Lumping and splitting
is something paleontologists do with the various specimens they find as they assign them names and nodes in the family tree of life. Jack Horner recently made news for lumping several pachycephalosaur genera together as distinct ontogenetic growth stages of the same genus and species. He did the same with Triceratops, which changed its appearance rather drastically while reaching maturity.
let’s look at three closely related specimens, Panphagia protos Martinez and Alcobar (2009) , Eoraptor lunensis (Sereno et al. 1993, 2014) and Pampadromaeus barberenai (Cabriera et al. 2011, Fig. 1). These three nest between basalmost theropods and basalmost phytodinosaurs (sauropodomorphs + ornithischians) in the large reptile tree. Others consider them basal or stem sauropodomorphs, but only because basal ornithischians are not included in their analyses.
The number of bones preserved
with each specimen varies, so all the bones cannot be compared with one another. What is preserved, however, appears to be more closely matched than many other specimens sharing the same generic name, like Pteranodon and Rhamphorhynchus. (Evidently dino-workers are splitters and ptero-workers are lumpers as, until recently, they preferred not to provide new names for distinct specimens, some of which were improperly considered juveniles of distinctly different larger specimens).
These three proto-phytodinosaurs (Fig. 1) are obviously similar. Sereno, et al. (2014) often refers to “the closely related Pampadromaeus and Panphagia” when writing about Eoraptor.
What are the differences?
Using the 228 characters of the large reptile tree is not enough to split and lump these three specimens. Only these three traits split them and cause loss of resolution.
- The anterior nasal is wider in Panphagia.
- The mandible tip does not descend in Pampadromaeus.
- The tibia is shorter than 2x the ilium length in Pampadromaeus
More resolution might come from adding taxa and more complete taxa, unless these three are indeed congeneric. Almost certainly there are obscure, but important differences not covered by the list of 228 rather obvious traits.
Enter Martinez et al. (2012).
Their strict consensus tree (51 taxa, 378 characters) was also unable to resolve relationships among these three and several other taxa. Their reduced consensus tree (eliminating poor specimens) nested them in ascending order: Phanphagia > Eoraptor > Pampadromaeus separated by single decay indices.
The position of Panphagia as basal to other sauropodomorphs is supported by nine unambiguous synapomorphies:
- pterygoid wing of the quadrate extending for more than 70% of the total quadrate length;
- presence of postparietal fenestra between supra occipital and parietals;
- supraoccipital wider than high;
- coarse serrations of the teeth angled upwards at 45◦;
- absence of a postzygodiapophyseal lamina in cervical vertebrae 4–8;
- weakly developed laminae in the neural arches of cervical vertebrae 4–8;
- minimum width of the scapula less than 20% of its length;
- posterior end of the fibular condyle of the tibia anterior to the posterior margin of proximal articular surface;
- and strongly laterally curved iliac blade in dorsal view.
The more derived position of Eoraptor is supported by three unambiguous synapomorphies:
- subtriangular cross-section of the ischial midshaft;
- supraacetabular crest of the ilium contacting the distal end of pubic peduncle;
- and sub triangular distal end of the ischium.
Pampadromaeus and more derived sauropodomorphs share four unambiguous synapomorphies:
- squamosal bordering the laterotemporal fenestra for more than 50% of its depth (62:0)
- length of the base of proximal caudal neural spines greater than half the length of the neural arch;
- transverse width of the distal humerus greater than 33 of its length;
- and length of the pubic peduncle of the ilium greater than twice the anteroposterior width of its distal end.
So the question remains,
are these several distinctions sufficient to split these three specimens? Are they indeed distinct genera? Or are they all three species of Eoraptor? (Eoraptor is the earliest of these three to be named.)
Cabreira SF, Schultz CL, Bittencourt JS, Soares MB, Fortier DC, Silva LR and Langer MC 2011. New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (advance online publication) DOI: 10.1007/s00114-011-0858-0
Martínez RN and Alcober OA 2009. A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha (pdf). PLoS ONE 4 (2): 1–12. doi:10.1371/journal.pone.0004397. PMC 2635939. PMID 19209223. online article
Martínez RN , Apaldeti C and Abelin D 2012. Basal sauropodomorphs from the Ischigualasto Format ion, Journal of Vertebrate Paleontology, 32:sup1, 51-69.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113