The discovery of a second juvenile ceratopsid
(Currie et al. 2016) raised an interesting point: “In phylogenetic analysis, if all characters are coded as seen, the two juvenile ceratopsids (a partial Triceratops skull and the UALVP 52613 juvenile, Fig. 1) nest together. However, when size or age dependent characters are [not scored], the new juvenile (Chasmosaurus) specimen groups with other adult Chasmosaurus specimens.”
So, does phylogenetic analysis fail us?
The new UALVP juvenile was recognized/identified as being closer to Chasmosaurus, just as the juvenile Triceratops was recognized as being closer to Triceratops, both on the basis of character traits and prior to analysis. But the Currie et al. unedited analysis takes us in another direction…
From the introduction
“The specimen comprises a nearly complete skeleton lying on its left side, lacking only the front limbs and girdle, which were lost many years ago into a large sinkhole….”
“The juvenile nature of this specimen is based on several lines of reasoning. At approximately 1.5 min total length, it is the smallest articulated ceratopsid skeleton that has ever been recovered. Immature bone textures on cranial bones (Brown et al., 2009), open neurocentral sutures throughout most of the vertebral column, incomplete fusion of sacral vertebrae, lack of fusion between caudal ribs and vertebrae, poorly formed articulations between limb bones, and many other characters confirm that this is an immature ceratopsid….”
“Of all the chasmosaurines from Dinosaur Park, it is most similar to Chasmosaurus belli and C. russelli.”
was made by expert and experienced assessment. The question is, why would the unedited Currie et al. analysis separate the juveniles from the adults and nest the juveniles together? They’re not exactly tadpoles or caterpillars, but they do change somewhat during maturation, following basic archosauromorph (including synapsid/mammal) growth strategies, that lepidosauromorphs (including pterosaurs) are less likely to follow.
When an adult Chasmosaurus
and the juvenile Chasmosaurus are added to the large reptile tree, using a character list NOT specific to ceratoposids, the juveniles nest with their respective adults, not with each other. And this happens despite the very few bones that represent the juvenile Triceratops (posterior face and shield only). Notably there are no other competing ceratopsid candidates in the present taxon list. All data was gleaned from online images. The adult data may be represented by chimaera mounts and chimaera drawings. If the Currie et al analysis was restricted to just an adult and juvenile Triceratops and just an adult and juvenile Chasmosaurus, would adults nest with juveniles as they do in the large reptile tree? We don’t know because that test was not run.
Here’s how the large reptile tree divides
the Chasmosaurus adult and juvenile from the Triceratops adult and juvenile (posterior skull traits only). Please feel free to provide better data or more precise readings for any of these interpretations. Some were difficult to figure from available sources. At present I do not include traits for parietal fontanelles or horn lengths, which are the easiest two traits that most commonly separate Chasmosaurus from Triceratops and are reflected in their juveniles.
- skull table: C: depressed terrace, medial and lateral crests; T: convex
- snout in dorsal view: C: not constricted; T: constricted
- orbit positon: C: postorbital > preorbital; T: subequal
- lateral rostral shape: C: convex, smooth curve; T: double convex
- nasals/frontals: C: nasals >; T: subequal
- antorbital fenestra: C: absent; T: without mx fossa
- orbit/upper temporal fenestra: C: orbit not > T: orbit >
- orbit position/skull: C: anterior half of skull; T: not
- orbit shape: C: round to square: T: taller than wide
- upper temporal fenestrae: C: not closed or slit-like; T: closed or slit-like
- frontal shape: C: not wider posteriorly; T: wider posteriorly
- frontal shape 2: C: without posterior processes; T: with posterior processes
- posterior rim of parietal: C: transverse; T: anteriorly oriented or curved.
- parietal skull table: C: forms a sagittal crest: T: broad
- squamosal descent: C: mid level; T: ventral skull (ventral maxilla)
- skull roof fusion: C: parietal fusion only; T: frontal fusion and parietal fusion
- jaw joint orientation: C: descends from ventral mx; T: in line with ventral mx, after jugal arch.
- last maxillary tooth: C: posterior orbit; T: mid orbit
- mandible ventrally: C: 2-tier convex; T: straight
- 2nd sacral rib: C: not: T: double wide laterally
- manus/pes: C: subequal: T: manus smaller
- ilium: C: posterior process >; T: not
- metatarsal 1:4 ratio: C: 1 not > than half: 4 T: 1> half of 4
- metatarsals 2-4: C: < than half the tibia; T: not
- pedal 3.1 vs p2.1: C: not > T: 3.1>
- metatarsals 2 and 3: C: aligns with mt1; T: aligns with pedal 1.1
- pedal 4 length: C: subequal to mt 4; T: > mt4
- pedal digit 3 vs 4: C: 4 narrower than 3; T: 4 is not narrower
Shifting the juvenile Triceratops
to the juvenile Chasmosaurus adds 12 steps. Doing the opposite adds 21 steps. Bootstrap scores are over 99-100 for the three nodes represented by the four taxa. I have not reviewed the scores or data in the Currie et al study, which obviously adds more ceratopsid traits.
Added < 24 hours after original publication Below is a new reconstruction of the Triceratops juvenile based on text measurements and an adult skull compared to the original reconstruction that does not appear to have correctly scaled the mandible to the skull elements.
Short notes for readers and critics
“Criticism of a writer is absolutely inevitable.” — Malcolm Gladwell.
Gladwell is one of the most respected and best-selling authors in current decades. Nevertheless, this interview on YouTube quotes several critics, many with scathing barbs. So, this give and take between writers and their critics is universal and ‘inevitable.’
On the other hand,
in Science, one either can or cannot duplicate experiments and observations. It should be cut and dried, but with errors and egos on both sides, it rarely is. Even so, most people think it is better to try/experiment with/refute alternate hypotheses. Aaaaaat least that’s the editorial policy at ReptileEvolution.com where occasional lack of talent and insight is sometimes overcome by tenacity, huge blocks of data and the ability to update online blunders.
Currie PJ, Holmes RB, Ryan MJ and Coy C. 2016. A juvenile chasmosaurine ceratopsid (Dinosauria, Ornithischia) from the Dinosaur Park Formation, Alberta, Canada. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1048348.