 2020. Evidence supporting predation of 4-m marine reptile by Triassic megapredator. online
Maisch M et al. 2015. Cranial osteology of Guizhouichthyosaurus tangae (Reptilia: Ichthyosauria) from the Upper Triassic of China. Journal of Vertebrate Paleontology 26(3): 588-597.

Publicity
https://www.livescience.com/triassic-sea-monster-ate-huge-reptile.html
https://www.livescience.com/24031-ancient-sea-monsters-predator-x.html

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