Notes on Limusaurus (Dinosauria, Theropoda) and its odd little hand

Added November 20, 2015, a report by Guinard G 2015 supports the present results.

Figure 1. Limusaurus in situ. The associated croc has been painted red here.

Figure 1. Limusaurus in situ. The associated croc has been painted red here. Their marriage was legal in only 16 states.  : )

Limusaurus inextricabilis (Xu et al. 2009; earliest Late Jurassic, Oxfordian; 1.7m in est. length; IVPP V 15923; Figs. 1, 2) is an herbivorous theropod with very tiny arms and hands in the lineage of proto-birds. Coincidentally, Limusaurus was buried next to the skeleton of a tiny Jurassic croc (in red above). Pol and Rauhut (2012) nested Limusaurus with Elaphrosaurus and Spinotropheus. Those taxa have not been added yet to the large reptile tree where Limusaurus nests with the oviraptor  Khaan and Juravenator. Limusaurus shares a ventral pelvis, but not long arms and large hands with these bird-like taxa. Like birds, Limusaurus has a pair of sternae. The coracoids appear to be transitional between disc-like and stem-like.

Figure 2. Limusaurus reconstructed. Both hands are shown and colorized. Digit 1= purple. Digit 2= pink. Digit 3 = Green. Digit 0=pale yellow. Digit 0 goes back to basal tetrapods, like Ichthyostega, and only appear here due to the vestigial nature of the manus in which it matured at an embryologically immature state compared to sister taxa.

Figure 2. Limusaurus reconstructed. Both hands are shown and colorized. Digit 1= purple. Digit 2= pink. Digit 3 = Green. Digit 0=pale yellow. Digit 0 goes back to basal tetrapods, like Ichthyostega, and only appear here due to the vestigial nature of the manus in which it matured at an embryologically immature state compared to sister taxa.

Limusaurus is notable for two main reasons:

  1. It is an herbivorous theropod from the earliest Late Jurassic
  2. It has four fingers when all sister taxa have three. This fact has added credence and confusion to the chick embryo digit identification issue we looked at earlier here

It’s well worth looking at both sides of this extra finger issue:
From the Xu et al. abstract: “Theropods have traditionally been assumed to have lost manual digits from the lateral side inward, which differs from the bilateral reduction pattern seen in other tetrapod groups. This unusual reduction pattern is clearly present in basal theropods, and has also been inferred in non-avian tetanurans based on identification of their three digits as the medial ones of the hand (I-II-III). This contradicts the many developmental studies indicating II-III-IV identities for the three manual digits of the only extant tetanurans, the birds. Here we report a new basal ceratosaur from the Oxfordian stage of the Jurassic period of China (156–161 million years ago), representing the first known Asian ceratosaur and the only known beaked, herbivorous Jurassic theropod. Most significantly, this taxon possesses a strongly reduced manual digit I, documenting a complex pattern of digital reduction within the Theropoda. Comparisons among theropod hands show that the three manual digits of basal tetanurans are similar in many metacarpal features to digits II-III-IV, but in phalangeal features to digits I-II-III, of more basal theropods. Given II-III-IV identities in avians, the simplest interpretation is that these identities were shared by all tetanurans. The transition to tetanurans involved complex changes in the hand including a shift in digit identities, with ceratosaurs displaying an intermediate condition.”

There were a long list of authors (see below) that agreed with the above, some of whom have come to our attention earlier for publishing various errors, along with their otherwise excellent work.

Unfortunately the Xu et al. abstract ignores the fact that basal tetrapods, like Acanthostega, had an additional digit medial to digit #1 (we’ll call this digit #0). Therefore digit #0 is part of the phylogenetic history of all tetrapods, despite the fact that it is almost never expressed in tetrapod adults, only embryos. Digit #0 is expressed in Limusaurus, in which the hand is a small vestige relative to those of sister taxa. Like other vestiges the hand of Limusaurus did not continue to develop normally as a hatchling and into adulthood. Rather the hand retained a shape found at a certain embryological stage, a stage that included digit #0.  That’s why the big metacarpal (in purple) has the morphology of metacarpal 1 in sister taxa (Fig. 3) and metacarpal #0 continues the shape of metacarpal #1 in cross section. In most tetrapods digit #0 is fused to metacarpal #0 or otherwise disappears before birth or hatching.

Figure 3. The manus of several theropods including Limusaurus. Here digit 1 is purple, digit 2 is pink and digit 3 is green. Note the presence of digit 0 in this vestigial hand, a holdover from basal tetrapods that has not been correctly identified by Xu et al. and others.

Figure 3. The manus of several theropods including Limusaurus. Here digit 1 is purple, digit 2 is pink and digit 3 is green. Note the presence of digit 0 in this vestigial hand, a holdover from basal tetrapods that has not been correctly identified by Xu et al. and others. Click to enlarge.

That’s why otherwise you only see digit #0 in embryos. Thus there is no “phase shift” of digit identity. There is only loss, fusion or absorption of digit #0, a factor missed by earlier workers.

Supportive work (2015) by Guinard G 2015.
reports the following: “There is controversy between paleontological and developmental data regarding manual digit identities of birds and their tetanuran ancestors (I, II and III vs. II, III and IV). Limusaurus should not be used as a reference concerning the identity of avian manuals digits. Evolutionary teratology supports identities I, II and III of the tetanuran manus via a frame-shift that did not occur in the Ceratosauria lineage.”

References
Guinard G 2015. Limusaurus inextricabilis (Theropoda: Ceratosauria) gives a hand toevolutionary teratology: a complementary view on avian manual digits identities. Zoological Journal of the Linnean Society (advance online publication) DOI: 10.1111/zoj.12329
Xu X, Clark JM. Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-K, Han F-L. and Guo Y 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature, 459(18): 940–944.

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