Hello r/DebateEvolution
Introduction
Creationists and proponents of intelligent design often claim that the fossil record of cetacean evolution shows no evidence of the gradual evolution of whales. These claims can be found in publications such as: [1], [2], [3],[4] .
As many of you now the whale evolution contain a big number of fossil, with many groups like:
- Pakicetidae: 50-48 Ma [5].
- Ambulocetidae: 53.5-48 Ma [5].
- Remingtonocetidae: 49-42 Ma [6]
- Protocetidae: 49-37 Ma [7]
- Basilosauridae: 46-33 Ma [8],[9]
- Neoceti: 35-0 Ma [10][11]
However, creationists argues that this fossils are not whale related mammals or "fully formed" whales. They claim than other terrestrial mammals not considered ancestral to whales have traits that we asociate with the primitive archaeocetes, for example:
- The triangular teeth
- Accessory cusps large
- Auditory bulla
With the basilosaurids they claim the opposite, argues that are aquatic whales and cannot use as transitional forms.
These arguments have major deficiencies. For example the paleontologist don't classifies the fossil acording a trait but sets of traits. And exists more traits that define the cetaceans.
Modern whales possess baleen in the case of mysticetes and a melon in the case of odontocetes; basilosaurids lacked both of these features. Therefore, they were not "fully formed whales," though we will discuss their nature later on.
In this part we will to treat the most basal archaeocetes (pakicetidae and ambulocetidae) they also are one of the most attacked by creationist fossils. Some of these characteristics can also aplies for the raoellids and the remingtonocetids.
1. Ear
One of the characteristics than differentiates the cetacean to other mammals is their capacity to hear underwater.
Modern cetacean have the next characteristics for can do it:
- Thickened medial bulla
- involucrum (cetacean synapomorphy, absent in any other mammal)
- Greatly enlarged mandibular foramen (the hole for the fat pad)
- Tympanoperiotic acoustically isolated
- Ossicular chain rotated. [12]
The development of the characteristic structures of cetacean ancestors occurred at an early stage of their evolutionary history; some even emerged before cetaceans themselves existed—for example, Indohyus (a raodellid) already possessed an involucrum [13]. Pakicetus retains this involucrum but still has a land-mammal ossicular chain and a small mandibular foramen, hearing mainly in air and using bone conduction underwater aided by its heavy bulla. In this stage the hearing in both air and water is unsophisticated [14].
Ambulocetus and remingtonocetids then show the first enlargement of the mandibular foramen and the development of the tympanic plate for fat-pad transmission [14]. In other words, the basilosaurid ear did not appear ex nihilo; it is the endpoint of a 10-million-year stepwise replacement of the external ear by the mandible.
land mammals: not Involucrum, Thickened medial bulla, etc.
Raeollidae: Involucrum, not small sigmoid process and small mandibular foramen. Early bone condution hearing [13]
Pakicetidae: Involucrum, Thickened medial bulla, Small sigmoid process and small mandibular foramen, no acoustic fat pad. Hearing in air, bone conduction underwater via heavy bulla.
Ambulocetidae: Greatly enlarged mandibular foramen, reduced lateral wall of bulla
Remingtonocetidae/protocetidae: tympanic plate, where fat pad contact, tympanoperiotic still attached to skull, but starting to isolate via air sinuses
Basilosauridae: Tympanoperiotic acoustically isolated, Ossicular chain rotated
Neoceti: Lost of funtional external auditory meatus [12]
In this scenario pakicetidae and ambulocetidae are intermideates between land artiodactils and fully aquatic whales. Creationist have argued that the sigmoid process of pakicetids is flat and small [4], but it don't invalidate the fact that still have a cetacean characteristcs and that their hearing is transitional.
2. Locomotion
Creationists often divide archaeocetes and other relatives of cetaceans into three groups:
- Land mammals: pakicetidae and raoellidae
- Semi-aquatic but not related with whales mammals: Ambulocetidae, remingtonocetidae and protocetidae
- Fully aquatic whales: basilosauridae
However, the picture is more complex: Pakicetus has typically been considered a terrestrial mammal due to its similarities to other artiodactyls, it has been suggested that it was no more amphibious than a tapir [15]. Creationists use this paper to claim that it's not a cetacean relative.
But, pakicetids exhibit some characteristics more consistent with a paleobiology linked to an aquatic environment:
- Osteoesclerotic bones: dense, thickened cortex acting as ballast, inconsistent with cursorial terrestrial mammals
- Dorsally placed orbits: positioned high on the skull for surface lurking [16], [17].
- Freshwater oxygen isotope signature: indicating prolonged time in water [18].
More recently, CMI claim that these semi-aquatic adaptations prove the trichotomy, but It couldn't be further from the truth. The reasons will discuss later on.
Ambulocetus, combine semi-aquatic and fully aquatic traits, for example its pelvis and anterior limb suggest than have a major capacity for walk in land than Remingtonocetids and protocetids [19]. But the rib cage present a different view, its rib thickness clustered with fully aquatic mammal like sirenians [20].
In the water, Ambulocetus moved using a pelvic motion—swimming with alternating strokes of its hind limbs, without using its forelimbs—and also undulated its tail while swimming. Modern otters provide a good analogy [21], although in their case, the foot remains the primary mechanism of propulsion.
The discoveries give us this evolutionary scenario:
Basal land artiodactil (e.g Diacodexis): not dense bones or another adaptations for the semi-aquatic life.
Raedollidae: osteosclerotic bones but less dense than true cetaceans; most terrestrial but related to the water [22].
Pakicetidae: Eyes in the upper part of the skull, foot powered swimmer; could live and walk easily in land, but weren't cursorial artiodactils.
Ambulocetidae: Osteosclerotic ribs as ballast., shorter limbs, large hind feet; Still with complete sacroiliac joint but with reduced ability for fast terrestrial locomotion; Pelvic paddling with alternate hindlimb strokes and lumbar undulation, ambush predator like a crocodile-otter hybrid.
Remingtonocetidae and protocetidae: Reduced hindlimbs, sacroiliac joint unfused or weakly connected. Long, narrow rostra, smaller orbits. Protocetids show tail-powered swimming beginning: caudal vertebrae with enlarged neural spines indicating strong tail musculature, forelimbs starting to act as flippers [23]. Still able to haul out, but poor walkers. More aquatic.
Basilosauridae (e.g. Basilosaurus, Dorudon): Forelimbs transformed into flippers with fixed elbow, hindlimbs vestigial and not connected to vertebral column [24]. Pelvic paddling abandoned for undulatory swimming with tail fluke [23].Cannot walk on land. Osteosclerosis reduced in limbs, pachyostosis in ribs. Fully aquatic.
Neoceti: Loss of hindlimbs; horizontal tail fluke; fully pelagic habitat.
3. Dentition
Teeth are indispensable for classifying fossil mammals; they also provide insight into their diet and habitat.
The cetacean synapomorphies incluide dental characters as:
- Incisors parallel with the toothrow
- protocones small
- accessory cusps large [25]
Creationist usually claim that this characteristics are present in other mammals (like mesonichya) and exist case in where we can't determine if this characters are present like ambulocetus.
However, this ignore that the mesonychya molars have considerably differences with the ancient archaeocetes molars:
While mesonychids possess triangular, laterally compressed upper molars adapted for shearing, early archaeocetes exhibit distinct protocone wear, double-rooted molars, and a gradual development of polydonty and the characteristic cetacean dental pattern [15],[17].
Pakicetids exhibit a greatly reduced trigon basin on the upper molars, and the talonid basin is virtually absent or non-basined in its conventional form, consisting only of a narrow heel with a single hypoconid. This condition differs from mesonychids, which lack a true talonid basin entirely and possess a hyper-carnassial trigonid without a basined structure [15].
On the other hand, as mentioned at the beginning, paleontologists do not rely on a single characteristic to group species, nor do they separate them if one is missing. If other characters are present that allow the species to be grouped and we have a strong phylogeny, it is possible to infer those traits that have not been preserved in the fossil record (like the ambulocetus incisors) . This is the phylogenetic bracketing method.
These differences are well documented and clearly distinguish both groups, despite superficial similarities.
Basal land artiodactyl (e.g. Diacodexis): 44 teeth, bunodont low-crowned molars for omnivory/herbivory, heterodont, diphyodont, both single and double-rooted. Grinding diet.
Raoellidae (e.g. Indohyus): Same dental formula: 11 teeth per jaw half, heterodont, diphyodont, both single/ double-rooted. Lower molar prim. bunodont but with early accessory cusps and heavier wear. Isotopic and microwear data show aquatic omnivory: aquatic plants + small fish. First step toward carnivory, but still grinding.
Pakicetidae (e.g. Pakicetus): Same formula: 11, heterodont, diphyodont, both rooted. Lower molar mult./prim. with large triangular trigonid and accessory cusps. Single cusp shearing instead of grinding. Tooth wear shows abrasion from fish scales, no plant grinding. Piscivore/carnivore,freshwater.
Ambulocetidae (e.g. Ambulocetus): Identical formula: 11, heterodont, diphyodont, both rooted. Teeth larger, thicker enamel, large canines, but molar morphology still prim. triangular with accessory cusps like Pakicetidae. No new cusp number. Shows dental stasis while skeleton becomes aquatic. Diet: ambush predator, durophagy, takes large fish and small tetrapods at water edge. Crocodile-like.
Remingtonocetidae and Protocetidae (e.g. Rodhocetus, Georgiacetus): Same formula: 11, heterodont, diphyodont, both rooted. Lower molar transitions to multiple cusps. Crowns become narrower, more cusps in line, less precise occlusion. Cannot chew efficiently, starts to pierce and swallow. More specialized piscivory.
Basilosauridae (e.g. Basilosaurus, Dorudon): Still heterodont and diphyodont, but mastication reduced. Molars with multiple cusps, large diastemata, no grinding facet. Last archaeocetes with functional heterodonty before loss. Apex predator: large fish, other marine mammals.
Neoceti: dietary bifurcation:
- Mysticeti (e.g Balaenidae, Neobalaenidae, Eschrichtiidae, Balaenopteridae): no teeth. Baleen filter feeding. Complete loss of mastication.
- Odontoceti (e.g. Physeter, Ziphiidae, Platanistidae, Delphinidae): homodont, monophyodont, all single-rooted, single cusp, more than 11 teeth per jaw half, polydont. Teeth for grasping only, no mastication. Suction and grip feeding [26].
Conclusion
Early archaeocetes, such as Ambulocetus and Pakicetus, exhibited a mosaic morphology that combined traits of primitive artiodactyls and basal cetaceomorphs with those of more advanced groups, such as remingtonocetids, protocetids, and pelagicetids.
They possessed many primitive characteristics but also displayed the emergence of distinctive cetacean traits: cetacean ear (involucrum) with a land-mammal hearing mechanism, dense bones for ballast but limbs still able to walk, and cetacean-like triangular teeth with accessory cusps in a terrestrial dental formula.
In Part 2 we will cover Remingtonocetidae and Protocetidae, where the fat pad, tail-powered swimming and the loss of terrestrial locomotion become evident.
References:
Creationist references:
[1] Answers in Genesis. (n.d.). Isn't the whale transitional series a perfect example of evolution? Answers in Genesis. https://answersingenesis.org/aquatic-animals/isnt-the-whale-transitional-series-a-perfect-example-of-evolution/?srsltid=AfmBOoqjCwqNXLc_OnMNOljVZcyjL7flACMaPArPuHrqH4kqFyqSAZX0
[2] Wieland, C. (2002). A whale of a tale? Creation Ministries International. https://creation.com/en/articles/a-whale-of-a-tale#addendum-2-4-january-2002
[3] Wells, J. (2018). From bears to whales: A difficult transition. Discovery Institute - Science and Culture Today. https://scienceandculture.com/2018/07/from-bears-to-whales-a-difficult-transition/
[4] Surtees, M. (2020). Whales designed or evolved: part 1—the fossils. Journal of Creation 37(3):121 - 127,https://creation.com/en/articles/whales-designed-or-evolved-1
Cientific literature:
[5] Bajpai, S., & Gingerich, P. D. (1998). A new Eocene archaeocete (Mammalia, Cetacea) from India and the time of origin of whales. Proceedings Of The National Academy Of Sciences 95 (26), 15464-15468. https://doi.org/10.1073/pnas.95.26.15464
[6] Bebej, R. M., Zalmout, I. S., El-Aziz, A. A. A., Antar, M. S. M., & Gingerich, P. D. (2015). First remingtonocetid archaeocete (Mammalia, Cetacea) from the middle Eocene of Egypt with implications for biogeography and locomotion in early cetacean evolution. Journal Of Paleontology, 89 (5), 882-893. https://doi.org/10.1017/jpa.2015.57
[7] Gingerich, P. D., Ul-Haq, M., Von Koenigswald, W., Sanders, W. J., Smith, B. H., & Zalmout, I. S. (2009). New Protocetid Whale from the Middle Eocene of Pakistan: Birth on Land, Precocial Development, and Sexual Dimorphism. PLoS ONE, 4 (2), e4366. https://doi.org/10.1371/journal.pone.0004366
[8] Köhler R. & Fordyce R. E. (1997).An archaeocete whale (Cetacea: Archaeoceti) from the Eocene Waihao Greensand, New Zealand. Journal of Vertebrate Paleontology 17 (3): 574-583. https://doi.org/10.1080/02724634.1997.10011004
[9] Buono, M. R., Fernández, M. S., Reguero, M. A., Marenssi, S. A., Santillana, S. N., & Mörs, T. (2016). Eocene basilosaurid whales from the La Meseta Formation, Marambio (Seymour) Island, Antarctica. _Ameghiniana, 53_(3), 296–315. https://doi.org/10.5710/AMGH.02.02.2016.2922
[10] McGowen, M. R., Spaulding, M., & Gatesy, J. (2009). Divergence date estimation and a comprehensive molecular tree of extant cetaceans. _Molecular Phylogenetics and Evolution, 53_(3), 891-906. https://doi.org/10.1016/j.ympev.2009.08.018
[11] Montgomery, S. H., Geisler, J. H., McGowen, M. R., Fox, C., Marino, L., & Gatesy, J. (2013). THE EVOLUTIONARY HISTORY OF CETACEAN BRAIN AND BODY SIZE. Evolution, 67 (11), 3339-3353. https://doi.org/10.1111/evo.12197
[12] Nummela, S., Thewissen, J., Bajpai, S., Hussain, T., & Kumar, K. (2007). Sound transmission in archaic and modern whales: Anatomical adaptations for underwater hearing. The Anatomical Record, 290 (6), 716-733. https://doi.org/10.1002/ar.20528
[13] Patel, S; Nanda, A C.; Orliac, M, and Thewissen, J.G.M. 2024. Cranial anatomy of Indohyus indirae (Raoellidae), an artiodactyl from the Eocene of India, and its implications for raoellid biology. Palaeontologia Electronica, 27(1):a21. https://doi.org/10.26879/1307
[14] Nummela, S., Thewissen, J. G. M., Bajpai, S., Hussain, S. T., & Kumar, K. (2004). Eocene evolution of whale hearing. Nature, 430 (7001), 776-778. https://doi.org/10.1038/nature02720
[15] Thewissen, J. G. M., Williams, E. M., Roe, L. J., & Hussain, S. T. (2001). Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature, 413 (6853), 277-281. https://doi.org/10.1038/35095005
[16] Thewissen, J. G. M., Cooper, L. N., George, J. C., & Bajpai, S. (2009). From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evolution Education And Outreach, 2 (2), 272-288. https://doi.org/10.1007/s12052-009-0135-2
[17] Uhen, M. D. (2010). The Origin(s) of Whales. Annual Review Of Earth And Planetary Sciences, 38 (1), 189-219. https://doi.org/10.1146/annurev-earth-040809-152453
[18] Clementz, M. T., Goswami, A., Gingerich, P. D., & Koch, P. L. (2006). Isotopic records from early whales and sea cows: contrasting patterns of ecological transition. Journal Of Vertebrate Paleontology, 26 (2), 355-370. https://doi.org/10.1671/0272-4634(2006)2626)
[19] Madar, S. I., Thewissen, J. G. M., & Hussain, S. T. (2002). Additional holotype remains ofAmbulocetus natans(Cetacea, Ambulocetidae), and their implications for locomotion in early whales. Journal Of Vertebrate Paleontology, 22 (2), 405-422. https://doi.org/10.1671/0272-4634(2002)022022)
[20] Ando, K., & Fujiwara, S. (2016). Farewell to life on land – thoracic strength as a new indicator to determine paleoecology in secondary aquatic mammals. Journal Of Anatomy, 229 (6), 768-777. https://doi.org/10.1111/joa.12518
[21] Thewissen, J. G. M., & Fish, F. E. (1997). Locomotor evolution in the earliest cetaceans: functional model, modern analogues, and paleontological evidence. Paleobiology, 23 (4), 482-490. https://doi.org/10.1017/s0094837300019850
[22] Cooper, L. N., Thewissen, J. G., Bajpai, S., & Tiwari, B. N. (2011). Postcranial morphology and locomotion of the Eocene raoellid Indohyus (Artiodactyla: Mammalia). Historical Biology, 1-32. https://doi.org/10.1080/08912963.2011.624184
[23] Gingerich, P. D., Antar, M. S. M., & Zalmout, I. S. (2019). Aegicetus gehennae, a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales. PLoS ONE, 14 (12), e0225391. https://doi.org/10.1371/journal.pone.0225391
[24] Uhen, M. D. (2009). Basilosaurids. In W. F. Perrin, B. Würsig & J. G. M. Thewissen (Eds.), Encyclopedia of marine mammals (2nd ed., pp. 91-94). Academic Press. https://doi.org/10.1016/B978-0-12-373553-9.00026-2
[25] Berta, A. (1994). What Is a Whale? Science,263 (5144), 180-181. https://doi.org/10.1126/science.263.5144.180
[26] Thewissen, J. G. M., & Williams, E. M. (2002). The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations. Annual Review Of Ecology And Systematics,33 (1), 73-90. https://doi.org/10.1146/annurev.ecolsys.33.020602.095426