SVP 12 – Lotosaurus bonebed redated to Ladinian

Hagen et al. 2015 discuss the sedimentology of the Lotosaurus (now dated to Ladinian, Late Middle Triassic, Fig. 1) bone bed. This was suspected here three years ago and thankfully suspicions are now confirmed.

Figure 1. Lotosaurus, a finback poposaur.

Figure 1. Lotosaurus, a finback poposaur.

From the abstract
Lotosaurus adentus is a highly unusual, sail-backed, edentulous poposauroid pseudosuchian archosaur* known primarily from a single site in Sangzhi County, Hunan Province, south China. This locality, the Lotosaurus Quarry, is traditionally dated to the Anisian and is distinctive in being a dense bonebed from which dozens if not hundreds of individual bones and occasional partial skeletons of Lotosaurus have been collected since it was discovered in 1970. The site appears to have formed in a fluvial-floodplain depocenter with sediment derived from multiple sources, rather than in a tidal flat setting as previously suggested. The presence of a population of unexpectedly young detrital zircons from the bone bed unit indicates that Lotosaurus is likely to be Ladinian in age, rather than Anisian as previously reported. This result is more congruent with the phylogenetic position of Lotosaurus, which lies among or just outside a grouping of derived poposauroids known from the Upper Triassic of North and South America.”

*Poposaur, yes, but there is no such thing a pseudosuchian, which is not a monophyletic clade, and only crocs and dinos are archosaurs.

Previous to this reediting of the sediment, Lotosaurus was a chronological outlier and this date change comes as good news.

References
Hagen CJ et al.  2015. Taphonomy, age, and geological context of the original Lotosaurus adentus (Archosauria, Poposauroidea) bone bed in the Middle Triassic Badong Formation, Hunan China. Journal of Vertebrate Paleontology abstracts.

Poposaurs to scale and the chronological Lotosaurus problem

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

Adding Sacisaurus (Fig. 1) as a basal member of the poposaur list adds a certain perspective.  It calls into question the early Triassic appearance of Lotosaurus (Fig.1) since all other poposaurs are Late Triassic. Either the geological setting for Lotosaurus was poorly calibrated, or these poposaurs all had a much earlier origin, in the Permian, which appears unlikely. The other possibility is that Lotosaurus is not a poposaur after all, but an offshoot of another Permian root. This might be interesting…

Poposaurs now nest as basal archosaurs. So the chronology problem goes away.

Figure 1. Poposaurs to scale and in phylogenetic order (top to bottom). Sacisaurus is at the base. Silesaurus and Lotosaurus are derived. Poposaurus is one of the largest, along with Lotosaurus.

Figure 1. Poposaurs to scale and in phylogenetic order (top to bottom). Sacisaurus is at the base. Silesaurus and Lotosaurus are derived. Poposaurus is one of the largest, along with Lotosaurus. Pisanosaurus, basal ornithischian, does not belong in this clade.

Poposaur mandibles

There’s still the question of Effigia’s mandible hanging out there.
The question is: “Is that a predentary or a dentary at the tip?” Fig. 1). Nesbitt (2007) says dentary. I say predentaries. Let’s look at the evidence.

To answer that,
I took a comparative survey of poposaur mandibles (Fig. 1), looking for evolutionary patterns and thereby strive to provide an update to the predentary/dentary question. Surprisingly, in the case of Effigia, when you add in the splenials, which neither Nesbitt nor I did before, the mandibular fenestra becomes substantially reduced. That may be similar to what one sees in Lotosaurus, in which the elements are not jumbled. And that provides more substance to the “predentary” argument. Other than Lotosaurus, the closest sister is Shuvosaurus, which is known from an incomplete mandible (Fig.1) showing similar patterns over the remaining portions. Shuvosaurus has something similar to what I saw in Daemonosaurus, that others consider something else. In any case, at some point, something interesting developed in front of the dentaries in certain phytodinosaurs.

The other question is,
when something similar to a predentary appears in front of the dentary, as in Sacisaurus (Figure 1), should it be considered a “beak” rather than a premaxilla? This bone may be paired, as it is in Sacisaurus, rather than a single median bone, as in the predentary of Heterodontosaurus (Fig. 1).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus and Effigia appear to share a common heritage of design.  In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus, Shuvosaurus and Effigia appear to share a common heritage of design. In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak). The extension of the angular to the predentary is unique to this clade.

If all these other mandibles had a premaxilla or beak (or the possibility of one), is there any reason to suspect that Effigia did not?

The original reconstructions of the Effigia mandible
introduced us to the largest mandibular fenestra I have ever seen relative to the size of the jaw. The new reconstruction reduces the fenestra length and, no doubt, produces a stronger jaw with the splenial (lavendar to iris blue bone) laminated to the medial side and edges.

Typically the mandibular fenestra splits the surangular from the angular,
as it does in Heterodontosaurus. However, in Lotosaurus the mandibular fenestra develops largely below the dentary with very little surangular and angular exposure. In Shuvosaurus the same pattern could play out, but unfortunately the key parts are missing (perhaps due to a very large mandibular fenestra?). This is a different pattern than in ornithischians, saurischians and theropods. And this pattern is also different from rauisuchians. Among euarchosauriforms, only in aetosaurs does the very large mandibular fenestra develop largely below the dentary. In others, the fenestra develops midway or beneath the surangular and it doesn’t get to the size seen in Effigia and Lotosaurus.

One final point
The suture between the two premaxillae in Effigia is convoluted like a puzzle piece. In this way they are locking themselves together, convergent with the central or fused premaxilla of ornithischians, but homologous with the premaxilla in Lotosaurus and Shuvosaurus.

If I’m wrong, show me some data. At this  point, at least it’s worth talking about.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ferigolo J and Langer MC 2006. “A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone”Historical Biology 19 (1): 1–11. online pdf.
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf

AMNH Effigia webpage

wiki/Effigia

The Skull of Lotosaurus (the finback poposaurid dinosaur)

The skull of Lotosaurus color coded.

Figure 1. The skull of Lotosaurus color coded. This toothless poposaurid nests with the herbivorous Silesaurus and Pseudolagosuchus (Fig. 2), neither of which have a fin. This is a DGS tracing.

Lotosaurus is interesting and mysterious because it is so big and so derived, yet appears so early (early Middle Triassic), essentially earlier than all other known dinosaurs. If phylogeny is a guide, then dinosaurs, notably theropods, originated earlier than this. Maybe there was a dino explosion in the early Triassic matching the placental explosion in the early Paleocene. We just haven’t found evidence for it yet.

Lotosaurus really needs a fresh new paper and a complete redescription. To that point, Nesbitt (2011) reports, A full description of Lotosaurus is currently underway.” We’ve seen recent papers on Arizonasaurus (Fig. 1, an unrelated rauisuchian) and Ctenosauriscus (Fig. 2, too soon to know what it is), but really nothing recent on Lotosaurus (Zhang 1975), which currently nests with poposaurid dinosaurs. It would be nice to know what’s real and what isn’t, how many specimens we have (Wiki says 10), and if new data changes hows it currently nests.

Earlier we looked at other finbacks and possible sister taxa. The skull of Silesaurus is a pretty close match to that of Lotosaurus, sans the teeth and adding some bulk. The rest of the changes in morphology appear to reflect the return to a quadrupedal stance along with greater bulk and loss of teeth.

Figure 3. Lotosaurus compared to sister taxa and other finback archosaurs.

Figure 3. Lotosaurus compared to sister taxa and other finback archosaurs.

Nesbitt (2003) reported on Arizonasaurus. He wrote, Characterisitics of the skeleton of Arizonasaurus show that it belongs to a poorly known group of Middle Triassic (240–230 Myr ago) archosaurs called the ctenosauriscids, and that ctenosauriscids are or are closely related to poposaurs. Furthermore, many characteristics of Arizonasaurus provide evidence that poposaurids and ctenosauriscids are derived rauisuchians.”

Dinosaurs are also derived from basal rauisuchians, but that’s not what Nesbitt meant. Nesbitt considered Arizonasaurus a derived rauisuchian, but the large reptile tree nested it close to the basal taxon, Vjushkovia. Nesbitt’s (2003) analysis did not include Lotosaurus, but his 2011 study did, nesting it between Poposaurus and Sillosuchus, Effigia and Shuvosaurus, with rauisuchians and far from Silesaurus, which Nesbitt (2011) nested just outside the Dinosauria. We earlier discussed problems with Nesbitt (2011) and his “strange bedfellows” in a nine-part  series. It’s worthwhile to also recall that certain poposaurs developed a new calcaneal tuber, convergent with the development of a calcaneal heel in crocodylomorphs. Such a structure traditionally removes poposaurids from the Dinosauria, but phylogenetic analysis puts them back in. Lotosaurus had a very small calcaneal tuber, if any. It’s hard to see on existing data.

I’ll be out for a week on family business. See you again after the 25th.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Butler RJ, Brusatte SL, Reich M, Nesbitt SJ, Schoch RR, et al. 2011. The Sail-Backed Reptile Ctenosauriscus from the Latest Early Triassic of Germany and the Timing and Biogeography of the Early Archosaur Radiation. PLoS ONE 6(10): e25693. doi:10.1371/journal.pone.0025693 Plos One paper
Nesbitt SJ 2003. Arizonasaurus and its implications for archosaur divergence
Sterling J. Nesbitt Proceedings of the Royal Society, London B (Suppl.) 270, S234–S237. DOI 10.1098/rsbl.2003.0066
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Weinbaum JC and Hungerbuhler A 2007. A Revision of Poposaurus gracilis (Archosauria: Suchia) based on two new specimens from the Late Triassic of the southwestern USA. Palaeontologische Zeitschrift 81(2):131-145.
Zhang F-K 1975. A new thecodont Lotosaurus, from Middle Triassic of Hunan. Vertebrata PalAsiatica 13:144-147.

wiki/Lotosaurus