Albian South Korean tracks do not match Monjurosuchus

Lee et al. 2020 describe
“a new quadrupedal trackway found in the Lower Cretaceous Daegu Formation (Albian) in the vicinity of Ulsan Metropolitan City, South Korea, in 2018. A total of nine manus-pes imprints show a strong heteropodous quadrupedal trackway (length ratio is 1:3.36). Both manus and pes tracks are pentadactyl with claw marks. The manus prints rotate distinctly outward while the pes prints are nearly parallel to the direction of travel. The functional axis in manus and pes imprints suggests that the trackmaker moved along the medial side during the stroke progressions (entaxonic), indicating weight support on the inner side of the limbs. There is an indication of webbing between the pedal digits. These new tracks are assigned to Novapes ulsanensis, n. ichnogen., n. ichnosp., which are well-matched not only with foot skeletons and body size of Monjurosuchus but also the fossil record of choristoderes in East Asia, thereby N. ulsanensis could be made by a monjurosuchid-like choristoderan and represent the first possible choristoderan trackway from Asia.

Not sure why they say they have a “well-matched”
foot skeleton and body size for Monjurosuchus. That does not appear to be true (Fig. 1). Other coeval mammal-mimic trackmakers, like Repenomamus, appear to match better (Fig. 1).

Figure 1. Novapes tracks from Lee et al. 2020 matched to little Monjurosuchus (lower left) and Repenomamus (upper right) and Repenomamus overall. Croc tracks are similar but the pes lacks digit 5.

Figure 1. Novapes tracks from Lee et al. 2020 matched to little Monjurosuchus (lower left) and Repenomamus (upper right) and Repenomamus overall. Croc tracks are similar but the pes lacks digit 5.

Images provided by Lee et al.
indicate digits of nearly equal length on both manus and pes. Unfortunately the choristoderan, Monjurosuchus (Fig. 1) is too small and digit 4 on both manus and pes the longest on a sprawling (not erect) hind limb. Not a good match.

A better match can be found
in the mammal-mimic Repenomamus. It is the correct size, shape and coeval with the trackmaker of Novapes. Repenomamus is not mentioned by Lee et al. 2020. A Repenomamus relative, Liaoconodon, better preserves the extremities, but the manus and pes are similar in size.

Repenomamus and Liaoconodon are found in
the nearby Yixian Formation, NE China, Albian, late Early Cretaceous, 125 mya. Novapes is also from the Albian, late Early Cretaceous, nearby in South Korea.

Novapes diagnosis from Lee et al. 2020:
Monjurosuchus (M: yes, no); Repenomamus (R: yes, no)

  1. Quadrupedal tracks with a pronounced heteropody; (M no; R yes)
  2. Pentadactyl manus impression with claw marks and semi-symmetrical outline (M yes; R yes)
  3. Manus wider than longer (M no; R yes)
  4. Divergence between digit I and V imprints ranges 180° to 210°; (M no; R yes)
  5. Digit IV imprint slightly longer than digit II; (M yes; R yes)
  6. Entaxonic manus (medial digits more robust than lateral digits); (M no; R no; Novapes no)
  7. Pentadactyl pes impression with claw marks and asymmetrical outline (i.e., lateral digits are more developed) (M yes; R yes)
  8. Longer than wide; (M yes; R yes)
  9. Webbing between the proximal portion of slender digits; (M ?; R?)
  10. The subequal digits III and IV imprints longer than others (M 4>3; R 4=3)
  11. Digit I imprint only 30% in length of the digit IV imprint); (M yes; R yes)
  12. The sole pad impression is elongate with a U-shaped “heel”; (M no; R yes)
  13. Entaxonic pes (M no; R no; Novapes no)

Ichnites are sometimes difficult to match to trackmakers, 
but some trackmakers can be eliminated. The possibility of a mammal-mimic trackmaker, like Repenomamus, should not be omitted from consideration.


References
Lee Y-N, Kong D-Y and Jung SH 2020. The first possible choristoderan trackway from the Lower Cretaceous Daegu Formation of South Korea and its implications on choristoderan locomotion. Nature Scientific Reports 10:14442 https://doi.org/10.1038/s41598-020-71384-1

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