Metacarpal 5 is the problem (Fig. 1).
Which bone is metacarpal 5?
Is it the long bone similar to metacarpal 4? That would make sense with most taxa, except Pectodens nests with other long-necked taxa, like Langobardisaurus and Tanystropheus. In those taxa metacarpal 5 is short and pedal 5.1 is metapodial (= very long).
Did taphonomy change things?
Or do we trust phylogenetic bracketing?
One more thing…
If the long bone is the metacarpal, then the phalangeal count matches sister taxa (4 phalanges). If the short bone is the metacarpal, then there is one extra phalanx. Did the preparator add a bone? Or did this taxon have an extra bone?
And take a look
at the width of the tibia + fibula. It’s the right width if the short bone is metacarpal 5. The width is not quite wide enough if the long bone is metacarpal 5.
you have to make a decision in paleontology. Sometimes you have to point your finger at a preparator’s mistake. Sometimes you make the mistake when you use your brain OR when you accept the data as presented.
What to do… what to do…
Here’s what I wrote a while back
Pectodens zhenyuensis (Li et al. 2017; IVPP V18578; Anisian, Middle Triassic; 38cm in length) was originally considered to be a diapsid and a possible protorosaur. Here Pectodens nests between Macrocnemus and Langobardisaurus. Originally the interclavicle, sternum and quadratojugal were overlooked. Note the large orbit, the long metarsal 5 and the perforated pubis. The elongate caudal transverse processes anchor powerful leg muscles.
With the short metacarpal 5
Pectodens cleanly nests with fewer autapomorphies at the base of the Langonbardisaurus/Tanystropheus clade.
Li C, Fraser NC, Rieppel O, Zhao L-J and Wang L-T 2017. A new diapsid from the Middle Triassic of southern China. Journal of Paleontology.7 pp. doi: 10.1017/jpa.2017.12