Field of Science

Jurassic Phytosaur??

A new paper by Maisch and Kapitzke (2010) describes a mandibular fragment from a phytosaur from marine shales in England. This is supposedly the first record of a phytosaur from the Jurassic (Hettangian) as the specimen was found in-situ beneath beds that provide the lowest occurrence of the ammonoid Psiloceras. Thus some phytosaurs, in this case a marine one from Europe, purportedly survived the end-Triassic extinction.

However, as Randall Irmis (who also informed me of this paper) reminded me, this is non-sensical regarding this specimen as the base of the Jurassic is presently defined by the first appearance of Psiloceras. Thus the phytosaur bearing strata are latest Triassic in age and not Jurassic.

I'm not adverse to the possibility that some basal pseudosuchians such as phytosaurs, aetosaurs, and rauisuchians may have survived into the Triassic, especially given our poor control on the determination of the Triassic-Jurassic boundary in non-marine strata.  Obviously crocodylomorphs make it through and we have no evidence for an abrupt global-wide non-marine extinction event for other pseudosuchians. 

The specimen mentioned by Maisch and Kapitzke appears to currently hold the title of the "last phytosaur", but unfortunately it does not provide clear evidence for the survival of this clade past the end-Triassic.

Maisch, M. W. & Kapitzke, M. 2010. A presumably marine phytosaur (Reptilia: Archosauria) from the pre-planorbis beds (Hettangian) of England. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 257: 373–379.

Abstract: A mandibular fragment of a longirostrine archosaur is decribed from the lowermost Jurassic (pre-planorbis beds, lowermost Hettangian) of Watchet, Somerset, England. The specimen is compared to both marine crocodilians (Thalattosuchia) and phytosaurs, groups which are either unknown (Thalattosuchia) or only doubtfully represented (Phytosauria) in lowermost Jurassic strata so far. The specimen shows striking morphological similarity to the Late Triassic phytosaur Mystriosuchus, but differs from known teleosaurid and metriorhynchid thalattosuchians. It is consequently
determined as aff. Mystriosuchus. It supports previous assumptions that phytosaurs crossed the Triassic-Jurassic boundary, at least in Europe. It also provides additional evidence that at least some phytosaurs, particularly the longirostrine forms, may have been facultative marine animals. The persistence of amphibious, piscivorous, longirostrine phytosaurs in the earliest Jurassic of Europe may have hampered the distribution of the ecologically similar teleosaurids, which are not known from strata that are older than the latest Sinemurian to date.

8 comments:

  1. "we have no evidence for an abrupt non-marine extinction event for other pseudosuchians"

    This is not quite true. Olsen et al. (2002) demonstrated that at least in the Newark Supergroup of eastern North America, footprints indicate that non-crocodylomorph pseudosuchians go extinct abruptly, and Whiteside et al. (2010) demonstrated that this coincides with the initial carbon isotope excursion and extinction in marine strata.

    Of course this is just one region - we have no idea if this pattern holds up globally, mainly because of poor age control for terrestrial Triassic-Jurassic sections.

    Olsen, P. E., D. V. Kent, H.-D. Sues, C. Koeberl, H. Huber, A. Montanari, E. C. Rainforth, S. J. Fowell, M. J. Szajna, and B. W. Hartline. 2002. Ascent of dinosaurs linked to an iridium anomaly at the Triassic-Jurassic boundary. Science 296:1305-1307.

    Whiteside, J. H., P. E. Olsen, T. Eglinton, M. E. Brookfield, and R. N. Sambrotto. 2010. Compound-specific carbon isotopes from Earth's largest flood basalt province directly link eruptions to the end-Triassic mass extinction. Proceedings of the National Academy of Sciences 107:6721-6725.

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  2. Duly noted and post modified. Thx.

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  3. "Megalosaurus" terquemi is also thought to be an Early Jurassic phytosaur, based on teeth from the Hettangian of France (Buffetaut et al., 1991). They are apparently similar to teeth described in-

    Huene and Maubeuge, 1954. Sur quelques restes de sauriens de Rhetien et du Jurassic Lorrains. Bulletin de la Societe Geologique de France. 6(4).

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  4. Key words there...thought to be based on teeth. I don't trust teeth. Especially not as the only support for such a game changing hypothesis such as basal pseudosuchians surviving into the Jurassic.

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  5. Frank DeCourten in Dinosaurs of Utah (1998) also seems to mention an occurence of a possible Early Jurassic phytosaur, a specimen found in Early Jurassic rocks outside of the dune sea that covered much of the American Southwest during this period. The identification may be a bit sketchy though.

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  6. Have not had a chance to read the paper yet, so the following comes with a grain of salt.

    If I understand the state of affairs, the pre-planorbis beds exist in a continued state of temporal purgatory. There seems to be general acceptance that elsewhere in the Lias Psiloceras c.f. ergatum extends below the traditional bottom of the Psiloceras planorbis beds at St. Audrie's Bay (bed #13). Isotopic correlation suggests that Psiloceras may extend even further down in Nevada, so at least some of the preplanorbis beds are likely to wind up Hettangian when all is said and done. I'm not sure which bed the phytosaur comes from, but if it is in the upper pre-planorbis it may well be Hettangian, though given the fragmentary nature I am wonder how/if the authors of the new paper rule out the possibility of a "zombie effect" (i.e. reworking).

    As far as the supposed marine habits go I am curious to know more what you and Herr Doktor Irmis think? Mystriosuchus occurs in the marine-ish Zorzino limestone along with a mix of presumed marine (thalattosaur, placodont) and terrestrial (drepanosaur, pterosaur) vertebrates. Do we have evidence from biogeography that phytosaurs were capable of at least periodically crossing marine barriers? Given what extant archosaurs (and for that matter, certain squamates) do, I am fairly happy to accept that phytosaurs would not have been opposed to the occasional sea cruise, but calling this new fossil "marine" seems a bit of a stretch.

    On a related note, in a past comment on this blog I countered Irmis skepticism about Odontochelys being marine by noting that terrestrial vertebrates were absent from the Wayao/Falang formation/member. However, I just returned from China and I was mistaken. There are at least rare terrestrial elements in that assemblage, so I find myself wondering more about Odontochelys now too.

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  7. Hi Neil,

    You make some very good points regarding biostratigraphy and the possibility of reworking in this case, both which do make the Jurassic age assigned to these beds and the specimen ambiguous. Regarding marine phytosaurs, I'm not aware of any Triassic worker stating that this isn't a possibility although I hven't closely examined all of the evidence myself. Again it is possible that we are looking at post-mortem transport, which is probably the case for the 'marine' 'rauisuchian' described a few years back by Li et al. (2006).

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  8. Thanks for the reply Bill.

    Qianosuchus, Sikkanisuchus, and Ticinosuchus are all interesting cases. Having just come from the Qianosuchus locality I will note that the critter is not all that uncommon within certain decidedly marine horizons and often occurs as associated or articulated specimens. Another marine locality fairly distant from the type locality but probably nearly equivalent in age has isolated archosaur teeth, possibly from Qianosuchus or a related form, but also abundant plant parts and (possibly freshwater) isopods. I don't really know what to make of any of this one way or the other, but it seems Qianosuchus must have at least been spending some time on the shoreline or around river deltas for multiple individuals to wind up in fairly deep water.

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