Summerville Ptero Tracks – Mickelson et al. 2004

Mickelson et al. (2004) described new pterosaur tracks from the Late Jurassic Summerville Formation in Utah. From their paper: “Footprint length varies from 2.0 to 7.0 cms. The ratio of well-preserved pes:manus tracks is about 1:3.4. This reflects a bias in favor of  preservation of manus tracks due to the greater weight-bearing role of the front limbs, as noted in other pterosaur track assemblages. The sample also reveals a number of well-preserved trackways including one suggestive of pes-only progression that might be associated with take off or landing, and another that shows pronounced lengthening of stride indicating acceleration. However, traces of a fifth pes digit suggest some tracks are of rhamphorynchoid rather than pterodactyloid origin, as usually inferred for Pteraichnus.”

Let’s take a closer look at which pterosaur might have made these tracks.

pterosaur tracks (pteraichnus ichnites)

Figure 1. A series of pterosaur tracks from Mickelson et al. (2004). Impressions made at the same time are colored the same. Note the distance between left manus and right pes is quite short, as if the pterosaur walked quite upright. Note the length of digits 1-4. It is rare that a pterosaur foot includes such a long digit 1. Pink arrow points to pedal track that includes fifth digit impressions.

The relative length of pedal digit 1 is typically shorter than digit 2 in pterosaurs. However, in a few, digit 1 is as long or nearly as long as the other digits, as shown in the Figure 1. The relative length of digit 4 is also not typical. Apparently the metatarsus was not closely appressed as divisions extend to the heel. Together these greatly reduce the list of possible trackmakers to Beipiaopterusn44, and possibly Huanhepterus and the closely related flightless pterosaurs.  Pterodaustro has a similar foot, but the manus is much smaller than in the Summerville tracks. The Crato “azhdarchide” (SMNK PAL 3830) also had coequal pedal digits, but the huge unguals are not good matches. Tiny Nemicolopterus has a similar pes but was half the size.

My guess is these tracks represent the concurrent flightless pterosaur mislabled Mesadactylus (Smith et al. 2004), which preserves neither manus nor pes. As in the more completely known flightless pterosaur, SoS 2428, the manual digits are quite asymmetrical. Unfortunately, the feet are the only part of the flightless pterosaur that remain unknown.

So, this guess is based entirely on phylogenetic bracketing, restoration, chronology and size, all close matches.

The relative positions of the left manus and right pes (Fig. 1) indicate an upright posture when walking (Fig. 2) rather than the more horizontal configuration favored by traditional paleontologists (Fig. 3).

I can’t say much about the progression of accelerating tracks, mentioned by Mickelson et al. (2004), other than it supports the bipedal take-off model, rather than the disputed forelimb take-off model.

Pterodactylus walk matched to tracks according to Peters

Figure 2. Click to animate. Plantigrade and quadrupedal Pterodactylus walk matched to Crayssac tracks

Walking pterosaur according to Bennett

Figure 3. Click to animate. Walking pterosaur according to Bennett. This is the traditional model. Note the forelimbs provide no forward thrust, but merely act as props.

A preponderance of manus-only tracks might represent floating pterosaurs poling through shallow waters.

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
Mickelson DL, Lockley MG, Bishop J, Kirkland J 2004. A New Pterosaur Tracksite from the Jurassic Summerville Formation, Near Ferron, Utah. Ichnos, 11:125–142, 2004

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