Mythbusting: Prorotodactylus

We looked at the Early Triassic ichnogenus, Prorotodactylus
(Ptaszynski 2000; Brusatte et al. 2010; Figs. 1,3) earlier here and here.

Today a fresh look
from images published in Brusatte 2018 (Figs. 1, 2, 8), used here for their scientific value, education and criticism.

Figure 1. From Brusatte 2018, colored here. The caption reports, "a handprint overlapping a footprint.' In reality the handprint was put down first, followed by the footprint. Given the presented lighting with shadows below, this is a natural cast of the original impression.

Figure 1. Image rom Brusatte 2018, colored here. The caption reports, “a handprint overlapping a footprint.’ In reality the handprint was put down first, followed by the footprint, as is typical of tetrapods moving forward. Given the presented lighting with shadows below, this is a natural cast of the original impression.

Following Brusatte et al. 2010, Wikipedia reports,
Prorotodactylus is a dinosauromorph ichnogenus known from fossilized footprints found in Poland and France. The prints may have been made by a dinosauromorph that was a precursor to the dinosaurs, possibly closely related to Lagerpeton.” …which is not a dinosauromorph in the LRT, but a dinosaur-mimic related to the gracile and long-legged chañaresuchid, Tropidosuchus (Fig. 3). A wide gamut phylogenetic analysis, like the LRT, has to precede any such mis-pronouncements based on tradition, chasing fame and…

Figure 2. Model of hypothetical trackmaker of Prorotodactylus alongside its creator, paleontologist, Grzegorz Niedzwiedzki (ghosted to bring out the model) from Brusatte 2018.

Figure 2. Model of hypothetical trackmaker of Prorotodactylus alongside its creator, paleontologist, Grzegorz Niedzwiedzki (ghosted to bring out the model) from Brusatte 2018. There is no reason to imagine such a trackmaker when we have a real trackmaker that matches the tracks, Diandognosuchus (Figs. 3,4). Compare this imaginary tetrapod to the real Tropidosuchus (Fig. 3), which lacks digit 5.

Taxon exclusion.
For one reason or another the best matches to Protorotodactylus were excluded to flavor the more exciting headline, “Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic” (Brusatte et al. 2010). Sometimes we have to lose our quest for glory, step back and test all available candidates, lest some amateurs some day pull the curtain back and reveal the error.

Tropidosuchus in its two variants. In the holotype (above) the humerus is more robust and pedal digit 4 is gracile, as in Chanaresuchus (Fig. 3). In the referred specimen of Tropidosuchus (below) the humerus is smaller and pedal digit 4 is longer than 3, as in Lagerpeton. The rise to a bipedal configuration appears to coincide with the change in pedal proportions.

Figure 3. Tropidosuchus in its two variants. In the holotype (above) the humerus is more robust and pedal digit 4 is gracile, as in Chanaresuchus (Fig. 3). In the referred specimen of Tropidosuchus (below) the humerus is smaller and pedal digit 4 is longer than 3, as in Lagerpeton. The rise to a bipedal configuration appears to coincide with the change in pedal proportions.

Figure 3. Manus and pes casts of Prorotodactylus compared to manus and pes of Diandongosuchus (Fig. 4).

Figure 3. Manus and pes casts of Prorotodactylus compared to manus and pes of Diandongosuchus (Fig. 4). Dotted blue line indicates a dragging digit 4 during the recovery phase of the step cycle.

Brusatte 2018 and Brusatte et al. 2010
considered the asymmetric (fourth toe longer than the third) pedal trackmaker of Prorotodactylus an Early Triassic dinosauromorph.

Wikipedia reports,
“Prorotodactylus tracks were probably made by a small dinosauromorph. [not true] The ichnogenus possesses several distinctively archosaurian features, such as narrow trackways and a pace angulation of 130°. [not unique to archosaurs] The pace angulation, or the angle made between two successive footprints, shows that Prorotodactylus had an erect stance rather than a sprawling one. Dinosauromorph [no such clade] characteristics include digitigrade prints (in which only the digits touch the ground), [not unique to this clade] bunched metatarsals, [not true] a reduction of the first and fifth digits, and the posterior deflection of the fifth digit. Prorotodactylus prints share several characteristics [and invalidating characteristics] with the dinosauromorph genus Lagerpeton from Argentina, indicating that the print maker was closely related to Lagerpeton. [bogus conclusion] The three central digits of the foot are parallel, a feature otherwise only seen in Lagerpeton. [not true] Digit IV is the longest digit in the foot of both Prorotodactylus and Lagerpeton. [and most tetrapods] In both animals, there is a progressive decrease in size from digits IV to II, with digit III angled relative to the midline. [not true, Fig. 3] The bunched metatarsals in Prorotodactylus are a synapomorphy of the clade Avemetatarsalia. [invalid clade]. The metatarsal pads, preserved only in deeply imprinted footprints, are united in a single unit. [not true] This makes the foot act as a single unit rather than a collection of splayed digits. [bogus hoopla] In ichnotaxa similar in appearance to Prorotodactylus, the digits are not parallel to one another and the posterior margin of the metatarsal pads is curved, making the digits splay. [not true, consider Cosesaurus].

 

FIgure 2. Diandongosuchus (2012) compares well with Prorotodactylus tracks. These legs are long enough to make overlapping tracks. Diandongosuchus is closer to the genesis of phytosaurs and their sisters, the chañaresuchids than to dinosaurs.

FIgure 4. Diandongosuchus (2012) compares well with Prorotodactylus tracks. These legs are long enough to make overlapping tracks. Diandongosuchus is closer to the genesis of phytosaurs and their sisters, the chañaresuchids than to dinosaurs.

Beside all the above, remember…
Brusatte et al. 2010 and Brusatte 2018 were working from invalid cladograms lacking many key taxa. In their traditional minds, the dinosaur-mimic chañaresuchid, Lagerpeton, was a dinosaur ancestor.

In the large reptile tree (LRT, 1344 taxa) ‘dinosauromorpha‘ is a junior synonym for ‘archosauria’ (birds + crocs + LCA and all descendants). Diandongosuchus (Fig. 4) was published two years after Brusatte et al. 2010. Basal archosauriforms are notoriously  preserved without hands and feet. So, in 2010 there was some excuse.

Figure 5. GIF animation of a very short-legged extant crocodilian demonstrating how the toes nearly touch the wrist during the step cycle. No legs more elongate than Diandongosuchus need to be imagined.

Figure 5. GIF animation of a very short-legged extant crocodilian demonstrating how the toes nearly touch the wrist during the step cycle. No legs more elongate than Diandongosuchus need to be imagined. Despite being a ‘sprawling reptile’ not the erect carriage of the hind limbs.

Wikipedia reports
“Trackways indicate that the maker of Prorotodactylus footprints was quadrupedal. However, the overstep of the hind feet beyond the front feet indicates that the forelimbs were reduced, a characteristic of bipedal animals. [not true]  Another Polish dinosauromorph ichnogenus, Sphingopus, [actually a rauisuchian] occurs later in the Triassic and is fully bipedal. The transition to bipedality probably occurred between Prorotodactylus and Sphingopus. [happened many times in tetrapods}. During this transition, body size also increased, as Sphingopus tracks are larger than those of Prorotodactylus.” [all this guesswork without a wide gamut phylogenetic analysis]

?The different shapes of the manus and pes of Prorotodactylus may show different forms of specialization. [not true] The forelimbs, which were reduced, may have been used for hunting, grasping, or manipulating. [not true]The bunched metatarsals of the hind feet may have enabled the metatarsals to act as a lever, along with the stylopodium, or upper leg, and the zeugopodium, or lower leg. [they nearly always act as a lever in nearly all tetrspods] This would have enabled facultative bipedalism in Prorotodactylus,[confuding an ichnogenus with an undiscovered extinct genus] and a wholly bipedal gait in later dinosauromorphs. [this line of basal archosauriforms is not in dino ancestry] Pace angulation is relatively high in Prorotodactylus, and increased as bipedalism becomes obligate in later dinosauromorphs” [faulty evidence, faulty conclusion]

Phylogenetic analysis can help.

The trackmaker of Prorotodactylus has to have the following traits:

  1. Manus smaller than pes
  2. Quadrupedal
  3. Manus semi-digitigrade, pes fully plantigrade
  4. Neither metacarpus nor metatarsus compact (some radiation)
  5. Hindlimb longer than forelimb
  6. All unguals relatively small, but sharp
  7. Five fingers and five toes
  8. Manual digit 3 is the longest
  9. Pedal digit 4 is the longest
  10. No digits are longer than the metacarpals/metatatarsals
  11. No digits/phlanages are long and gracile.

Distinct from Diandongosuchus,
in the trackmaker of Prorotodactylus:

  1. The manus is relatively longer
  2. The tarsus is not so wide
  3. Manual digit 2 is longer than digit 4
  4. Pedal digit 1 is aligned with the metacarpophalangeal hinge

Some of these problems are fixed
when making comparisons to the manus and pes of Early Triassic ancestor, Proterosuchus (Fig. 6), coeval with Prorotodactylus, but the manus bones are not complete.

Figure 7. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta. Note the longer metatarsals than toes. Pedal digit 5 does not reach the substrate.

Figure 7. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocitta. Note the longer metatarsals than toes. Pedal digit 5 does not reach the substrate.

What about Sphingopus?
We looked at that large ichnite earlier here. It most closely resembles rauiisuchid pedes because five digits impressed on a digitigrade pes and pedal digit 5 was hooked. This is unlike proximal dino ancestors, like PVL 4597 (Fig. 7). Why do these hypotheses get published without hard evidence and analysis, when we have the hard evidence and analysis?

Synaptichnium

Figure 1. Synaptichnium compared to a slightly altered pes of Proterosuchus. Note a reduction of one phalanx in pedal digit 4 to match one less pad in the ichnite. The last two (or three phalanges) of pedal 4 are unknown in Proterosuchus.

Let’s get real.
Phylogenetic analysis determines which taxa were in the lineage of dinosaurs. You can run an analysis on good ichnites.

We know what the last common ancestor of crocs and birds looked like.
It’s PVL 4597 (Late Middle Triassic; Fig. 7), a bipedal basalmost archosaur originally attributed to Gracilisuchus.

Figure 8. Imagined trackmaker of Prorotodactylus from Brusatte 2018.

Figure 8. Imagined trackmaker of Prorotodactylus from Brusatte 2018. This is wishful thinking, not phylogenetic analysis and bracketing.

Let’s put this Prorotodactylus myth to bed,
or at least attribute it to a basal archosauriform, because it was not impressed by a dinosaur ancestor. Insist that your scientist/authors back up their hypotheses with comprehensive evidence that considers all other possibilities before they set the world on fire with another ‘origin of dinosaurs’ headline that overlooks the validated proximal outgroups. The LRT tests all other candidates, not only at the origin of dinosaurs, but at the origin of reptiles, bats, turtles, snakes, pterosaurs, both types of whales, and every other taxon in the LRT.


References
Brusatte SL, Niedźwiedzki G and Butler RJ 2010. Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic. Proceedings of the Royal Society B. 278 (1708): 1107–1113.
Brusatte S 2018. The rise and fall of the dinosaurs. A new history of a lost world. Wm. Morrow. An imprint of HarperCollins Publishers. 404pp.
Li C, Wu X-C, Zhao L-J, Sato T and Wang LT 2012. A new archosaur (Diapsida, Archosauriformes) from the marine Triassic of China, Journal of Vertebrate Paleontology, 32:5, 1064-1081.
Ptaszynski T 2000. Lower Triassic vertebrate footprints from Wiory, Holy Cross Mountains, Poland. Acta Palaeontologica Polonica45 (2): 151–194.
Stocker MR, Nesbitt SJ, Zhao L-J, Wu X-C and Li C 2016. Mosaic evolution in phytosauria: the origin of longsnouted morphologies based on a complete skeleton of a phytosaur from the Middle Triassic of China. Abstracts of the Society of Vertebtate Paleontology meeting 2016.

https://en.wikipedia.org/wiki/Prorotodactylus

https://pterosaurheresies.wordpress.com/2012/08/29/diandongosuchus-not-a-basal-poposauroid-a-basal-phytosaur/

https://pterosaurheresies.wordpress.com/2016/10/28/you-heard-it-here-first-four-years-ago-diandongosuchus-is-a-stem-phytosaur/

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