Manus dominated pterosaur tracks from Early Cretaceous, Gansu, China

A recent paper by Li et al. (2015)
reveals a manus dominated pterosaur trackway associated with minor sandstones in the mud-dominated sequence in the Hekou Group, Gansu, China. They report the tracks are mostly random and “likely reflect differential registration depths of manus and pes and/or sub optimal preservation conditions.”

How long will the alternate explanation be ignored?
We’ve got a better explanation for manus-dominated tracks (Fig. 1). Pterosaurs could float and pole themselves around using their hands (but see below!). Not sure why this alternate explanation continues to be avoided. It also explains the randomness of the tracks. The pterosaurs were rotating above the beds of bottom-dwelling menu items.

The authors did not match a specific trackmaker to the tracks (see below) despite having all the clues necessary to do so (Fig. 2). The manus is about the length of the pes. The pes is extremely narrow with pedal digits 1-4 about equal in length and pedal digit 5 is a vestige at best. The time is Early Cretaceous. The place is China. Jidapterus, a protoazhdarchid, is a good match (Peters 2011).

Invertebrate traces and burrows
are found throughout the track strata providing a good clue to azhdarchid and protoazhdarchid diets. These were probing pterosaurs, picking up burrowing invertebrates with they long beaks (Fig. 3). Not scavengers….Not baby dinosaur predators.

The first pterosaur tracks reported from China (Peng et al. 2004, Zhang et al. 2006, Fig. 4 ) represent a single individual slightly larger than the Gansu trackmaker actually making a directed track. By the dimensions and shape, this trackmaker was likely also a Jidapterus-like azhdarchid.

Li et al. report,
“The small sample size also makes it difficult to discern systematic features across a significant number of tracks, so the material is referred to only as Pteraichnus isp.” That’s a shame because I was able to identify the trackmaker within minutes. It’s not that hard given the clues and resources, like “the catalog of pterosaur pedes for trackmaker identification” (Peters 2011). I mean…that’s why I wrote and illustrated it, just for cases like this.

Li et al. report,
“The lack of pes tracks, predominance of, or greater depth of manus tracks at many pterosaur track sites could reflect the greater proportion of body weight (relative to foot surface area) born by the pterosaur forelimbs.” Then they go the other way, “However, this manus-emphasis trait may also depend on substrate consistency. For example, in specimens of Pteraichnus isp. from the Wenxiyuan tracksite, Shandong Province, the pes impressions are as deep or deeper than the manus impressions.” 

Li et al dismiss the floating hypothesis
by reporting, “The random distribution of the deep tracks could perhaps indicate that the pterosaurs were semi-floating and making irregular contact with bottom of a non-marine shallow-water environment. However, we consider this unlikely, given the remnants of typical walking trackway configurations.” Pterosaurs weren’t jumping into the pool. They were walking in from the shore.

They also reference Hone and Henderson who, “employed digital models to imitate the swimming strokes of pterosaurs, and their results suggest that many pterosaurs did not regularly rest on the surface of the water and, if immersed, would need to take off rapidly.” That paper and its hypotheses were thoroughly debunked here.

Li et al. reference Lockley et al 2014
when they report, “It is possible that buoyant or semi-floating pterosaur trackmakers touched the substrate with their forelimbs, while their shorter hindlimbs floated and perhaps paddled in the water above. This would require deliberately lowering the wrist to the substrate, below the level reached by the feet, and thus, as noted below, seems like an improbable interpretation. Much track evidence suggests that the reverse was typically the case: i.e., swim track assemblages usually show only the traces of hind footprints, mostly with associated elongate drag or scratch marks [Lockley et al. 2014]. In such cases, this indicates water depths were roughly equal to the length of the pterosaurs’ legs. Given that it is known that pterosaur manus tracks are often deeper, more common and thus more easily preserved than pes tracks, in non-swim track assemblages, it is most parsimonious to infer that the irregular to random distribution of many of the manus tracks at the Yangouxia pterosaur tracksite are most simply explained by variable preservation. Pterosaur tracks are typically associated with shorelines and shallow water and are often represented by quite high-density assemblages with variable combinations of manus and pes tracks. Thus, while we cannot discount the possibility that some manus tracks are associated with swimming trackmakers, this cannot be proven, and it is more parsimonious to infer that the irregular distribution patterns are the result of variable preservation.” This is an odd sort of logic IMHO. Variable preservation? Manus only tracks were somehow selected by geological factors (see Fig. 6)? That doesn’t make sense over one instance, let alone several now known worldwide.

Lockley et al. 2014 Dakota Group tracks
Lockley et al. attribute long scratch marks to pterosaurs (Fig. 5) and this may be so.

If so,
in morphology and size they represent a different sort of pterosaur than the Gansu trackmaker. The Dakota Group trackmakers from the Mid Cretaceous appear to be most closely related to Shenzhoupterus, Sinopterus, Tapejara and Tupuxuara, of which, based on the small claw size, Shenzhoupterus appears to be the closest match. We looked at Tapejara floating and swimming earlier here.

Feeding hypotheses
Lie et al. report, “The association of the Y-PS1-2 tracks with abundant invertebrate ichnofossils (P. tubularis)  raises the possibility that the pterosaurs were feeding on the invertebrate trackmakers. There are many sites where pterosaur tracks occur with invertebrate traces and Lockley and Wright reported such associations in the late Jurassic of the western USA and Garcia-Ramos et al. [52] reported pterosaur footprints associated with Lockeia at the Tazones tracksite, in Asturias (northern Spain). However, there is no feeding trace that has been observed on these pterosaur tracks surface, probably due to the preservation or the special feeding way (such as filter feeding of ctenochasmatids).” Actually little peck marks might be present. Hard to tell in the invertebrate burrow beds.

Trackmaker candidates according to Li et al. 
Li et al. could not rule out a non-pterodactyloid trackmaker because the impression of “pedal digit V is rarely impressed clearly and unambiguously.”  They consider dsungaripterids and Huanhepterus (hands not known, feet only figured in lateral view in the literature) without considering azhdarchids, like Jidapterus, and without referencing the catalog of pterosaur pedes for trackmaker identification (Peters 2011).

How pterosaurs produced manus only tracks
has been illustrated by Mark Witton (Fig. 6, from Li et al. 2014). I only hope he was squirming when he did this, as it defies all logic. Note the sediment behind the walking pterosaur is not preserving its pedal tracks. Nor are the manus only tracks random.

This is bad science: rejecting great solutions and publishing magic (hind limb levitation) or selective geology (harder and less compliant matrix only beneath the pedes). Evidently whenever my views are considered that appears to ‘poison the waters’ so any other solution, no matter how bizarre, is embraced. That’s too bad that it has come to this.

References
Hone DWE, Henderson DM 2014. The posture of floating pterosaurs: Ecological implications for inhabiting marine and freshwater habitats. Palaeogeography, Palaeoclimatology, Palaeoecology 394:89–98.
Li D-Q, Xing L-D, Lockley MG, Piñuela L, Zhang J, Dai H, Kim J Y, Persons S and Kong D 2015. A manus dominated pterosaur track assemblage from Gansu, China: implications for behavior. Science Bulletin 60(2):264-272.
Lockley MG, Cart K, Martin J et al 2014. A bonanza of new tetrapod tracksites from the Cretaceous Dakota Group, western Colorado: implications for paleoecology. New Mexico Museum of Natural History Science Bulletin 62:393–409
Peng BX, Du YS, Li DQ et al. 2004. The first discovery of the Early Cretaceous pterosaur track and its significance in Yanguoxia, Yongjing County, Gansu Province. J Chin Univ Geosci (Earth Sci) 29:21–24.
Peters D 2011. A catalog of pterosaur pedes for trackmaker identification. Ichnos 18(2):114-141.
Zhang JP, Li DQ, Li M et al. 2006. Diverse dinosaur-, pterosaur-, and bird-track assemblages from the Hakou Formation, Lower Cretaceous of Gansu Province, northwest China. Cretaceous Research 27:44–55.