Do ceratopsid juveniles (phylogenetically) nest together?

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.”

Figure 1. Chasmosaurus juvenile UALVP 52613 specimen.

Figure 1. Chasmosaurus juvenile UALVP 52613 specimen lacking forelimbs due to  taphoniomic loss down a nearby sinkhole.

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.”

This interpretation
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.

  1. skull table: C: depressed terrace, medial and lateral crests; T: convex
  2. snout in dorsal view: C: not constricted; T: constricted
  3. orbit positon: C: postorbital > preorbital; T: subequal
  4. lateral rostral shape: C: convex, smooth curve; T: double convex
  5. nasals/frontals: C: nasals >; T: subequal
  6. antorbital fenestra: C: absent; T: without mx fossa
  7. orbit/upper temporal fenestra: C: orbit not > T: orbit >
  8. orbit position/skull: C: anterior half of skull; T: not
  9. orbit shape: C: round to square: T: taller than wide
  10. upper temporal fenestrae: C: not closed or slit-like; T: closed or slit-like
  11. frontal shape: C: not wider posteriorly; T: wider posteriorly
  12. frontal shape 2: C: without posterior processes; T: with posterior processes
  13. posterior rim of parietal: C: transverse; T: anteriorly oriented or curved.
  14. parietal skull table: C: forms a sagittal crest: T: broad
  15. squamosal descent: C: mid level; T: ventral skull (ventral maxilla)
  16. skull roof fusion: C: parietal fusion only; T: frontal fusion and parietal fusion
  17. jaw joint orientation: C: descends from ventral mx; T: in line with ventral mx, after jugal arch.
  18. last maxillary tooth: C: posterior orbit; T: mid orbit
  19. mandible ventrally: C: 2-tier convex; T: straight
  20. 2nd sacral rib: C: not: T: double wide laterally
  21. manus/pes: C: subequal: T: manus smaller
  22. ilium: C: posterior process >; T: not
  23. metatarsal 1:4 ratio: C: 1 not > than half: 4 T: 1> half of 4
  24. metatarsals 2-4: C: < than half the tibia; T: not
  25. pedal 3.1 vs p2.1: C: not > T: 3.1>
  26. metatarsals 2 and 3: C: aligns with mt1; T: aligns with pedal 1.1
  27. pedal 4 length: C: subequal to mt 4; T: > mt4
  28. 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.

Figure 4. A new reconstruction of the Triceratops juvenile with the mandible and squamosal scaled to text measurements and shaped to adult elements compared to the original (Goodwin et al.) reconstruction which appears to have shortened the mandible.

Figure 4. A new reconstruction of the Triceratops juvenile with the mandible and squamosal scaled to text measurements and shaped to adult elements compared to the original (Goodwin et al.) reconstruction which appears to have shortened the mandible.

A YouTube video, Dinosaurs Decoded, shows Mark Goodwin reassembling the juvenile Triceratops skull. Click here to watch.

_______________________

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.

References
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.

 

 

5 thoughts on “Do ceratopsid juveniles (phylogenetically) nest together?

  1. We have a number of juvenile skulls intermediate between adult size and the UCMP baby, which are also more complete. Reconstructing the missing portions of the baby based on a single adult size skull is ridiculous. And btw, you have not understood the anatomy of the adult skull and miss traced it to produce your reconstruction. The posterior process of the premaxilla is not as long or broad, in that particular specimen or any other. This is yet further evidence that DGS cannot accurately derive the correct anatomy of a specimen.

    As for the potential discrepancy between the length of the dentary as stated in the text and what you have derived from the reconstruction, there is no discrepancy. In the reconstruction, the dentary is quite a bit foreshortened because the lower jaw is broader where it articulates with the skull than it is at the symphyseal region. Its triangular from a dorsal or ventral perspective.

    • Also, are those the only traits that you have coded for the between the Chasmosaurus and Triceratops? Are juvenile and adult coded the same state for each trait? If so then it’s not surprising that they have nested together, from the program’s perspective you have just added the same specimen twice. If this is the case, you are not testing anything.

  2. Browsing through your ceratopsian character list here, I have found some issues I’d like to bring up. Give me a couple of days however as it’s a lot of literature to go through to confirm/refute them.

    • This is taking longer than I wanted to devote to it. I’m sorry but apart from being objectively incorrect, in most cases your characters are so vaguely constructed that it is difficult to understand what feature you are even trying to express. In most instances I’ve just tried to guess.

      Anyway here are the first 7 that i’ve gone through. My comments are in square brackets.

      #1) skull table: C: depressed terrace, medial and lateral crests; T: convex [No idea what you are referring to.]

      #2) snout in dorsal view: C: not constricted; T: constricted [This is difficult to confirm for triceratops, as I can’t locate any actual photos of a triceratops skull in dorsal view. I do have a number of photos in ventral view, and a number of illustration from Hatcher’s monograph. Hatcher’s illustrations show a smooth concave profile converging on a point and where the maxilla articulates to the the premaxilla the profile diverges slightly before converging again. But this is shown only in some of his specimens, eg YPM 1823, 1822, USNM 4928. Others appear to lack it, YPM 1821, and BSP T. brevicornis. If this is what you are refering to, I suspect it is more of an taphonomic effect than anything else. Some specimens of Chasmosaurus as seen in dorsal view in Campbell et al 2016 show this as well.]

      #3) orbit position: C: postorbital > preorbital; T: subequal [I’m not sure what exactly this is referring to. It reads like you are attempting to compare the length of the skull in front of the orbit to the length behind, but you call it orbit position so IDk. As you have it constructed, This character is ontogenetically influenced. In both taxa, both the frill and snout elongate as the animal ages. If you are keeping up on the latest research on triceratops, this character is arguably also stratigraphically influenced. With Triceratops from the bottom of the Hell Creek formation typically having a more elongate frill than Triceratops from the top of the Hell creek.]

      #4) lateral rostral shape: C: convex, smooth curve; T: double convex [Again, I don’t know what you are referring to. Are you referencing the Rostral bone itself? The rostrum, as in the tip of the snout? Or rostral, as in the direction? You need to be very specific. Vagueness is rampant throughout all of your characters and it makes it very difficult to understand them. Anyway, Double Convex is a specific shape that doesn’t exist in a lateral view of the skull. If you are referring to the way the profile of the skull in dorsal view almost converges to a point then widens slightly at the premaxillary maxillary junction before finally coming to a point at the rostral, then this describes the exact same condition as character #2.]

      #5) nasals/frontals: C: nasals >; T: subequal [Difficult to assess without actual measurements from a number of skulls. Godfrey and Holmes 1995 doesn’t even have these measurements, and most subsequent ceratopsian papers follow those landmarks. Just eyeballing, I would expect the nasal to be greater in adults in both taxa, though especially Triceratops given its longer face. However, given that the snouts elongate I would also expect some ontogenetic variability.]

      #6) antorbital fenestra: C: absent; T: without mx fossa [Should this maybe be two separate characters, ie one for presence of antorbital fenestra, and one for presence of maxillary fossa? Also is it maybe a typo and you mean antorbital fossa? Regardless, your codings for this character are wrong. All ceratopsians have an antorbital fenestra. In some specimens it does get obliterated due to preservational issues and perhaps maybe old age. No idea as to what you are calling a maxillary fossa, there is a depression in the maxilla leading to the antorbital fenestra, so there is an antorbital fossa]

      #7) orbit/upper temporal fenestra: C: orbit not > T: orbit > [This is nonsensical. Why would you compare the size of the orbit to the upper upper temporal fenestra? The two do not have similar shapes. I think you might be confusing the parietal fenestrae with the upper temporal fenestra. The upper temporal fenestra is just a pocket behind the postorbital and bordered by the squamosal and parietal. Parietal fenestrae are openings entirely within the parietal bone. The upper temporal fenestra is typically obscured by crushing, but in good specimens it seems to be narrow and elliptical best as i can tell, and in at least one dimension larger than the orbit for both taxa.]

  3. Thanks for all your hard work, Chris. #7 I compare orbit size to utf size in all 690 taxa. So I do so again with Triceratops. #6 Mx fossa = antorbital fossa present on maxilla, as opposed to lacrimal. #3 orbit position, as before, I compare all taxa. As this changes in juveniles, it should lump them and more characters will have to split them. #1 note that three bony ridges radiate along the dorsal frill in Chasmosaurus. These are not so obvious or not present in Triceratops. Interesting point about the jaw length. I’ll double check the length change when applied to a triangle. Major take-away is the original authors were able to identify the two specimens as distinct genera. My analysis did the same, which is to be expected.

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